Keithley 228aref schematic

Model 228A Voltage/Current Source

All references to the Model 228 apply also to the Model 228A

01985, Keithley Instruments, Inc.

All rights reserved.

Cleveland, Ohio, U.S.A.

Document Number: 228A.904.01

WARRANTY

Keithley Instruments, Inc. warrants this product to be fox from defects in ma&al and workmanship for a period of I year
from date of shipment.

Keithley Instruments, Inc. warrants the following items for 90 days from the date of shipment: probes, cables, rechargeable
batteries, diskettes, and documentation.

During the warranty period, we will, at our option, either repair or replace any product that proves to be defective,

To exercise this warranty, write or call your local Keithley representative, or contact Keithley headquarters in Cleveland, Ohio.
You will be given prompt assistance and return instructions. Send the product, transportation prepaid, to the indicated service
facility. Repairs will be made and the product returned, transportation prepaid. Repaired or replaced products are warranted for
the balance of the original warranty period, or at least 90 days.

LIMITATION OF WARRANTY

This warranty does not apply to defects resulting from product modification without,Keithley’s express written consent, or
misuse of any product or part. This warranty also does not apply to fuses, software, non-rechargeable batteries, damage from
battery leakage, or problems arising from normal wear or failure to follow instructions.

THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING ANY
IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. THE REMEDIES PRO­VIDED HEREIN ARE BUYER’S SOLE AND EXCLUSIVE REMEDIES.

NEITHER KEITHLEY INSTRUMENTS, INC. NOR ANY OF ITS EMPLOYEES SHALL BE LIABLE FOR ANY DIRECT,
INDIRECT, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OF ITS
INSTRUMENTS AND SOFTWARE EVEN IF KEITHLEY INSTRUMENTS, INC., HAS BEEN ADVISED IN ADVANCE
OF THE POSSIBILITY OF SUCH DAMAGES. SUCH EXCLUDED DAMAGES SHALL INCLUDE, BUT ARE NOT LIM­ITED TO: COSTS OF REMOVAL AND INSTALLATION, LOSSES SUSTAINED AS THE RESULT OF INJURY TO ANY
PERSON, OR DAMAGE TO PROPERTY.

Safety Precautions

The following safety precautions should be observed before using
[his product and any associated instrumeniarion. Although some in­stmmen~s and accessories would normally be used with non-haz­ardous voltages. rhere are situations where hazardous conditions
may be present.

This product is intended for use by qualified personnel who recog-

nize shock hazards and are familiar with the safety precaurions re­quired to avoid possible injury Read the operating information
carefully before using the product.

The types of product users are:
Responsible body is Ihe individual or group responsible for the use

and maintenance of equipment, for ensuring rhat the equipment IS
operared within its specifications and operating limits. and for en-

surin~ rhat operators are adequately trained.
Operators use the product for its intended function. They must be

trained in electrical safety procedures and proper use of the instill­n,ent. They must be protected from electric shock and contact with
hazardous live circuits.

Maintenance personnel perform routine procedures on the product
to keep it operating, for example, setting the line voltage or replac-

ing consumable materials. Maintenance procedures are described in
the manual. The procedures explicitly state if the operator may Peru
form them. Otherwise, they should be performed only by service
personnel.

Service personnel are trained to work on live circuits, and perform

safe installations and repairs of products. Only properly trained ser­vice personnel may perform installation and service procedures.

Users of this product must be pnxecred from e,cc,ric shock at all
times. The responsible body must ensure lhar users are prevenred
access and/or insulated from every connection point. In some cases.
connections musk be exposed 10 polenrial human contacI. Producl
users in these circumsmnces must be rained IO protect themselves

irom the risk of electric shock. If the circuit is capable of operatins
BI or above 1000 VOIIS. no conductive part of the circuit may be
exposed.

As described in rhe International Eleclrotechnical Commission

(IEC) Standard IEC 664. digital multimeter measuting circuits

(e.g.. Keithley Models I75A, 199. 2000,200l. 2002, and 2010) are

Inslallation Category II. A,, orher i”strume”,s’ s;gnaI lerminals are

Installation Category I and must nor be connected to mains.

Do not connect switching cards direcdy 10 unlimited power circuits~

They are intended to be used with impedance limited sources.

NEVER connect swiiching cards directly 10 AC mains. When con­necting sources to switching cards. install protective devices 10 lim­it fault currem and voltage to rhe card.

Before operating an ins,rumen,. make sure rhe line cord is conncct­ed to a properly grounded power receptacle. lnrpcct the connccim~
cables, test leads. and jumpers for possible wear. cracks. or breaks

before each use.

For maximum safety, do not touch the product, test cables. or an)
orher instruments while power is applied IO the circuil under :es,.
ALWAYS remove power from ihe entire lest system and discharge
any capacitors before: connccring or disconnecting cables or jump­ers, installing or removing switching cards. or making imemal

changes, such as installing or removmg]umpers.

