Keithley 241 Service manual
NOOEL 241
CONTESTS
CONTENTS
7
section
SpEi-IF*C*TIi)NS ____________--_---_----------------------- ii
1. GESE&AJ DESCRIPT~OA----------------------------------------
2. OPERATIOS--------------------------------------------------
3. CIRCUIT
4. MAINTENANCE------------------------------------------------
5. REPMCEI\BLE PARTS------------------------------------------
SCHEMATIC--------------------------------------------------
~ESCR~pT~ON----------------------------------------
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0472
i
SPECIFICATIONS
SPEClFlCATlONS
OUTPUT:
Voltage 0 to 1OW vdfs de in 0.01~volt steps.
Current: 20 milliamperes dc maximum.
Polrrlly: Posiiive or negative.
Floaling: WI volts maximum ofi chassis ground.
ACCURACY: aO.05gk of dial setting or 4 millivolt. whichever iS
greater.
RESOLUTION: A “Trim” potentiometer permits interpolation
between sleps with a resolution of betler than 1W microvoI1s.
TEMPERATURk COEFFICIENT OF REFERENCE: &lo Ppm/T.
LINE RLGULATION: *O.WS% or 1 millivolt for 10% line change.
LOAD REGULATION: ~O.OOSqb,rom”o,oad tofull bad.
RIPPLE AND NOISE: Less than 1 millivolt rms above 5 cps.
OUTPUT IMPEDANCE: Less than 0.05 ohm at dc.
RECOVERY TIME: No load to full load. less than 1 second lo
OVERLOAD PROTECTION: Ovtwt is disconnected within 50
millisecondsif currsn~axseedi.pploximately24 milliamperes.
CONNECTORS: Output: Tefkninrulated UHF type.
POWER: 105.125 or210.250 VD,~S. 60.60 cps. IOS watts.
DIMENSIONS. WEIGHT:l’high x Wwidsx12’deep; netweight.
25 pounds.
ACCESSORIES SUPPLIED: Mating co”“estors.
ACCESSORIES AVAILABLE:
Model 1491 End Frames:
adapts Model 241 for bench “se
MODEL 241
.
ii
0372
MODEL 241
GENERAL DESCRIPTIOS
SECTION 1.
GENERAL.
l-l.
secondary standards that employ mechanical choppers
and standard cells.
and calibration, long term stability LO voltage or
temperature variations, and immunity to shock and vibration. The 241 complements the popular Keithley
240 Supply. offering
current output and floating operation.
APPLICATIONS. Include calibrati.” of meters,
1-2.
transducers, and paver supplies: testing insulation,
diode, and capacitor leakage resistances; measuring
dc amplifier gain. linearity, drift and common mode
EjeCtiO”.
voltsge reference for analog camputors, and excitation
potential for photo-cells and ion chambers.
FEATURES.
1-3.
Extreme stability of 0.005% per hour is gained
8.
by using, as a reference standard, a highly stable
zener diode with a” ambient temperature coefficient
of less than 0.001% per degree C.
by rough handling or shock and cannot be damSed in
ordinary use.
designed photo chopper of indefinite life, havi”8 “a
The Model 241 is the successor to dc
If offers freedom from adjustment
greater
It can also be used as a
The cmparat”r device is a Keifhley-
accuracy, regulation,
voltage
1t is unaffected
GENERAL DESCRIPTION
comparator,
moving parts and requiring no maintenance.
b. High output accuracy within 0.05% is assured
through the “se of encapsulated wire-wound resistors
with 0.02% divider accuracy.
C. From O-1000 volts at up to 20 milliamperes -
plus, minus or floating - ca” be dialed in 100, 10, 1.
0.1 and 0.01 volt steps with 5 calibrated panel
switches. “TRIM” potentiometer permits interpolation
between steps with B reeolutia” of better than 100
microvolts.
d. Noise and hum are belw 1 millivolt nos. Lo”
line transients - excellent line regulation - make the
241 ideal for capacitor and diode testing. meter cali-
bration.
e. Overload protection is accomplished by a fastacting relay circuit which disconnects the output witbin 50 milliseconds at about 24 milliamperes. A “RESET”
front panel button reatores operation.
f. Other features include “STAM)BY” which removes
voltage from the output, making possible connections
while the instrument is operating; bench or rack op-
eration.
0472
GENERAL DESCRIPTION
MODEL 241
TABLE l-l.
Front Panel Controls
I
Control Functional Description
OUTPUT Switch Sl
OUTPUT VOLTS
s3
S4
S5
S6
s7
CALIBRATE Control It109
RESET Switch SZ
OVERLOAD Indiearor DS-3
Power Indicator US-1
READY Indicator DS-2
TRIM Control P.151
sets 0"Lp"t on.
