Keithley 225 Service manual

Instruction Manual

Model 225

Current Source

Publication Date: August 1976

Keithley Instruments, Inc.

Cleveland, Ohio, U.S.A.

Document Nufnber: 29039

WARRANTY

Keithley Instruments, Inc. warrants this product to be free from defects in material and workmanship for a period of 1 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 Wmanty period, we will, at

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.

OUT

option, either repair or replace any product that proves to be defective.

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.

CONTENTS

MODEL 225

CONTENTS

SP*CIFICATIONS---------------------------------------------------------------------

1. GENERAL DESCRIPTION------------------------------------------------------------

2. OPERATION----------------------------------------------------------------------

3. CIRCUIT DESCRIPTION------------------------------------------------------------

4. ACCESSORIES--------------------------------------------------------------------

5. SER”ICING----------------------------------------------------------------------

lj. CALIBRATION--------------------------------------------------------------------

7. REPLACEABLE PARTS--------------------------------------------------------------

SCH~rIC------------------------------------------------------------------------

ILLUSTRATIONS

Figure NO.

Title Prone Panel.

2 Front Panel Controls

Rear Panel Controls. 4 Connections. 5 6

Resistive Load

Currenf Sink 7 Capacitive Load. 8 9

Inductive Load

Floating Supply.

Volt Limit Characteristic.

Modulation

12 Block.Diagram.

13 14 15 16

17 18 19

Component Layout, PC-227 (Switch Board).

Component Layauc, PC-228 (Mother Board).

Component Layout,

Chassis - Top View

Accuracy Check ...........................

Regulation Check Modulation Check

20 Inductive Load

21 22 23

current Mode "S Voltage Made Waveforms Top Cover Assembly Bortom Cover Assembly.

............................

........................

............................

..........................

...........................

..........................

.............................

...........................

Switch 5101.

.........................

..........................

........................

Check

.........................

.......................

.....................

...................

..............

...............

Page

1 3 3 4 6 6 6 6 6

7 8

13 13

14 14 18 18

18 21 21

0676

iii

CAUTION

MODEL 225

CAUTION

IF THE DECADE CURRENT DIALS ARE

SET

TO O-O-O and if the Output Selector switch is not at STANDBY, a COMPLIANCE VOLTAGE up to 100 volts may be present at the oucpuc terminals. For r&x"rn operator safety the Model 225 Current Source should always be set to STA!IDBY mode when no current output is needed. dial seteings and/or Voltage Compliance.

The Output Selector switch (S105) should also be set to

This will ensure that no transient currente will be

STANDBY

prior to changing current

generated.

WARNING

This insrrument is no? approved for use in "hazardous locations" as defined in the National Electrical Code, Classes I, II, and III.

Class I: Those locations in which flameble gases or vapors are or may be present in the air in

quantifies sufficient to produce explosive or ignitable mixcures.

Class II: Those locations that are hazardous because of the presence of combustible dust. Class III: Those locations that are hazardous because of the presence of easily ignitable fibers

or flyings.

0676

MODEL 225

OUTPUT:

RESOLUTION: Three significant figuresfrom lo-’

ACCURACY: z 0.5% of reading. 2 0.05% of full

STABILITY: -0.02% of reading, _cO.O05% of

NOISE: Less than 0.01% rms of full rangeabove

SPECIFICATIONS

D.C. Current: lo-’ ampere full range to lo-’ ampere in seven decade ranges, 3.digit in. line readout. Voltage: 100 voltsmaximum. Compliancelimit continuously variable from 10 to 100 volts.

Polarity: Positive or negative. Floating: c 500 volts maximum off chassis ground, less than 5 ppm of full range change in output current per volt.

to 10-l ampere. “Trim” potentiometer permits

0.02% of full range or better resolution.

ra”ge.

full range on the 10-l to 1O-6 ampere range,

(e-0.1% of reading 20.02% of range on the 10-T ampere range) the first hour or in sub.

sequent E-hour periods after a l.hour warm-

up and at reasonably constant ambient tern.

perature.

