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410C
Service manual
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Keithley 410C, 410 Service manual

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MODEL 410
and
MODEL 410C
MICRO-MICROAMMETER
KEITHLEY INSTRUMENTS, INC.
CLEVELAND, OHIO
SECTION I INTRODUCTION
Model 410
The Keithley Model 410 Micro-microammeter is a line operated vacuum tube electrometer designed and constructed especially for measuring small currents. j x 13-13
ampere,
Full scale ranges are from 10-3 to
The fsaturos include full-scale voltage drop at the lnptit of less than five millivolts, zero drift of less than 2% of full scale in eight hours, good accuracy and calibration stability, and simpli­city of operation.
It also has an output which will drive a O-l
or 0-3 milliampere recorder as weli as the numerous potentiometer ­rebalance recorders; one output terminal is at ground, making it convenient to connect cathode ray oscilloscopes or pen-driving amplifiers,
similar to the Brush and Sanborn equipment.
The major panel controls are the rsngo switch (amperes full­scale) and the zero. Minor controls are the Zero Check, used to short circuit the input and in setting the zero, Meter Polarity
Tar providing up-scale readings for currents flowing in either di­rection, and an ON-OFF power switch.
The mater dial is illuminated,
and these bulbs serve as the pilot light. Model 41OC The Keithley Model 41OC is identical to the Model 410, except that
the panel meter is provided with contacts which can be set to
close at any predetermined meter pointer deflection.
The delicate
contacts of the meter operate a relay in the 410~7, and the relay contacts (SPDL') are available for external switching functions through an AN connector on the rear of the chassis.
KEITHI,EY TNSTRUMENTS
- I-1 -
CZVELAND, OHIO
SECTION II DESCRIPTION
Tw3nt.y ovorlapging current ranges, from 10 x. 10-Y qsre. to 3 x lnmpers are selected by the Amperes Full 3rraJ.e s&t&, :ccm&eB left of the Meter.
3 x 10-7 is within 25, 10 x 10-g through 3 x lo-]? is wJtiLn il$‘,.
Iqut Impedance is controlled by negative fee&%=$ f~>rm Cra ,:‘rqmn so that the voltage drop across the input terminslr 0 &raw than 5,
millivolts for full-scale meter deflection.
zero setting; if the meter needle is not set at ‘1,erq -nzzH zero 3ztqrti
c urrznt
scale are added to the five millivolts.
It is~UHE’nnector. with tafldn insulation, and sagas a s,tanda& tzf ion insulated mating plug. be extremely well insulated to prevent the leakage zrl C&a am&!1 ‘:3-s.
A cap is provldod for kasping dirt out when tha in&rumenm iy szore&
, about 3 millivolts constant potential per ReroenJ of 5;:
The Input Connector is located on t.he back :s.ce c3 $Be, s.%ssi~,
The accuracy of the ranges from 1C; x 1S-” throm@
TX3 3s .c&sedi 2n ~rr&Iprl’
The plug and 1es.d warsa, cm’ c&bLe sfamz&a
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Input Switch Labelled ZERO CKSCK is located ta, tihe Irf5 of %ltm RangeSwitch. move spurious charges,
for zeroing the meter with the Zero Control.
Grid Current is less than 5 x lo-l4 ampere, amdJ vs. 3&s
limitofmeasurement of a vacuum tube electrometzr.~
of full-scale on the most sensitive range.
Zero Drift is less than 25 of full scale im e&~&m hourn on a~Il?l rmrqes
exceptxx hours. These drifts include warmup from a cold s%xd. M’ * *;ro iiL.mm 74lmml­Up can be provided, the drift will be one half to amm f:oUHib @ z&ams amounts ,
Zero Control- The Zero knob is located to t&s r&g&t ti She stemerr a& is Usedor zeroing thexer with zero input cur&. EtY3ec*~&e%,7~ wxm input current can be obtained by depressing the Ee.mm: ~%mti bu%?,onl,. %e input must not be short-circuited.
and makes it impossible to zero ths meter.
When depressed, it effectively sbn?a $ti >npun ?a, re+-
ampere
----
and p&Ides tho scro inp.Xi cnrwlid. rsfersnnn
Twla is &mti: 1%:
where it is less than 4% 03 s?tiI sa.?im ;nz &@&
This upsets thm neg@5ve fmsd&m& pti
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411,
A
4'OC
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KEITmY INSTRUMENTS
&
It is recommended that the meter pointer not be set anywhere but zero
on the meter scaie with zero input current,
because with the feedback used,
a dc potential is developed across the input whenever the output and the
panel meter are not zero for zero input current. Recorders, of course, can
be biased to any part of their seals for zero Volts at the Model 410 o-utput.