Exercise extreme caution when a shock hazard is present. Lethal
voltage may be present on cable connector jacks or test fixtures. The
Am&an Xational Standards Institute (ANSI) states that a shock
hazard exists when vo,,age levels grearer than 30V RMS. 42.4V

peak, or 60VDC are present. A good safety practice is to expect
that hazardous voltage is present in any unknown circuit before
measuring.

Do not louch any objea rhat could provide a currcm path to the
common side of the circuit under rest or power line (earth) sround.
Always make measurements wirh dry hands while rrandinp on a
dry. insulated surface capable of wirhstanding [he 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.

The WARNING heading in a manual explains dangers that might
result in personal injury or death. Always read the associated infor­mation very carefully before performing the indicated procedure.

Do not exceed the maximum signal levels of the instruments and ac­cessorics, 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 rype and rating
for continued protection against fire hazaid.

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

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

lfa 0,

mew is present, connect it to safety earth ground using the

wire recommended in the user documentation.

The h

symbol on an instrument indicates that the user should re-

fer to the operaring instructions located in the manual.

Then 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 IO
avoid personal contact with these voltages.

The CAUTION heading in a manual explains hazards that could
damage the instrument. Such damage may invalidate the warranty.

Insrmmentation 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 circuils, including the power transformer, test
leads, and input jacks, must be purchased from Keithley lnstru­merits. Standard fuses, with applicable national safely approvals,
may be used if the rating and type are the same. Other componenrs
that are not safety related may be purchased from other suppliers as
long as they are equivalent to the original component. (Note that se­lected pans 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 [he 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 inm 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.

Rev. 10199

CHAPTER 1

Introduction

This booklet has been written to help the user understand the operation and specifications
of the Model 228 Voltage/Current Source. The chapters in this booklet have been written
to aid the user in some applications. Terms relating to the V/I source such as: stability, line
regulation, load regulation and sensing are defined in the glossary. For a complete explana­tion of the instrument including front panel controls and IEEE-468 bus operation refer to
the Model 228 Instruction Manual (Document Number 228-901-01).

The Model 228 is a power supply that can source OT sink electrical power. These unique

features set it apart from ordinary power supplies. Actually, because of the Model 228’s
precision and wide dynamic range, the term SOURCE is used to differentiate it from or­dinary power supplies. Most power supplies do what the name implies they supply power­usually voltage or current. The Model 228 is capable of precise control of voltage or current
while sourcing or sinking power.

In general, instruments that are defined as power sources supply electrical power to a con­nected load. Some power sources can also act as a load for a” external source. This is generally
known as sinking power. There are numerous applications for a” instrument that can sink
power as well as supply power. For example, the Model 228 can charge then discharge a
battery at a controlled rate.

There are three features that differentiate the Model 228 from power supplies:

1. Multiple Ranges

2. Four Quadrant Operation

3. Constant Voltage/Co”sta”t Current

Glossary

Accuracy-Accuracy is defined as how close the actual output value reflects the programmed

value. Display monitor accuracy is defined as how close the display monitor reflects
the actual output value.

Auto Cal-The A/D converter in the Model 228 measures various gains and offsets within

the instrument. With this information, the microprocessor calculates calibration fac­tors which it uses when programming the output. Power on, Program 6 and Program
9 all start the Auto Cal sequence.

Battery Back Up-Values programmed into the 100 step memory are saved even when the

AC power is turned off. This is possible because the values are stored in CMOS memory
which is powered by a rechargeable battery.

Common Mode Voltage-Common mode voltage is the potential difference between

“earth ground and normally common” output terminal of the Model 228. This voltage

is normally generated when connecting two power supplies in series.

Compliance-Compliance is defined as the extent to which an output parameter (voltage

or current) deviates from the programmed value without compromise of the output

parameter. e.g. For constant voltage operation, current control yields to voltage control

as long as the actual output current is within the range of +ISE~ING (the program­med setting) to -IsETTING (the compliance setting). Outside this range, the current
control circuit attempts to take control of the output away from the voltage control circuit.

Constant Voltage/Current-A constant current source can force current through a device

under test almost independently of the load resistance. A constant voltage source can

deliver a voltage across a device almost independently of the load resistance.

Current Monitor-The Model 228 output current is sensed internally and converted to the

*lV full scale voltage. This voltage is routed to the A/D converter and circuitry but,

also is available to the user via the current monitor output terminals.

Dwell Time-Dwell time is the time spent on a specific memory location. Dwell time is used

in the Single Step, Single Cycle and Continuous memory control modes. The range

of dwell time is 20msec to 1000sec.

Floating-Floating is the term used to describe a condition where a common mode voltage

exists between earth ground and the instrument or conductor of interest.

Four Quadrant Operation-In four quadrant operation, voltage or current can be either

positive or negative thus, four combinations are possible. Positive polarity of voltage

and current is defined as “source quadrants”, where power is delivered from the Model

228 to the user’s load. Opposite polarity of voltage and current (+V, -I) or (-V, +I) is de-

fined as “sink quadrants”, where power from the user is dissipated in the Model 228.

