Keithley 2790 User Manual

Model 2790 SourceMeter

®

Switch System

User’s Manual

A GREATER MEASURE OF CONFIDENCE

Model 2790 SourceMeter® Switch System

User’s Manual

©2002, Keithley Instruments, Inc.

All rights reserved.

Cleveland, Ohio, U.S.A.

Third Printing, September 2003

Document Number: 2790-900-01 Rev. C

Manual Print History

The print history shown below lists the printing dates of all Revisions and Addenda created
for this manual. The Revision Level letter increases alphabetically as the manual undergoes
subsequent updates. Addenda, which are released between Revisions, contain important change
information that the user should incorporate immediately into the manual. Addenda are
numbered sequentially. When a new Revision is created, all Addenda associated with the
previous Revision of the manual are incorporated into the new Revision of the manual. Each new
Revision includes a revised copy of this print history page.

Revision A (Document Number 2790-900-01)............................................................. March 2002

Revision B (Document Number 2790-900-01) ............................................................. March 2002

Revision C (Document Number 2790-900-01) ...................................................... September 2003

All Keithley product names are trademarks or registered trademarks of Keithley Instruments, Inc.
Other brand names are trademarks or registered trademarks of their respective holders.

S

afety Precautions

The following safety precautions should be observed before using this product and any associated instrumentation. Although
some instruments and accessories would normally be used with non-hazardous voltages, there are situations where hazardous
conditions may be present.

This product is intended for use by qualified personnel who recognize shock hazards and are familiar with the safety precautions
required to avoid possible injury. Read and follow all installation, operation, and maintenance information carefully before using
the product. Refer to the manual for complete product specifications.

If the product is used in a manner not specified, the protection provided by the product may be impaired.

The types of product users are:

Responsible body

ment is operated within its specifications and operating limits, and for ensuring that operators are adequately trained.

Operators

instrument. They must be protected from electric shock and contact with hazardous live circuits.

Maintenance personnel

voltage or replacing consumable materials. Maintenance procedures are described in the manual. The procedures explicitly state
if the operator may perform them. Otherwise, they should be performed only by service personnel.

Service personnel

trained service personnel may perform installation and service procedures.

Keithley products are designed for use with electrical signals that are rated Measurement Category I and Measurement Category
II, as described in the International Electrotechnical Commission (IEC) Standard IEC 60664. Most measurement, control, and
data I/O signals are Measurement Category I and must not be directly connected to mains voltage or to voltage sources with
high transient over-voltages. Measurement Category II connections require protection for high transient over-voltages often as­sociated with local AC mains connections. Assume all measurement, control, and data I/O connections are for connection to
Category I sources unless otherwise marked or described in the Manual.

Exercise extreme caution when a shock hazard is present. Lethal voltage may be present on cable connector jacks or test fixtures.
The American National Standards Institute (ANSI) states that a shock hazard exists when voltage levels greater than 30V RMS,

42.4V peak, or 60VDC are present.

circuit before measuring.

Operators of this product must be protected from electric shock at all times. The responsible body must ensure that operators
are prevented access and/or insulated from every connection point. In some cases, connections must be exposed to potential
human contact. Product operators in these circumstances must be trained to protect themselves from the risk of electric shock.
If the circuit is capable of operating at or above 1000 volts,

Do not connect switching cards directly to unlimited power circuits. They are intended to be used with impedance limited sourc­es. NEVER connect switching cards directly to AC mains. When connecting sources to switching cards, install protective de­vices to limit fault current and voltage to the card.

Before operating an instrument, make sure the line cord is connected to a properly grounded power receptacle. Inspect the con­necting cables, test leads, and jumpers for possible wear, cracks, or breaks before each use.

When installing equipment where access to the main power cord is restricted, such as rack mounting, a separate main input pow­er disconnect device must be provided, in close proximity to the equipment and within easy reach of the operator.

For maximum safety, do not touch the product, test cables, or any other instruments while power is applied to the circuit under
test. ALWAYS remove power from the entire test system and discharge any capacitors before: connecting or disconnecting ca-

is the individual or group responsible for the use and maintenance of equipment, for ensuring that the equip-

use the product for its intended function. They must be trained in electrical safety procedures and proper use of the

perform routine procedures on the product to keep it operating properly, for example, setting the line

are trained to work on live circuits, and perform safe installations and repairs of products. Only properly

A good safety practice is to expect that hazardous voltage is present in any unknown

no conductive part of the circuit may be exposed.

5/03

bles or jumpers, installing or removing switching cards, or making internal changes, such as installing or removing jumpers.

Do not touch any object that could provide a current path to the common side of the circuit under test or power line (earth) ground. Al­ways make measurements with dry hands while standing on a dry, insulated surface capable of withstanding the voltage being measured.

The instrument and accessories must be used in accordance with its specifications and operating instructions or the safety of the
equipment may be impaired.

Do not exceed the maximum signal levels of the instruments and accessories, as defined in the specifications and operating in­formation, and as shown on the instrument or test fixture panels, or switching card.

When fuses are used in a product, replace with same type and rating for continued protection against fire hazard.

Chassis connections must only be used as shield connections for measuring circuits, NOT as safety earth ground connections.

If you are using a test fixture, keep the lid closed while power is applied to the device under test. Safe operation requires the use
of a lid interlock.

If a screw is present, connect it to safety earth ground using the wire recommended in the user documentation.

!

The symbol on an instrument indicates that the user should refer to the operating instructions located in the manual.

The symbol on an instrument shows that it can source or measure 1000 volts or more, including the combined effect of
normal and common mode voltages. Use standard safety precautions to avoid personal contact with these voltages.