Output is provided for driving rscorders. The amplifier will develop 5 volts without upsetting the circuits.
for full-scale meter deflection, and 5 milliamperes can be drawn
The OUTiVT connector is at the rear of the chassis, The connection details and suitable outp,Jt attenuators are discussed in OPERATION, Section III.
Response Speed of the 410 depends upon the current range being used and also upon ths capacitance of the external circuitry. ranges the speed is limited by the amplifier respons
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approximately 1,000 cps. the speed has been reduced to about 1.0 second by the addition of capacitors across the range resistors.
On thi, ranges from 3 x lo-
On the three most oenoitivo ranges, shunt cap-
acitance across the input limits the response speed.
On the less sensitive
which is from dc to
8,
to 10 x 10-l* ompereo
Because of the method of application of tim negative feedback, the slowing effects of capacitance from the high input terminal to ground hs.ve been greatly reduced, but are still significant.
Tnble I below gives typical response speeds; viz; the
time constant of the response t,o a step functior..
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Rang.3 s No significant external capacitance
Undamped
3 x 10-13 2.2 seconds 1 x 10-q 1.0 ” 1 x 10-11
0.15 ”
0.10 ”
: x” ~~-~ 1 x 10’ 0.01 ”
-8
0.05 ”
TYPICAL BESPONSE SPEEDS (to read 67% of final value)
Damped
2.2 seconds
1.0 ”
1.0 ”
0.5 0
0.5 I’
0.5 u
I
with 5000 mmf across
I
Undamped 4 seconds
2 I’
.5 :; .2
.l ‘I .05 I’
Damped
4
seconds
2 ‘I
1.0 I’
1.0 ”
1.0 ‘I
1.0 ‘I
If the maximum speed of response is desired, the capacitors shunting the range resistors may be removed; however the increased response to spurious ac signals may interfore with recording, as mentioned in Section I.
Amplifier Noise is principally power frequency, and is 30 millivolts
ITM max at the OUtpUt t.smhISh, irrespective of the current range. From the most general point of view, grid current and amplifier zero drift are also background noise; these have already been discussed,
TABLE I
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KEITBLEY INSTRUMENTS
- ‘l-I-2 -
CLEVETELAND, OHIO
Circuit Description
The circuit diagram DR 10867-c is enclosed at the back. The amplifier consists of two 58% electrometer tubes operated as a long-tail pair, with a substantiel amount of in-phase rejection. Further in-phase rejection is obtained by supplying Vl and V2 screens from 7.3 and Vk. A triode connected &X6 is used as the cathode follower output stage.
Negative feedback from the output (directly, or through a low impedance
divider) is accomplished through the shunt resl.star to the grid of the in­put electrometer tube. It is this feedback which keeps the input voltage drop low.
Tho open loop volts.g~o gain of ths amplifier, meas’ired from the first
stage grid to the feedback connection which would normally be connected to
is
the low impedance end of the shunt resistor, sensitive ranges, alternates between 1500 and 500 on the middle ranges, and is 150 on the 3 x lo-13 ampere range.
To insure low drift, the feedback-voltage (the voltage drop across
the high resistance range resistors) is made large and alternat
1.0 volt and 3.0 volts for most of the ranges. On the 10 x 10­range and those less sensitive it is 5.0 volts, while ampere range (the instrument’s most sensitive) it is 0.3 volt. The alter-
nation of the feedback voltage is used to economize on the very expensive high megohm resistors, so that only one Is used for every decade of
m*as;ired current. each current range.
Table II, below, gives the value of Ep and RS for
This assures a low input drop.
about 2500 on the
on
the 3 x lo-13
1~~s
s between
8
ampere
Range
Ampares Full Scale
10 x 10-4 3 x 10-4 10 x 10-5 3 x 10-5
10 x 10-6 3 x 1~6 10 x 10-V 3 x 10-7 10 x 10-g 3 x 10 10 x 10-g 3 x 10-9 10 x 10-10
3 x 10-10 ;Oxxl,'!;:
10x10-12 ~"xx':;~:3
3 x 10-13
-a
:
3
1
0.3
TABLE: II
RS
Z67K
5OK
166.7~ 500K
1.667 5 Meg
16.67
::aMeg
108 109 109 1010 1010 1011 1011 1012 1012 1012
Meg Meg
Resistor Accuracy $
0.1 1 I. 1 1 1 1 1 1 1
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3
410 &
4lOC
KEIT’LUXY INSTRUMENTS
- 11-3 -
CLEVELAND, OEIO
SECTIOll III OPERATION
Simplicity of operation is an outstanding characteristic of the Model
413.