5 HZ.

SPECIFICATIONS

LOADREGULATION: t0.005% offullrangefrom

no load to full load on the lo-’ to 10-6 am. pere range, 50.05% on the lo-’ ampere range (with FILTER “OFF”).

LINE REGULATION: k 0.005% Of full range for

10% change in line voltage.

OVERLOAD PROTECTION: Voltage limited tocorn.

pliance voltage setting of polarity selected. Automatic recovery from overload.

MODULATION: Transformer input permitsmodw

lation of current from 50 Hz to 500 Hr.

Input 2 approximately 500 ohms.

FILTER: For operation with inductive loads up to

100 millihenries having greater than 10.6L/R ratio. Limits shunt output impedance to 1

microfarad shunted bygreaterthan 1010ohms.

ENVIRONMENT: 50% relative humidity limit at

25’C.

CONNECTORS: Output (front and rear panels):

BNC

POWER: 105.125, 210.250 volts (switch se.

lected), 50.60 Hz; 25 watts.

DIMENSIONS, WEIGHT: 5%” high x 8%” wide x

10” deep; net weight, 8 pounds.

0571

225 CURRENT SOURCE

.IWE

GENERAL DESCRIPTION

MODEL 225

SECTION 1.

1-I. GENERAL. full scale ranges frarn 10-7 to 10-l

1-2. FEATURES.

The Model 225 is a current source with

ampere.

GENERAL DESCRIPTION

c. Accuracy. The three decade dials adjust the out-

put current to +.05% of full range and +.5% of reading.

d. Polarity. The OUTPUT SELECTOR Switch provides

polarity selection without changing connections c" the

a. Resolution.

The thpe decade dials plus a "Trim"

load.

potentiometer provides a resolution of .02% of full range.

e. Floating Capability. The instrument LOW can be

floated up t" 2500 volts above chassis.

b. Compliance Voltage. A front panel Compliance Cantral provides a" adjustable maximum compliance voltsee from 10 to 100 volts.

f. Modulation. The ""tput current can be modulated

over a frequency range from 50 t" 500 Hz.

TABLE l-l.

Front Panel Controls

CQ"trOl Punctianal Description

Power Switch (S202) Controls line power to instrument. Decimal point serves

as a pilot lamp. OUTPUT SELECTOR Switch (SlO5) VOLTAGE COMPLIANCE Control

Selects positive or negative polarity or standby pasitio".

Sets the maximum compliance voltage which can be developed

(R117> at the OUT terminal.

Paragraph

2-4, a

2-4, b 2-4, c

LIMIT Indicator (DSlOl)

When lighted, the instrument is in Voltage mode. Range Switch (SlOl) Selects the full scale range. The decimal point is auto-

matically positioned by the Range switch. Selectar Switches (S102,

Selects the magnitude af current in 3 decade positions.

s103, SlO4) FINE Control (R13S) Adjusts current. FILTER Switch (5106)

When set to "ON", the switch connects a 1 p capacitor at the OUT terminal.

TABLE 1-2.

Rear Panel Controls end Termirals

Control or Terminal

Power Cord (P201) LINE

VOLTAGE

Switch (S201) FUSE (F201) OUT Receptacle (3104) LOW Terminal (5201) GROOND Terminal (520)

Pravides line power to instrument. Sets instrument for 117 or 234 V line operatia". Type 3AG. slow slow. Output high cannectio". Output law connection. Ground (chassis) connection.

Functional Description Paragraph

117 v: 1,4A; 234 Y: l/S‘4

2-2, e 2-4, d

2-4, e

2-4, f 2-4, s

2-1, a 2-1, a 2-1, a

2.1, a

!4ODUlATE Receptacle (5101) Connection to modulation transformer.

Shorting Link

Provides cbnnection between LOW and GROUND.

2-1, b

0571

MODEL 225

GENERAL DESCRIPTION

0571

OPERATION

MODEL 225

SECTION 2.

2-l.

CONNECTIONS.

8. OUtPUt conneerors. Front Panel.

1. panel are BNC type receptacles (Keithley cs-15).