The symbol indicates a connection terminal to the equipment frame.

The

WARNING

information very carefully before performing the indicated procedure.

The

CAUTION

ranty.

Instrumentation and accessories shall not be connected to humans.

Before performing any maintenance, disconnect the line cord and all test cables.

To maintain protection from electric shock and fire, replacement components in mains circuits, including the power transformer,
test leads, and input jacks, must be purchased from Keithley Instruments. Standard fuses, with applicable national safety ap­provals, may be used if the rating and type are the same. Other components that are not safety related may be purchased from
other suppliers as long as they are equivalent to the original component. (Note that selected parts should be purchased only
through Keithley Instruments to maintain accuracy and functionality of the product.) If you are unsure about the applicability
of a replacement component, call a Keithley Instruments office for information.

To clean an instrument, use a damp cloth or mild, water based cleaner. Clean the exterior of the instrument only. Do not apply
cleaner directly to the instrument or allow liquids to enter or spill on the instrument. Products that consist of a circuit board with
no case or chassis (e.g., data acquisition board for installation into a computer) should never require cleaning if handled accord­ing to instructions. If the board becomes contaminated and operation is affected, the board should be returned to the factory for
proper cleaning/servicing.

heading in a manual explains dangers that might result in personal injury or death. Always read the associated

heading in a manual explains hazards that could damage the instrument. Such damage may invalidate the war-

Table of Contents

1Test System Overview

Introduction ................................................................................ 1-2

Key features ........................................................................ 1-2

Technical terms ................................................................... 1-3

Model 2790 configurations ........................................................ 1-4

Test system ................................................................................. 1-5

Inflator tests ................................................................................ 1-6

Shunt bar test ....................................................................... 1-6

Bridgewire test .................................................................... 1-8

Insulation resistance (HIPOT) test (7751 and 7753 only) .. 1-9

2 Keithley 7751, 7752, and 7753 Switching Modules

Card configuration ...................................................................... 2-2

I-source – channels 21, 22, 24, and 27 ................................ 2-4

V-source and I/V amplifier (7751/7753 only) –

Cable discharge (7751/7753 only) – channel 20 ................. 2-4

Source readback – channels 13 and 25 ............................... 2-5

Interlock – J106 ................................................................... 2-5

General purpose terminal blocks – J105, J107, and J108 ... 2-5

Safety warnings ................................................................... 2-6

Setting source output levels ........................................................ 2-6

Remote programming ......................................................... 2-8

Math ........................................................................................... 2-9

Front panel operation ........................................................ 2-11

Remote programming ....................................................... 2-11

Opening and closing channels .................................................. 2-13

Front panel operation ........................................................ 2-13

Remote programming ....................................................... 2-14

Viewing closed channels .......................................................... 2-15

Open interlock errors ............................................................... 2-16

Cable discharge circuit (7751 and 7753) ................................. 2-17

Connection log ......................................................................... 2-19

Using memory patterns ............................................................ 2-20

Firmware revision level ..................................................... 2-20

Overview ........................................................................... 2-20

Front panel operation ........................................................ 2-20

Memory pattern commands .............................................. 2-21

Setting up and executing memory patterns ....................... 2-21

Memory pattern command options ................................... 2-22

Memory pattern scanning ................................................. 2-22

Memory pattern log ........................................................... 2-23

channels 21, 22, 23, 28 .................................................. 2-4

3 Keithley 7702 Switching Module

Safety precautions ...................................................................... 3-2

Card configuration ...................................................................... 3-3

Typical connections .................................................................... 3-5

Connection log .................................................................... 3-7

Opening and closing 7702 channels ........................................... 3-9

Channel assignments ........................................................... 3-9

Front panel operation ........................................................ 3-10

Remote programming ........................................................ 3-11

Viewing closed channels .......................................................... 3-12

Using 7751/7753 V-source with 7702 module ......................... 3-13

4 Switching Module Wiring and Installation

Connections and wiring .............................................................. 4-2

Screw terminals ................................................................... 4-2

Cabling requirements .......................................................... 4-3

Wiring procedure ................................................................. 4-4

Zippertubing® shield .......................................................... 4-8

Guarding .............................................................................. 4-9

Switching module installation .................................................. 4-10

Module slot considerations ................................................ 4-10

Power supply restrictions with the 7753 module .............. 4-10

Installation procedure ........................................................ 4-10

5 Inflator Testing

Introduction ................................................................................ 5-2

Test procedures .................................................................... 5-2

Closed channels for test circuits .......................................... 5-3

Test system ................................................................................. 5-3

RS-232 interface (PLC or PC) ............................................. 5-4

GPIB (PC only) ................................................................... 5-4

Test signal connections ............................................................... 5-5

Interlock ...................................................................................... 5-9

Shunt bar test ............................................................................ 5-10

Test circuit ......................................................................... 5-11

Test procedure ................................................................... 5-12

I-source readback .............................................................. 5-12

Bridgewire tests ........................................................................ 5-14

Test circuit ......................................................................... 5-14

Test procedure ................................................................... 5-15

I-source readback .............................................................. 5-16

Insulation resistance (HIPOT) tests (7751/7753 only) ............. 5-17

Contact checks .................................................................. 5-18

V-source readback ............................................................. 5-22

Cable discharge ................................................................. 5-23

HIPOT test ........................................................................ 5-24

Test procedure – HIPOT ................................................... 5-26

I-source readback ..................................................................... 5-28

Memory patterns test ................................................................ 5-30

Test circuit ......................................................................... 5-30