Impedance-The effect of R, L, or C in series or parallel with the output. This causes the

output voltage or current to vary slightly when load changes occur.

Load-The device to which power is delivered.

Load Line-The load line is the operating line on the V vs I graph for a particular load.

Load Regulation-Load regulation is the ability of the Model 228 to keep the output voltage

or current constant when the load changes.

Modulation (External)-External modulation is a low frequency (DC to 600H.z) low voltage

(rlOV) signal that is supplied by the user and is superimposed on the output signal

of the Model 228.

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Operate-Operate programs the output to the user selected values. Standby programs the

output to OV, OA. During operate, the two displays show actual measured values of
voltage and current. During standby, the programmed settings are displayed.

Quick Disconnect Board-An output board that contains the OUTPUT, SENSE, EXTERNAL

MODULATION and CURRENT MONITOR terminals. This board fits into the re­cessed slot on the rear panel of the Model 228.

Response Time-Response time is the time the Model 228 microprocessor takes to respond

to a signal on the External Trigger input. Response time is measured from trigger in­put until the output has change 99% of the difference between the old memory step
and the new memory step. A range change, polarity change or change between voltage
and current extends the response time.

Self Test-During the power on sequence the Model 228 tests memory (RAM and ROM),

the AID converter control circuitry and power supplies. During operation, the Model
228 monitors temperatures, power supplies and the AID converter. If an error is
discovered, the Model 228 attempts to protect the user and itself.

Sense (Local and Remote)-Remote sensing maintains regulation at the load instead of the

output terminals of the Model 228 therefore, compensating for the test leads IR voltage
drop. Local sensing maintains regulation at the output terminals of the Model 228.

Source (Vice Sink-Source is a condition where in the Model 228 delivers power to the user’s

load.
Stability-Stability is the ability to keep from changing.
Trigger IN & OUT-The TRIGGER IN and TRIGGER OUT connectors are female BNC con-

nectors that accept or output a negative going greater than 10~s~ TTL pulse. The trigger

input pulse starts the memory control mode. The trigger output pulse appears at the

TRIGGER OUT connector at the end of each programmed dwell time.

Standby (Vice Operate)-The standby mode programs the Model 228 to output OV, OA and

displays the programmed settings instead of the actual values. Polarity and range

changes are delayed until the OPERATE button is pressed. Modulation is disabled

in the standby mode.

Sink (Vice Source)-Sink is described as the ability of the Model 228 to dissipate power sup-

plied by the user’s circuit. This includes discharging batteries, inductive loads and

capacitive loads.

Sink Only-Sink only reduces internal dissipation so that a full 1OOW can be dissipated by

the Model 228 for long periods of time, even at high ambient temperatures, with no

derating. Since this function reduces the Model 228’s sourcing capability (= 1.5A) it
should only be used when sourcing is not required.

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CHAPTER 2

Safety Precautions

The following information outlines general safety precautions that should be observed before,
during and after operating the Model 228.

1. Before operation, ground the Model 228 through a properly earth grounded receptacle.
Failure to ground the instrument may result in severe injury or death in the event of
a short circuit or malfunction.

2. Never assume the output is at a safe potential while the AC line is connected.

3. Never come into contact with the output connections while the instrument is turned on.

4. Always set up the test circuit while power is turned off. Do not come into contact with
any part of the test circuit while power is on.

5. Always place the instrument in standby after the measurement or test is complete.

6. Use cables for the output that have appropriate current and insulation rating. For ex­ample, if 1OV at 10A is to be produced, or dissipated, then the cables must be rated for
that amount. Also, use insulated lugs for connections on the quick disconnect board.

7. The Model 228 is capable of producing several times its current rating for short periods
of time. Keep this in mind when choosing a load. Brief bursts of high current are still
enough to damage other inskumentation and cause serious injury.

8. Do not operate the Model 228 with the top and/or bottom covers removed. Lethal poten­tials are present throughout the mainframe. The covers must also be in place to allow
proper air flow through the instrument. Proper air flow is required to cool the instru­ment during operation. If proper cooling is impeded the instrument may overheat.

9. When connecting active circuits, observe proper polarity (sink mode). A reversed polarity
may allow the instrument to operate at the current limit of the output fuse (20A).

10. When using the Model 228 to sink power from an external source refer to the example
program and Program 2 (Sink) in Section 3 of the Model 228 Instruction Manual.

11. The MODULATION- and the I MONITOR- terminals on the quick disconnect board

are electrically shorted.

12. The OUT- terminal is at a potential similar to the MODULATION and I MONlTOR
terminals. Shorting the OUT terminal to the MODULATION and/or I MONITOR ter-

minals shorts the Model 228’s current limiting circuitry. The current limiting circuitry
will not function properly if the terminals are shorted. Grounded equipment can cause
this problem.

Example: A grounded function generator connected to the modulation input of the Model

228 with a grounded oscilloscope connected across the output terminals of the Model 228.

13. When using multiple Model 228’s do not connect the modulation inputs together or the
current monitor outputs. Remember, that the OUT- terminals and these other terminals
are at similar potentials.

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