Sets output in 1oov increments.
sees oucpu’t in IOV increments.
sets OUtpUt in 1v increments.
sets OUrpUt in O.lV increments.
sets O”CP”L in p.01v increments.
Adjusts output calibration.
Overload reset control.
Indicates overlo& condition.
Indicates parer on.
Indicates ready condition.
kdjuata output up to 15 mV.
I
READY
INDICATOR CONTROL
DSZ RI09
2
CALIBRATE
TRIM
CONTROL INDICATOR SWITCH
R151(SS)
FIGURE 2.
POWER OUTPUT
DS 1
Front Panel
Sl
RESET
SWITCH
s2
OVERLOAD
INDICATOR
DS3
0472
WDEL 241
(2t
OPERATIOS
SECTION 2. OPERATION
2-1. OPER4TING CONTROLS.
8. output volts.
the panel permit setting the voltage directly from
zero to 1000 volts with en accuracy of 0.05% or 1
millivolt end e resolution to 10 millivolts.
b. Trim.
tinge is possible with the TRIM control which Is below the wltage dials.
net to the OFF position when not in use to avoid inBCCUrate o"tp"t.
C. Calibrate. The calibration Potentiometer is
available from the front panel and is covered with en
acorn nut to prevent inadvertent operation. The instrumenr is set to well within 0.05% accuracy et the
factory. However, since the linearity of the output
is within 0.02%, if the user he8 a potentiometer of
thie accuracy available, it is possible to see the
calibration more closely for specific needs. This
control, however, should not be touched unless proper
calibration facilities ere available. Bee mainten-
*nce section).
d. Power Off - Standby - Output On. This switch
turns on the power, places the instrument in STANDBY
ower on but potential disconnected from the outp"t
minals), and turns on the output terminals.
2-2. CONNECTIONS, POLARITY AND FLOATING OPERATION.
The output connectors ere located et the rear of the
instrument.
the positive and one for the negative line. A sharting cap ie provided so that either the positive or
negative Line may be grounded to the cese. Thus, if
the shorting cap grounds the negative line, the remining cmmecror has its center terminal at plus pclarity, end the negative terminal is on the shell of
Extrapolation between 10 millivolt eet-
Two output connectore ere used, ‘x,e for
The five skirted dials ecrose
The TRIM control should be
the connector. If the shell grounds the other con-
nector, the oppoeite holds true. Floating operation
is permiesible provided that potential of either line
to the cede doe8 not exceed 1500 volts. For floating
operation, remove the ehorting cap and use two aep-
arate coaxial cables, one for plus end one, for minus.
Due to the high potentials the ueer is cautioned to
"se connectors end shielded coaxial cable only.
2-3.
PRELIMINARY OPERATING PROCEDURE. The Model
241 Regulated High-Voltage Supply is shipped complete
with tubes and fuses and is accurately calibrated at
the factory. Plug the power cord into a source of
proper voltage and frequency. Unless otherwise spec-
ified, the unit is wired for 117V. 50 to 60 cps. For
220 volt operation change the jumpers on the tre,,sformer primary ae indicated in the schematic diagram.
After the connections have bee,, made, turn inetrument
to STANDBY. The pilot light will go on but the READY
light will be delayed by the internal time-delay relay for 30 seconds. After rhie time, the READY light
will go on, end the instrument is ready to operate.
If the instrument is immediately turned to the ON
position, the OVERLOAD light will go on with the READY
Light, and it will be necessary to operete the RESET
button to make voltage appear et the output terminals.
2-4. OVERLOAD
supply is exceeded by more than approximately 4 milli'
amperes, the overload circuit will disconnect the output in about 50 milliseconds turning on the red OVERLOAD light. To restore operation press RESET button.
If the OVERLOAD light refuses to go out, or the relay
charters when the reeee button is pushed, either the
supply has become defective or the overload is still
present. DO NOT PRESS RESET BUTTON MORE
TARILY TO AVOID DAMAGE
CIRCUIT.
If the current rating of the
THAN
TO
POWER SUPPLY.
MOMEN-
0472 3
CIRCUIT DESCRIPTION
MODEL 241
SECTION 3. CIRCUIT DESCRIPTION
0
3-L. GENERAL,.
In Figure 3, two cascaded electronic
series regulators furnish e high degree of load end
Line regulation. The first regulator reduces the effect of input voltage change while the second eegu-
e. The remainder of the circuit is e high gain
feedback amplifier. Point 35, es mentioned above, is
meinteined et ground by the feedback action. This
occurs because of the following:
lator provides e means for varying the output voltege.
i
It provides e Low output impedance, end very close regulation.
Reeis,toee Rl end R2 compere the output voltage against e silicon voltage etsnderd. By varying
Bl the voltage may be msde to essume eny voltage be-
tween zero end 1000.