Test procedure ................................................................... 5-31

6 Model 2790 DMM Ohms Functions

Introduction ............................................................................... 6-2

Low ohms measurements .................................................... 6-2

Fuse resistance considerations ............................................ 6-2

High ohms measurements ................................................... 6-3

4-wire ohms ................................................................................ 6-3

Offset compensated ohms .......................................................... 6-5

Keithley 7702 module ................................................................ 6-6

Keithley 7751, 7752, and 7753 modules .................................... 6-8

A Specifications

(Models 2790, 7751, 7752, 7753, and 7702) ............................ A-1

Ohms specification calculations (7751/7752/7753) .................. A-8

7751/7752/7753 I-source ................................................... A-8

7751/7753 V-source ........................................................... A-9

List of Illustrations

1Test System Overview

Figure 1-1 System connections ............................................................... 1-5

Figure 1-2 Shunt bar testing .................................................................... 1-7

Figure 1-3 Bridgewire testing .................................................................. 1-8

Figure 1-4 HIPOT testing (7751/7753) ................................................... 1-9

2 Keithley 7751, 7752, and 7753 Switching Modules

Figure 2-1 Simplified schematic of Keithley 7751, 7752, and 7753

switching modules ............................................................ 2-3

Figure 2-2 Setting I-source and V-source output levels ........................... 2-7

Figure 2-3 Enabling ohms math function .............................................. 2-11

Figure 2-4 Opening and closing channels ............................................. 2-14

Figure 2-5 Viewing closed channels ...................................................... 2-15

Figure 2-6 Cable discharge circuit ......................................................... 2-18

3 Keithley 7702 Switching Module

Figure 3-1 Simplified schematic of Keithley 7702 switching module .... 3-4

Figure 3-2
Figure 3-3

Figure 3-4 Current connections (DCI and ACI) ...................................... 3-6

Figure 3-5 Voltage connections (DCV or ACV) ...................................... 3-7

Figure 3-6 andkeys ................................................................................ 3-10

Figure 3-7 CLOSE key .......................................................................... 3-10

Figure 3-8 OPEN key ............................................................................ 3-11

Figure 3-9 Viewing closed channels ...................................................... 3-12

Figure 3-10 Using 7751/7753 V-source with 7702 module

Ω

2 and thermistor TEMP connections .................................. 3-5

Ω

4 and RTD TEMP connections ........................................... 3-6

(dual multiplexer) ........................................................... 3-14

4 Switching Module Wiring and Installation

Figure 4-1 Screw terminal access ............................................................ 4-3

Figure 4-2 Model 7751/7752/7753 screw terminal channel designations 4-5

Figure 4-3 Model 7702 screw terminal channel designations ................. 4-6

Figure 4-4 Wire dressing (7751/7752/7753) ........................................... 4-7

Figure 4-5 Zippertubing® shield (ZTZ-*-SH3 series) ............................ 4-8

Figure 4-6 Installed Zippertubing® shield .............................................. 4-9

5 Inflator Testing

Figure 5-1 System connections ................................................................ 5-3

Figure 5-2 Test connections – shunt bar and bridgewire

testing – single stage inflator ............................................ 5-5

Figure 5-3 Test connections – shunt bar, HIPOT, and bridgewire

testing – single stage inflator ............................................ 5-6

Figure 5-4 Test connections – shunt bar and bridgewire bar

testing – dual stage inflator ............................................... 5-7

Figure 5-5 Test connections – shunt bar, HIPOT, and bridgewire

testing – dual stage inflator ............................................... 5-8

Figure 5-6 Interlock connections ............................................................. 5-9

Figure 5-7 Test circuit – shunt bar ......................................................... 5-11

Figure 5-8 Test circuit – bridgewire tests ............................................... 5-14

Figure 5-9 Test circuits – contact check – single stage inflator ............. 5-19

Figure 5-10 Test circuits – contact check – dual stage inflator ................ 5-19

Figure 5-11 V-source readback circuit ..................................................... 5-22

Figure 5-12 Test circuit – HIPOT ............................................................ 5-25

Figure 5-13 I-source readback circuit ...................................................... 5-29

Figure 5-14 Test circuits – memory patterns – dual stage

inflator contact test ......................................................... 5-30

6 Model 2790 DMM

Ohms Functions

Figure 6-1 Using W2 function to measure resistance .............................. 6-3

Figure 6-2 Using W4 function to measure resistance .............................. 6-4

Figure 6-3 Using Model 2790 4-wire ohms with 7702 module

(low ohms measurements) ................................................ 6-6

Figure 6-4 Using Model 2790 2-wire ohms with 7702 module

(high ohms measurements) ............................................... 6-7

Figure 6-5 Using Model 2790 4-wire ohms with 7751/7752/7753

module (low ohms measurements) ................................... 6-8

Figure 6-6 Using Model 2790 2-wire ohms with 7751/7752/7753

module (high ohms measurements) .................................. 6-9

List of Tables

2 Keithley 7751, 7752, and 7753 Switching Modules

Table 2-1 Commands to set amplitude for V-source and I-source ......... 2-8

Table 2-2 Commands to select ohms calculations ............................... 2-12

Table 2-3 Commands to control 7751/7752/7753 module channels .... 2-14

Table 2-4 Connection log Model 7751/7752/7753 .............................. 2-19

Table 2-5 Basic memory pattern example ............................................ 2-22

Table 2-6 Memory pattern log ............................................................. 2-23

3 Keithley 7702 Switching Module

Table 3-1 Connection log Model 7702 .................................................. 3-8