The OVP receptacle (5103) provides conr,ection to the Model 225 Output High. (The center terminal is Output High while the outer shield is connected to chassis ground.) Connections to the output load can be made using the OUT receptacle

when the Model 225 is not floating. (For a more complete discussion of floating operation, refer to

paragraph 2-2 f.) The LOW receptacle (JlO5) provides

a connection to the Model 225 Output Low.

center terminal is Output Low while the outer shield is connected to chassis ground.)

asac Panel.

panel is also a BNC type. Connections can be made

to either the front panel receptacle s the rear panel receptacle with satisfactory results. The rear panel LOW terminal (3104) is a binding post (Keithley part no. BP-1lG) which is connected to the

center terminal of 5105 (on the front panel). The

rear panel GROUND terminal (3201) is a binding past (Keithley part’no. BP-118) which is connected to

chassis ground. LOW to chassis ground connections.

b. Modulate Connector. The NOD”UTE receptacle

0101) is a BNC type connector (Keithley part no. CS-

15). This connector can be used for modulating the output current using an ac modulation signal. The center terminal is connected to a transformer-coupled modulation circuit while the outer shield is connected

to chassis ground.

c. Cables. Connections should be made using coaxial cables which are low-noise types with graphite coating between dielectric and shield braid. For custom length cables, Keithley Part No. SC-9 Low Noise Coaxial Cable should be used. A mating BNC connector

is available by ordering Keithley part no. CS-44.

The connectors on the front

part

no.

(The

The OUT receptacle on the rear

The shorting link (supplied) permits

OPERATION

2-2.

MEASUREMENT CONSIDERATIONS.

a. Accuracy. The accuracy is specified in terms of the front panel dial setting or reading as well as the full range selected. (The FINE current control

must be set accuracies.) Since the total accuracy of the Model

225

is the s of the readina accuracy and the full

a accuracy, the user should select the lowest

full range for best possible accuracy. Table 2-l.

shows the total accuracy for typical dial settings.

(The accuracy on the lOOkA range is +.05% of 1000 or

o.s”A plus 2.5%

Output

lhrL-elLt setting

1 x lo-6A 1 x lo-6A O-1-0 100 $ + .055 NA 1 x

104A

1 x lo-6A

When the OUTPUT SELECTOR is set to + or - and VOLTAGE When the OUTPUT SELECTOR is set to + or - and VOLTAGE COMPLIANCE is set to 100, up to 100 volts will be COMPLIANCE is set to 100, up to 100 volts will be present at the output terminals. present at the output terminals.

b. Stability. The stability is specified in terms of the front panel dial setting or reading as well as the full range selected. specification is the e of the readinR stability and the full range stability, the user should select the

lowest full range for best possible stability.

c. Line Regulation. as +..005% of full range for a corresponding 10% change in line voltage.

d. Load Rezulation.

ified a8 2.005% of full range far a change from no load to full load.

tion corresponds to an output compliance voltage change

from 0 volts to 100 volts.

“0” position to obtain the stated

of 1000 or %A

for a coca1 of 5.5nA.)

TABLE 2-l.

Accuracy

Decade Dial

For Typical Dial Settings

RANGE

Setting

Total Accuracy

(FINE c0ntr01

set to “0”)

o-o-1 1000 &k4 + .505 VA

1-o-o

10-O-O 1000 UA + .0055 p.4

10 @

+ .OlO *

CAUTION CAUTION

Since the m stability

The line regulation is stated

The load regulation is spec-

The no load to full load varia-

FIGURE 4. Co”*ectm”s.

e. Output Impedance.

The effective output impedance of the Model 225 is a function of the load regulation specification and the high to low insulation

resistance. The output impedance for each Range setting is given in Table 2-2. The insulation resistance is 2 x 1012 ohms with the FILTER Switch OFF, but is 1 x lOlo

ohms with the’FILTER Switch ON. There-

fore the maximum output impedance is 2 x lOl2 on 100 nA and 1OOOti ranges with the FILTER Switch OFF. The effective output impedance R. ~a” be decermtned by the

following equations.

0676

MODEL 225

TABLE 2-2.