The current supply to the
ref-
erence Is doubly regulated. Relays RYl end RY2 ere
overload protectors and RY3 end RY4 provide a 30-sec-
and time delay on turn on.
1. If point 35 is not et ground, the light modulator convert8 the voltage iato en ec signal which
is amplified by the two sections bf VlO. The eignel is then demoduleted by synchronous demodulator
D13-D16.
The resultant dc signal is filtered by
R147 end Cl24 and applied to pin 7 of Vll. The signal is further amplified by VL1 end V5 end applied
to the control grid of V3. The signal will be of
such polarity as to return point 35 to exactly zero
3-2. DETAILED CIRCUIT DESCRIPTION.
volts.
VlO
At frequencies higher then e few cycles,
and the essociated modulator are not operative
end in such e ceee the error signa& proceeds directly
to
8. Refer to,DR L3333D at the rear of thle manual.
The high voltage output of the transformer. Tl is
rectified and supplied to the plste of V2, the series
tube in the pre-regulator. V4 compares a fraction of
pin 2 of VLl end by-passes the modulator. CL3D
by-psssee the comparison divider end increases the
signal et high frequencies, which peeees into pin 2
of vu.
the pee-regulated volcege from the divider network
R127. to R129 and R131 to the 150 volts of V6. This
regulator supplies approximately 1200 volte to the
final regulator.
2. Several safeguards are incorporated
into
the
circuit et this point. 012 end RLO-7 ere connected
between ground and the summing node, point 35. D12
is e silicon diode which is normally open but con-
b. The final regulator consists
amplifier V5 difference amplifier
stabilizing amplifier consisting of light modulatora
(Keithley 15101, a two stege amplifier
ma+Lstor D13 to D16.
of
series tube V3,
VU,
end B drift
VlO,
and de-
ducts if the euraning point is carried more then 0.7
volt positive due to e transiene. This protects the
voltage reference diode.
RYL is the overload re$
It is shunted by R136 to eet its pull in sensitiv
et approximetely 24 milliamperes. When en overload
f
occure end RYL does pull in, it breaks the self-
C. A very stable 9 volt potential is obteined st
point 29 from the zener reference standard. The zener
element is operated et ite rated current end obtains
this curreat regulated to better than 1% from the casceded VR tubea V7,
VS.
end
V9.
To accomplish this,
RI11 is provided to adjust the rated current through
the zener, the link beween points 27 end 28 is removed
and e 1/4X toler&ce 10 ohm resistor is put at this
point.
With an accurate voltmeter, Rlll is adjusted
until the voltage scross the 10 ohm reeistor correeponds to the product of rated current x 10 ohms.
holding contact on RY2 causing it to open.
To
re-
etore operation, RESET switch SW2 is pushed to mo-
mentarily reclose the RY2 self-locking
circuit.
The
OFF, STAND BY, WTPUT ON switch, SWL, also operates
RYZ opening it In the STANDBY position by interrupting the holding
circuit
ot RY2 by means of its first
deck between points 25 end 38. SW1 re-energizes RY2
in the OUTPUT ON position. RY4 is a 30-second therme1 time-delay which ie recycled by RY3. If RY3 is
not energized, RY2 cannot close, end the circuit remains in the overload or standby position.
d. The voltage from the zener is compared against
the output voltage by means of RlOS, RL09, end RllO
(these resistors correspond tp Rl of Figure 1). Since
the ection of the feedback amplifier is to maintain
point 35 exactly et ground; resistors RLOS through RllO
ere adjusted so that exactly 1 milliempere of current
flows into point 35 from the zener. This current must
now flow into R151 end through R153 to R182. Since
these reeietore ere connected between point 35, which
the feedbeck meintsins at ground end the high voltage,
the output voltage must aeeume a value of 1 milliampere
times the value of resistance chosen by the voltage
dials which select resistors RI51 and R153 through
RLS2. It is so arranged that the output voltage equals
one volt per thousand ohms connected between point 35
end ground.
4
3. After the signs1 is amplified by Vll. it is
further amplified by V5.
This tube ia srrenged to
eupply driving voltage to V3 eo that the supply ten
cover a range of 0 to 1000 volts. To do this the
place supply must vary over a range of about minue
50 to plus 950 volts. Therefore, the cathode of V5
is returned to minus LO5 volts. The screen of V3 is
supplied with e floating 105 volts bootstrapped to
the cathode of V3.
In this way a conetent ecreen-tOcathode voltage is maintained, end the tube operates
es a pentade. The advantage of this connection la
that much less grid swing is required over the op,-
eratina range. and the ac plate impedance of V2 is
markedly increased. This results in e higher degree
of output isolation from input transients.
8
0472
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