Table 3-2 Commands to control 7702 module channels ...................... 3-11

5 Inflator Testing

Table 5-1 Test procedure – shunt bar ................................................... 5-13

Table 5-2 Test procedure – bridgewire ................................................. 5-15

Table 5-3 Test procedure – contact checks for single stage inflator .... 5-20

Table 5-4 Test procedure – contact checks for dual stage inflator ....... 5-21

Table 5-5 Test procedure – V-source readback .................................... 5-23

Table 5-6 Test procedure – HIPOT ...................................................... 5-26

Table 5-7 Command sequence for memory patterns test example

Table 5-8 Command sequence for memory patterns test example

using scanning ..................................................................... 5-31

using individual memory pattern recall ............................... 5-33

6 Model 2790 DMM

Ohms Functions

Table 6-1 Commands to set offset compensated ohms .......................... 6-5

Test System Overview

•

Introduction

Model 2790 to test inflators. Covers some key features of the Model 2790 and
defines/explains technical terms associated with airbag inflators.

•

Model 2790 configurations

tions available from Keithley.

•

Test system

•

Inflator tests

and insulation resistance (HIPOT).

— Summarizes the switching modules that are used with the

— Explains the various mainframe/module combina-

— Shows a typical test system used to test inflators.

— Explains the three basic tests for inflators: shunt bar, bridgewire,

1

1-2 Model 2790 SourceMeter® Switch System User’s Manual

Introduction

When used with one or two switching modules, the Model 2790 can be used to test inflators.
With a 7751/7752/7753 module, the Model 2790 can safely measure the resistance of
bridgewires and shunt bars. With a Model 7751 or 7753 module, the Model 2790 can use the
high voltage method to measure the insulation resistance of an inflator. The 7702 module can
be used for ohms testing that requires 20/40 channels of 4-wire/2-wire measurements.

Key features

•

Programmable I-source

programmable current source (0 to 50mA). This constant current source is used to
measure the resistance of bridgewires and shunt bars.

The Model 2790 has a low-ohms math function: SxIohms. It calculates (using the
programmed current and measured voltage) and displays the resistance (in ohms)
of the bridgewire or shunt bar. X is the slot number of the module being used.

•

Programmable V-source

mable voltage source (50 to 500V) and an I/V converter to measure the insulation
resistance of an inflator. Current through the insulator is applied to the I/V con­verter where it is converted to a voltage that is measured by the Model 2790.

The Model 2790 has a high-ohms math function: SxVohms. It calculates (using the
programmed V-source voltage and measured I/V converter output voltage) and dis­plays the resistance (in ohms) of the insulation. X is the slot number of the module
being used.

•

Safety features

bility of inflator ignition and damage to the module due to excessive current.

•

Interlock

the sources (I-source and V-source) from the switching matrix. Details on the
interlock are provided in Section 5.

•

Fuses — Current limiting fuses for the current source of the Model 2790

DMM and the I-source of the 7751/7752/7753 module.

•

I-source current limiting

for the I-source of the 7751, 7752, and 7753 modules.

•

V-source current limiting

— The Model 2790 has built-in safety features to reduce the possi-

— The interlock feature of a 7751/7752/7753 module disconnects

— The 7751, 7752, and 7753 modules have a built-in

— The 7751 and 7753 modules have a built-in program-

— An active, high-speed current limiting circuitry

— An impedance circuit limits current to <1mA.

Model 2790 SourceMeter® Switch System User’s Manual 1-3

Technical terms

The following defines and explains technical terms associated with airbag inflators:

•

Initiato

action. Sometimes referred to in classic military terms as a “squib”. The term
“squib” may be used to refer to the core element of an initiator (explosive coated
bridgewire) or to the entire initiator.

•

Airbag inflator

ing a stored chemical or pressurized inert gas (or both), and an electrical initiator.

•

Hybrid inflator

to generate the gas charge for bag inflation.

•

Airbag or Airbag module

textile bag and mounting frame (or enclosure).

•

E-check

modules. Consists of two basic resistance measurements:

•

HIPOT

using high voltage. The intent is not to break down the insulator, but to measure the
leakage current that flows through the insulator.

•

Body check (contact check)

housing must be verified. This is necessary because an open connection would
result in a reading that appears to be the resistance of the insulator, but would
instead be a high resistance reading that is the result of an open circuit.

•

Shorting clip (shunt bar)

(electro-static discharge) or other accidental discharge, every inflator module has a
spring loaded clip that shorts the initiator/squib pins together when a connector is
not installed. The connection cables also include shorting clips since many inflator
modules have cables attached when manufactured such that the open end of the
cables also have shorting clips. These clips are pneumatically pushed open during
E-check testing just after the clips resistance (as a short) is measured.

r — Pyro-technic device used to trigger the main gas charge generation

— The gas generating assembly, typically made of steel, contain-

— An inflator that uses both pressurized gas and a chemical reaction

— The fully functional device including initiator, inflator,

— Industry standard term for electrical testing of airbag inflators and

–Test continuity of the bridgewire (typically 2 to 3 ohms) using a constant current

method (typically 10 to 50mA).

– HIPOT test — Measure the insulation resistance (≥100MΩ) between the

bridgewire and the inflator housing using an applied voltage (typically 500V).

— Used in this manual to describe the high resistance measurements made

— Prior to HIPOT testing, connections to the inflator

— For handling and transport safety against ESD

1-4 Model 2790 SourceMeter® Switch System User’s Manual

Model 2790 configurations

Model 2790 test configurations available from Keithley include the following:

2790-H configuration (one 7751 module)

This test system includes a single 7751 module. This module has a programmable 50 to
500V V-source and I/V amplifier for HIPOT testing. It also has a programmable 0 to
50mA constant current I-source to measure the resistance of shunt bars and bridgewires.