Output Impedance

Range setting

100 m.4

10 mA

1000 liA

100 ,,A

O”tput Impedance*

2 x 10; n 2 x log n 2 x lOlOG 2 x 1OlI R

10 UA

1000 nA

100 nA

2 x l0l2 “, “, : :;12 n

*With Filter switch set to off. I

For the 100 mA range,

R,, = 103n and % regulation = 0.005%

since I regulation = 100 x

I 1

Then % -

The Model 225 will deliver the dialed current for a load resistance from 0 ohms to WK. The val’ue of w is determined by the dialed current I and the compliance voltage setting VC where Rw.K =

WI.

If the load resistance exceeds RMAx then

the Mcdel 225 will automatically switch into Volt-

age Limit mode indicated by the LIMIT lamp (DSlOl)..

f. Floatiw Operation. The instrument can be floated

“I) to +500 volts off chassis nround with less than 5

ppm of-full range change in &put current per volt off ground. on the rear panel m,,st be m.

For floating operation, the shorting link

In this case, two

coaxial cables should be used to connect to output

high and low.

The outer shell of the front panel O!Jf and LOW receptacles are always at chassis ground for safety when flokting the instrument.

filter capaci-

tor C217 (,05 ~0 is connected between the LOW and

GROUND binding posts to minimize line frequency pickup. The LOW to GROUND isolation is approximately lo7 ohms.

typical example of floating operation is shown in

Fig”re 9. In this example, the Model 225 can be used

with a voltage supply such 8s Keithley Model 240A

extend the maximum compliance voltage to +600 volts.

The Model 225 can be adjusted for a compliance voltage

of +500 2100 volts de. The maximum current is 10 milliamperes (for the Model 24OA).

c. contra1 setti”*s.

The output current is selected by three decade switches and a seven position Range Switch. Refer to the discussion of the fronr panel concrok in paragraph 2-4 before power is turned on.

2-4. FRONT PANEL CONTROLS.

a. Power Switch (S2022. This switch controls the

line-power to the instrument.

b. OUTPUT SELECTOR Switch (SlO5). This switch has four positions designated “-“, and “+“. polarity.

The + and - positions set the output current

The two “STANDBY” positions divert ehe out-

“STANDBY”,

put current as shown in Figure 5. The output load is

not disconnected at any position of the switch. (The

shunting effect should be considered, since a small

voltage can be developed across the lo-milliahm shunt and therefore across the output terminal.)

c. VOLTAGE COMPLIANCE Control (R1171. This control

is a continuously variable potentiometer which can be

see for a compliance voltage from lo~volts to 100

volts.

d. Range Switch (SlOl). This switch selects the

full scale ranpe from 10-7 BmDePe to 10-l amoere.

The dial is deiignated in re&s of engineering Airs; that is, nA, IJA, and mA. The full range is determined by the position of the lighted decimal point end the designated engineering unit.

e. Selector Switches (SIOZ, SlO3. S104L. These switches select the magnitude of ‘the o”tput current in three decades. The position

the dials determines

the significant digits from O-O-O to V-9-9.

f. FINB Control (Rl38)-. This control is the inner

dial of a dual concentric control. The fully clock-

wise position “10” represents 1 digit added t., the

right-hand dial (S104). The fully counter clockwise position is the “0” or off position. When the FINE Control is

set

to “0”. the output current is determined

only by the calibrated dials within rated specifica-

tion. This control cm be used to “trim” the output

current to within +.02% of full range, although the

trim dial is adjustable t., within 2.005% of full range.

NOTE

For example, if the three decade dials were set

for

9-9-9 on the 1000 !,A range, then the FINE control could be set to 1500 clockwise to obtain a current .Y”tp”t of approximately 999.5 +4.

2-3. PRELIMINARY PROCEDURES.

a. Power.

Check the 117-234V Switch for proper linevoltage setting. Check the fuse for proper rating. Set the Power Switch to off before connecting the power

card.

b. Connections.

The output load should be cwnected

as described in paragraph 2-l.

0972B

Since the stabilitv specification is +.02% (or better) of reading, the useebie resolution w&d be ~i.2 ,,A. If the FINE control was set to “O”, then the output would be 999.0 ti 2.5 ,JA.

g. FILTER Switch (S106). This switch should be

used whenever the Model 225 must deliver current to

an inductive load.