First ccnnect the input to a current source, and the output to a re-
corder or external indicator, if desired.
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Then:
a) Plug the power cord into a 110 volt 60 cps outlet.
Not-2
that because a Sola resonant regulating transformer is used, the power fre-
quency, as weil as voltage, must be the proper value.
b) Turn the amperes Full Scala to the 10 x 10-k position. c) Turn the power‘ switch to ON. d) After a few minutes warmup,
set the panel meter to zero with
ths ZERO control.
e) Advance the instrument’s sensitivity with the re.nge switch, until a usable deflection is obtained on the panel meter. read directly.
Attention should be paid to the METER polarity switch, so
The current is
that an up-scale deflection is obtained.
f) Pariodically check the zero setting by operating the ZERO
CEXCK switch and receroing the meter if necessary.
Input, using cabling
--
The current source should be connected to the input connector wit,h the high impedance side of the current source associated with central conductor of the connector.
.
teflon insulated coaxial cable, and the connector should have teflon insula-
tion.
Amphenol type 83-756 or equivalent is recommended. During preparation
The lead-in cable should be polyethylene, polystyrere, or
of cable and connectors, it is essential that all high impedance surfaces be
kept scrupulously clean to avoid leakage.
With graphite coated cable, it is necessary to avoid tracking graphite onto the high impedance surfaces of the cut end of the insulation and the teflon surface of the connector.
Movement of the cable during measurement should be avoided since this will cause Spurious needle movements, because of capacitance changes and generation
of static charges.
CBID CUPBFXT in the Model 410 is less than 5 x lo-14 ampere - usually
.
about= X@Gnpere . after carefully shielding the high impedance input conductor such as by
It can be read directly on the 3 x lo-13 ampere range
screwing the connector cap on.
The grid current can be subtracted algebraically from the total current read on the meter to give the correct current in the circuit being measured ­on the most sensitive ranges.
.
PBCOPDING:
the chassis for recording.
The Model 410 is provided with a connector on the rear of
The output for full-scale meter deflection is
+5’volts. The maximum current that may be drawn from the output terminals
is 5 milliamperes.
This output is suitable for driving one and five mil-
liampere recorders as well as recorders employing an amplifier. Cinch-Jonas
S-202-B is the chassis connector, P-202~CCT is the mating plug.
Terminal
#l is ground.
- III-1 -
KEITHL?lX INSTBDMgNTS
cmLAND, OHIO
Tsblc III gives resistance to be used in series with one and five
milliampere recording milliammatera,
to make the recorder full-scale de-
flection equal the panel meter full-scale deflection.
TABLE TII
Recorder
1 m.a. 5 m.a.
The exact aeriss reaiatanco
varies from recorder to recorder, and a
Serisa Resistance
3.3 to 3.7K 323 to 940
portion of the series resistance should be adjustable ao that the recorder may be calibrated exactly against the panel mater.
A suitable voltage divider for more
sensitive recorders can easily be
made, keeping in mind that 5 volts appear at the output terminal.3 for full-
,
scale deflection of the panel meter,
and that a 1000 ohm divider will not draw too much output current and will be sufficiently low impedance to con­nect to amplifier inputs.
I
The Speed of Reaponae, or the time constant of an input transducer
and mico-nio%%ueter, depend8 upon the speed of response of the cir-
s
,
.~
cuitry of the instrument find also upon the capacitance of the current
source and its connecting cable.
Because of the wag the negative feed­back ia applied in the Model 410, the external Input capacitance is not nearly a.8 important aa in systems using a voltmeter acroaa a shunting
.
resistor, and quite large capacitances can be tolerated without having an
impossibly slow response.
Thus, a cable run from an ion chamber to the
micro-microammeter is permissible.
The internal time constant of the Model 410 depends upon both the
frequency response of the amplifier stages and the time constanta of the
high megohin range reaiatora and the associated distributed capacitancea, These change from range to range on the 410, the speed decreasing aa the
sensitivity is increased.
Table I in Section II, Description, gives
quantitative values.
t216 Volta.
chaaa& to provide 4216 volts for polarizing an ion chamber,
A connector has been mounted on the back face of the
The poton-
tial ia derived from 2 bB2 voltage regulator tubea, and is well filtered, The supply can be short circuited without damaging it, The chaaaia con-
nector ia Cinch-Jones SlOl, and PlOl is the mating plug.
- III-2 -
K!CITELEf INSTRTJMgNTS
CLEVELAND, OHIO
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