2790-HH configuration (two 7751 modules)

This test system includes two 7751 modules. When used with a “left and right” type
station, this two-module, high-voltage system allows higher throughput for increased
efficiency.

• When used with a “left and right” type station, this two-module, high-voltage system
allows two inflators to be tested. It provides higher throughput for increased effi­ciency. When soaking is required for HIPOT testing, one inflator can be tested while
the other inflator is soaking (high voltage applied).

•For a dual inflator, the two high-voltage module system allows parallel “soak and
test” of each half.

2790-HL configuration (one 7751 and one 7752 module)

This test system includes a 7751 module and a 7752 module. The high voltage module
(7751) can be used for HIPOT testing, while the I-source of the second module (7752) can
be used to test other non-inflator type circuitry (i.e., pushbutton switches and resistors) on
the steering wheel. The I-source allows accurate resistance measurements up to 100Ω.

2790-L configuration (one 7752 module)

This test system includes a single 7752 module. It is applicable for testing that does not
require high voltage HIPOT testing, but still needs the programmable I-source for low
ohms testing. Insulation resistance (up to 120MΩ) can be measured using the Ω2 function
of the Model 2790.

2790-H/L/A + 7702 configuration (one 7751/7752/7753, one 7702 module)

This test system includes a 7751, 7752, or 7753 module and a 7702 module. The 7702
module can be used for non-inflator related ohms testing that requires 20/40 channels of
4-wire/2-wire measurements. Typical tests using the 7702 module include burn-in/life/QA
testing of multi-pin connectors, wiring harnesses, power distribution/fuse centers,
switches and relays, and backplanes.

Model 2790 SourceMeter® Switch System User’s Manual 1-5

2790-A configuration (one 7753 module)

This test system includes a single 7753 module. This module has a programmable 50 to
500V V-source and I/V amplifier for HIPOT testing. It also has a programmable 0 to
50mA constant current I-source to measure the resistance of shunt bars and bridgewires.

Supplementary system components

For added versatility or as spares, a Model 2790 mainframe, 7751, 7752, 7753, and 7702
modules can be purchased separately. However, this special order may require a longer
delivery time and additional handling charges.

Test system

A typical test system for airbag inflators includes a Model 2790 that is controlled by a
PLC or PC, and a device handler/test station for the inflator. The Model 2790 has two slots
for switching modules. Modules for testing inflators include the Models 7751, 7752, and

7753. A Model 7702 module can also be used with the Model 2790.

NOTE Only one Model 7753 module can be installed in a Model 2790, and it should be

installed in slot 1. The 7753 can be installed in slot 2, but measurement accuracy
might be adversely affected because cards are always calibrated in slot 1.

Figure 1-1 shows a typical test system using a programmable logic controller (PLC) or a

PC. When using a PLC, communications with Model 2790 are done over the RS-232
interface. When using a PC, communications with the Model 2790 can be provided using
the GPIB or RS-232.

Figure 1-1

System connections

PLC

or PC

(RS-232)

OR

PC

(GPIB)

KPCI-488

Interface

RS-232

GPIB

Keithley 2790

MATH

2.790000

DIGITAL I/O

RS-232

IEEE-488

Device Handler Control

Slot 1

7751, 7752, or

Ω

7753 Module

Interlock

Slot 2

Test

System

Expansion

Test Signal

Cabling

Device Handler

Mechanical
Connection

Test Station

(with Interlock)

Inflator Under

Test

Interlock

1-6 Model 2790 SourceMeter® Switch System User’s Manual

Inflator tests

There are three primary tests for an inflator:

• Shunt bar test

• Bridgewire test

• Insulation resistance (HIPOT) test

NOTE Interlock – The three primary tests can only be performed if the interlock of the

7751/7752/7753 module is enabled. With interlock open (disabled), the I-source
of the 7751/7752/7753 module and the V-source of the 7751/7753 module are
disconnected from the switching module matrix. Details on the interlock are pro­vided in Section 5.

The following information provides an overview of the three tests that are used
to test an inflator. The actual detailed procedures for these tests are provided in

Section 5.

Shunt bar test

NOTE A shunt bar test is typically tested again after all other inflator tests are completed.

As a final test, it verifies that it is safe to install the inflator in a vehicle.

A shunt bar (shorting clip) is used to short the bridgewire(s) to prevent accidental ignition
when handling or installing the inflator. However, an oxide can form between the shunt
and the inflator terminals. It is this high-resistance oxide that diminishes the ability of the
shunt bar to act as an effective short across the bridgwire(s). Without a good contact short,
detonation could occur. The measured resistance of a shunt bar is typically between 10 to
100mΩ.

One method used for the shunt bar test is shown in Figure 1-2. In this case, the dry circuit
mode of the 7751/7752/7753 clamps voltage at 20mV (1mA maximum) for the constant
test current of the I-source. This low voltage will not pierce the oxide. With the low ohms
math function (SxIohms) selected, the R
play of the Model 2790.

reading can be read directly from the dis-

SHUNT

Model 2790 SourceMeter® Switch System User’s Manual 1-7

Figure 1-2

Shunt bar testing

Keithley 2790 SourceMeter Switch System

Keithley

7751,7752,

or 7753
Module

I

SOURCE

(1mA max)

20mV

Dry
Ckt

Shunt Bar

Under

Test

(R

SHUNT)

v

2790

DMM

RSHUNT =

MEAS

V
ISOUR

NOTE If required, a more accurate resistance measurement of the shunt bar can be

achieved by disabling the dry circuit and using a test current of 50mA. An even
more accurate resistance measurement can be achieved using the Ω4 function of
the Model 2790. Keep in mind that this is an additional test and does not replace
the test using the dry circuit.