OPERATION MODEL 225

OPERATING CHARACTERISTICS. The Model 225 Current

2-5.

Source has a bipolar output, a constant cnreent up to the maximum compliance setting, and automatic crossover to constant voltage mode.

a. Bipolar Output.

accept current and as such it has a bipolar output.

1. Current Source. The instrument can be used as a current source with a resistive load as shown in Figure 5. current set by the front panel controls. Tbe voltage developed across the load resistance IO x RL.

a current sink as shown in Figure 6. In this case.

the polarity of the current is such that cw’eent

is flowing into the Model 225. The output voltage “0 = E - IORL, where IC is selected by the front panel dials, E is the external voltage source, and RL is the load resistance.

b. Constant Current Mode. Tbe Model 225 will de-

liver the dialed current for resistive, capacitive, and inductive loads unless the voltage required at the output terminals exceeds the compliance voltage.

1. Resistive Load. deliver current to a resistive load from 0 ohms to w where k = VC/IO and VC is the compliance

voltage

In this case, the

Current Sink.

setting.

The

Model 225 can deliver 0’

Model 225 delivers B

RL is V, =

The instrument can be used as

The instrument can be used to

-‘Iff!?++iJ ___----------

FIGURE 5.

Resistive Load.

FIGURE 6.

Current Sink.

Capacitfve Load. The instrument can also be

used to charge a capacitive load as shown in Figure

The capacitor will be charged to the compliance

voltage set on the front panel.

Inductive Load. The instrument can

deliver current to an inductive load. When the ratio

of load inductance than 10-6, the FILTER Switch m”st be set to “ON” for stability. citor across the output as shown in Figure 8.

:I I I

The FILTER Switch connects a 1 pF cap*-

L to load

FIGURE 8.

resistance R is greeter

Inductive Load.

used

FIGURC 7.

FIGURE 9. FIGURE 9.

Capacitive Load.

Floating Supply. Floating Supply.

0972

MODEL 225

OPERATION

C. constant “Oltsge Mode.

jwted for a compliance voltaza from 10 to 100 volts. The voltage limiiing will o&r if the load resistance is such that IORL - "C.

marically switch into a constant voltage mode (the LIMIT lamp will be lighted) such that the output volt-

age will not exceed the compliance setting. If the

Mcdel 225 is used as a current sink, a,, output voltage

of 130 volts could be developed regardless of the con-

pliance setting. The voltage limiting characteristic can be shown as in Figure 10.

2-6.

M!JDULATION. For convenience in making dynamic

measurements, the Model 225 has a MODULATE input (JlOl)

so that the current can be modulated in the 50 to 500

HZ range.

a. Circuirry. The Model 225 uses a transformer

coupled input with 500 ohms impedance.

b. Characteristics. The peak-to-peak modulation

can be adjusted up to a maximum of 40% of full range from 50 CD 100 Hz decreasing to about 8% of full range at 500 Hz (with a 10 volt rms sine wave input). The modulation circuit is not calibrated so that the user

m"st check with a knwn load resistance. Within a

selected w the modulation is independent of decade

dial setting.

can affect the modulated current signal so that the

modulation calibration should take into account this

CapC.CitaWX.

C. "Olcage Complia*ce.

compliance can cause clipping of the modulated cur-

rent as shovn in Figure 11.

The capacitance of the output cables

The Model 225 can be ed-

The instrument "ill auto-

The serring of the voltage

1.3

F1mP.E 10.

RL-+

Volt Limit Characteristic

FIGURE 11.

Modulation.

NOTE

When the 225 output is modulated the average

value or dc level is determined by the dial set-

rings. The outpllt is essentially a sine-wave at the input drive frequency. The peak-to-peak

modulation amplitude depends on both the magni-

tude and frequency of the drive signal so that the exact modulation waveform will have to be

checked tith an actual output load connected. Clipping of the output signal may occur depending on the compliance voltage setting. The modulation feature is especially useful when making dynamic impedance measurements on semiconductor devices, etc.

0972

+ 26 hidden pages