NOTE Use this 2-step process to detect the presence of oxide build-up and more

accurately measure the resistance of the shunt bar:

1. Test the shunt bar using dry circuit ohms (channel 24 closed) and a test

current of 1mA. A resistance >100mΩ indicates the presence of oxide
build-up.

2. Accurately measure shunt bar resistance without dry circuit (channel 24

open) and a test current of 50mA or use the Ω4 function of the Model 2790.

Section 6 explains how to use the Ω4 function of the Model 2790 with the

7702 or 7751/7752/7753 module.

1-8 Model 2790 SourceMeter® Switch System User’s Manual

Bridgewire test

The air bag inflator has two terminals connected to a bridgewire, which is coated with a
primer that ignites when sufficient current passes through the wire. The bridgewire’s resis­tance is typically from two to three ohms (2Ω being the most common). To avoid unintended
ignition, the level of test current applied is generally 50mA or less.

As shown in Figure 1-3, a fixed current is sourced through the bridgewire. The voltage
across the bridgewire is then measured by the DMM of the Model 2790.

Figure 1-3

Bridgewire testing

Keithley 2790 SourceMeter Switch System

Keithley

7751,7752,

or 7753
Module

I

SOURCE

Bridgewire

Under

Test

(RBRID)

v

2790

DMM

RBRID =

MEAS

V
ISOUR

With the low ohms math function selected, the R

reading can be read directly from

BRID

the display of the Model 2790.

I-source readback — Before performing the bridgewire test, you can verify the actual
current flowing in the test circuit by using the I-source readback circuit.

NOTE A more accurate ohms reading can be achieved using the Ω4 function of the

Model 2790 (1mA fixed). See Section 6 for details.

Model 2790 SourceMeter® Switch System User’s Manual 1-9

Insulation resistance (HIPOT) test (7751 and 7753 only)

WARNING The Models 7751 and 7753 are provided with outputs that are poten-

tially hazardous if not connected properly. It is the responsibility of the
customer to operate instruments in a safe manner. Be sure to read and
follow all installation and operating instructions before attempting to
use the 7751/7753 HIPOT function.

The leakage resistance (R

) between each of the initiators and the housing must be

HIPOT

verified. As shown in Figure 1-4, this is performed by applying a voltage, typically 500V,
between the bridgewire and the housing. The resultant test circuit current is applied to an
I/V amplifier, where it is converted to a voltage. This voltage is then measured by the
DMM of the Model 2790.

NOTE If not using a 7751/7753 module in your test system, resistance up to 120MΩ

can be measured using the Ω2 function of the Model 2790.

Figure 1-4

HIPOT testing (7751/7753)

Keithley 2790 SourceMeter Switch System

Keithley 7751/7753 Module

RF

–
+

I/V Amplifier

(7751/7753)

SOUR

V

x RF

VMEAS

500V

x R

MEAS

V

v

RF = 200kΩ (7751)

F = 20kΩ (7753)

R

F

2790

DMM

+

V

SOURCE

(500V)

RHIPOT

RHIPOT = –

= –

1-10 Model 2790 SourceMeter® Switch System User’s Manual

With the high ohms math function (SxVohms) selected, the R

reading can be read

HIPOT

directly from the display of the Model 2790.

Before measuring R

, there are a couple of preliminary verification tests that should

HIPOT

be performed first:

• Contact checks — To properly perform the HIPOT test, contact must be made to
the air bag inflator housing. If good contact is not established, the measured high
resistance will not represent the isolation resistance. Contact is checked by per­forming continuity checks through the V-source test leads and the connection
points. The resistance reading for a contact should be less than 20 ohms (depending
on cabling).

• V-source readback — Before performing the HIPOT test, the output voltage of the
V-source should be verified. The V-source readback circuit allows you to directly
measure the output voltage using the DMM of the Model 2790.

2

Keithley 7751, 7752, and 7753

Switching Modules

• Card configuration — Provides the simplified schematic of the Keithley 7751,

7752, and 7753 switching modules. Summarizes the individual components of the
modules.

• Setting source output levels — Explains how to set the output levels for the

I-source (7751/7752/7753) and V-source (7751/7753).

• Math — Covers the ohms math functions used to calculate low resistance when

using the I-source of the 7751/7752/7753, and high resistance when using the
V-source and I/V amplifier of the 7751/7753 module.

• Opening and closing channels — Explains how to open and close channels for the

7751, 7752, and 7753 modules.

• Viewing closed channels — Explains how to display closed channels on the

Model 2790.

• Open interlock errors — Explains the errors associated with an open interlock.

• Cable discharge circuit (7751 and 7753) — Explains the cable discharge circuit. It

dissipates voltage at the input channels. This unwanted voltage could ignite the
inflator.

• Connection log — Use to record connection information.

• Using memory patterns — Describes the memory patterns feature, which can be

used to simplify repetitive test procedures.

2-2 Model 2790 SourceMeter® Switch System User’s Manual

NOTE Module wiring and installation into the mainframe of the Model 2790 is covered

in Section 4.

Details on using the Keithley 7751, 7752, 7753 modules to test inflators are
provided in Section 5.

The edit keys (left, right, up, and down arrow keys) are used to display menu
items and output levels. With the desired menu item or output level displayed, it
is selected by pressing ENTER.

For remote programming, the <clist> parameter is used as follows:

<clist> = (@SCH)

where: S = Mainframe slot number (1 or 2)

CH = Switching module channel number of source (2 digits)

Examples: (@101) = Slot 1, Channel 1

(@101, 114, 118) = Slot 1, Channels 1, 14, and 18

Card configuration

CAUTION External energy sources should never be applied directly to a 7751,

7752, or 7753 module for any reason. Damage to the module and any
connected devices under test can result due to the application of
improperly selected or performing external energy sources.

The simplified schematic for the 7751, 7752, and 7753 is provided in Figure 2-1.

• Channels 1-12 — Input channels for DUT (inflator).

• Channels 13-19 and 25 — Routes test signals to backplane of Model 2790.

• Channels 20-24, 27, and 28 — Control individual components of the 7751/7752/7753.

Model 2790 SourceMeter® Switch System User’s Manual 2-3

Figure 2-1

Simplified schematic of Keithley 7751, 7752, and 7753 switching modules

Ch. 25

Ch. 13

Vsource
Ch. 28
(7751/7753
Only)

50-500V

Bank 1

J101

Bank 2

J102

Bank 3

J103

Bank 4

J104

Source

HI

Sense

Source

LO

Sense

Source

HI

Sense

Source

LO

Sense

Source

HI

Sense

Source

LO

Sense

Source

HI

Sense

Source

LO

Sense

+
–

Ch. 1

Ch. 2

A1

Ch. 3

B1

Ch. 4

Ch. 5

A2

Ch. 6

B2

Ch. 7

Ch. 8

A3

Ch. 9

B3

Ch. 10

Ch. 11

A4

Ch. 12

B4

Ch. 14

Ch. 15

Ch. 16 Ch. 18

Ch. 17

Ch. 23

Ch. 19

Source Enable

10kΩ

10kΩ

200kΩ (7751)

20kΩ (7753)

–

+

+5V

HI

Input

LO

HI

Sense

LO

Interlock
+5V
Interlock

Guard
Guard

Source HI

Source LO
Sense HI
Sense LO

I/V IN
I/V OUT
I/V LO

Vsrc HI
Vsrc LO
Isrc HI
Isrc LO

Guard
Guard

I/V
Current/Voltage
Amplifier
(7751/7753 Only)

To Model
2790
Backplane

J106

J105

J108

J107

Isource
Ch. 27

0-50mA

Ch. 24

20mV

Dry

Ch. 22

Ckt

Ch. 22 Open: I-source Select
Ch. 22 Close: V-source Select

Ch. 21

1Ω

Ch. 20

100kΩ

Cable Discharge

(7751/7753 Only)

2-4 Model 2790 SourceMeter® Switch System User’s Manual

I-source – channels 21, 22, 24, and 27

The programmable I-source (0 to 50mA) is used as the test current to measure low resis­tance (inflator bridgewires and shunt bars).

• The I-source is designated as channel 27.

• Opening channel 22 selects the I-source.

• Closing channel 21 connects the I-source.

• Closing channel 24 connects the dry circuit clamp for the shunt bar test (20mV,
1mA limit).

NOTE “Ohms specification calculations” in Appendix A explains how to determine the

accuracy specification for source levels not specified in the “7751/7752/7753
Source/Switch Module Specifications” sheet.

V-source and I/V amplifier (7751/7753 only) – channels 21, 22, 23, 28

The programmable voltage source (50 to 500V) and I/V amplifier are used to measure
high resistance (insulation resistance of an inflator).

• The V-source is designated as channel 28.

• Closing channel 22 selects the V-source.

• Closing channel 21 connects the V-source.

• Closing channel 23 connects the I/V amplifier.

WARNING Do not connect V-source Hi to earth ground. The 7751/7752/7753 mod-

ule uses a floating ground. This floating ground is not connected to
safety earth ground. Not only will it create a shock hazard by floating
up to 500V, it will also effectively connect a 1nF capacitor across the
V-source, providing energy storage that could ignite an airbag under
fault conditions.

NOTE “Ohms specification calculations” in Appendix A explains how to determine the

accuracy specification for source levels not specified in the “7751/7752/7753
Source/Switch Module Specifications” sheet.

Cable discharge (7751/7753 only) – channel 20

A cable discharge circuit is used to dissipate charge build-up on the inputs to prevent acci­dental ignition of the inflator. For details, see “Cable discharge circuit”.

• Closing channel 20 discharges voltage on the input channels.

Model 2790 SourceMeter® Switch System User’s Manual 2-5

Source readback – channels 13 and 25

The actual output of the selected source can be verified by using source readback, which
routes the selected source back to the DMM of the 2790 for measurement. I-source and
V-source readback are explained in Section 5.

• Opening channel 22 and closing channels 21, 25, and 18 connects the I-source
sense resistor to the DMM of the Model 2790.

• Closing channels 22, 21, 13, and 18 connects the V-source to the DMM of the
Model 2790.

Interlock – J106

A 7751/7752/7753 module has an interlock to disconnect its I-source and V-source from
the rest of the switch matrix. In order to use the module sources, interlock must be
enabled. The interlock switch of a test station is connected to the J106 terminals of the
module. When the interlock switch is open (i.e., safety shield open), the sources cannot be
connected to the switch matrix. Details on interlock are provided in Section 5.

NOTE The Interlock switch does not affect the output of J107. Voltage can still be present

at those terminals even if the interlock is disabled. See “General purpose terminal

blocks – J105, J107, and J108” for additional details.

General purpose terminal blocks – J105, J107, and J108

There are quick-disconnect terminal blocks available to the user to provide access to various
switching module components. These are provided for general purpose test applications that
are not necessarily related to airbags.

J105 (matrix backplane and guard)

The backplane terminals provide access to the relay matrix backplane. The matrix backplane
is the primary “artery” for signal routing though the module.

The guard terminals are typically used for shielding when using the V-source and I/V con­verter for high ohms measurements. Guard connections are explained in Section 4.

J107 (source outputs and guard)

The I-source and V-source (7751/7753) are always on and can be accessed by the user at
J107. The programmed output is routed to these terminals.

The guard terminals are typically used for shielding when using the V-source and I/V
converter for high ohms measurements. Guard connections are explained in Section 4.

2-6 Model 2790 SourceMeter® Switch System User’s Manual

J108 (I/V converter)

The user also has access to the input, output, and ground of the I/V converter at J108.

Safety warnings

WARNING Integrated safety features protect the 7751/7752/7753 module from its

built-in I-source and V-source. However, if an external source (such as
the 12V car bus) is connected to the front panel inputs of the Model 2790
or through a 7702 module, these safety features are compromised, creat­ing the very real possibility of igniting the airbag. Incorrect channel clo­sures in the test system could inadvertently apply the external source to
the inflator and ignite it. Damage to the 7751/7752/7753 module may
also occur if an external source is connected to it.

WARNING Since the Model 2790 system is not an intrinsically safe device, it is the

responsibility of the user to ensure that external protection be pro­vided, either by an inherently safe electrical barrier and/or a safety bar­rier around the DUT or airbag, to prevent injury in case of detonation.

Setting source output levels

I-source and V-source (7751/7753) output levels are set from the CONFIG item of the
CARD menu. The menu structure, shown in Figure 2-2, is accessed by pressing and
releasing SHIFT and then pressing CARD.

• I-source — Set output from 0 to 50mA. In the menu structure, the I-source is desig­nated as channel 27. The default setting is 1mA.

•V-source (7751/7753 only) — Set output from 50 to 500V. In the menu structure,
the V-source is designated as channel 28. The default setting is 50V.

NOTE When using the V-source as an independent source, limit 7751 output current to

50µA or 7753 output current to 500µA to achieve rated accuracy. Typical short
circuit current is <1mA.

Model 2790 SourceMeter® Switch System User’s Manual 2-7

Figure 2-2

Setting I-source and V-source output levels

Press

Press

Select

SLOT1: 77xx

7751
7752
7753

7751
7753

I-C27:01.00 mA

V-C28:050.0 V

* 7753 should be installed in slot 1 for rated accuracy.

SHIFT

CARD

CONFIG

77xx =7751/7752/7753

Select module

Set I-source

output

Set V-source

output

SLOT2: 77xx

I-C27:01.00 mA

V-C28:050.0 V

7751
7752
7753*

7751
7753*

2-8 Model 2790 SourceMeter® Switch System User’s Manual

Remote programming

As shown in Table 2-1, the amplitude command is used to set the outputs of the sources.
Notice that the channel number of the V-source (28) and I-source (27) must be included in
the command string.

Table 2-1

Commands to set amplitude for V-source and I-source

Commands Description Default

SOURce:VOLTage[:LEVel][:IMMediate]

[:AMPLitude] <NRf>, <clist> Set V-source output; 50 to 500 (V).

50V

<clist> = (@128) Slot 1

= (@228) Slot 2

[:AMPLitude]? <clist> Query V-source output level.

SOURce:CURRent[:LEVel][:IMMediate]

[:AMPLitude] <NRf>, <clist> Set I-source output; 0 to 50 (mA).

0.001A

<clist> = (@127) Slot 1

= (@227) Slot 2

[:AMPLitude]? <clist> Query I-source output level.

Note: Selecting an ohms calculation enables source tracking. Selecting any other math function disables

source tracking.

Model 2790 SourceMeter® Switch System User’s Manual 2-9

Math

There is a shortcut math function to calculate low resistance when using the I-source of a
7751/7752/7753 module, and one to calculate high resistance when using the V-source and
I/V amplifier of a 7751/7753 module.

Low ohms calculation (SxIohms)

• SxIohms – Calculates and displays low resistance reading for 7751/7752/7753
module in slot x (where x =1 or 2).

This ohms function is a shortcut for the mX+b calculation. When selected, the following
“m” and “b” factors are used:

m = 1 / I

SOUR

b = 0

Calculation: R = mX + b
= (1/ I
= V

where: I

SOUR

MEAS

is the programmed current level of the I-source of the 7751, 7752, or

SOUR

/ I

) x V

SOUR

MEAS

+ 0

7753.

1 / I

V

MEAS

is the “m” factor for the calculation (mX+b).

SOUR

is the voltage measured by the DMM of the Model 2790. It is the “X”

variable for the calculation (mX+b).

When the SxIohms calculation is selected, the Model 2790 measures voltage (X) and
calculates mX+b using the above factors. The displayed reading will be in ohms (Ω). This
selection also sets DMM to DCV, 1V range.

NOTE SxIohms cannot be selected if the I-source is set to 0mA. Since “m” = 1 / I

SOUR

dividing 1 by 0 would result in an erroneous reading (positive infinity).

When using the low ohms calculation, the highest DCV measurement range that can be
selected is 1V. Attempting to select a higher range will cause error -222 (Parameter out of
range). Attempting to turn Autoranging on generates error -221 “Settings conflict” over
the GPIB or a “NO AUTORANGE” message on the display if done from the front panel
AUTO key.