Keithley 151 Service manual
INSTRUCTION MANUAL
MODEL 151
MICROVOLTMETER
CONTENTS
SECTION
INTRODUCTION
SPJXIFICATIONS
Ranges
Accuracy
Input
Overvoltage
Zero Adjust
Noise
Zero Drift
Response
Input Rejection
In-Phase Rejection
Impedance
output
Tube Complement
power
Connector3
Accessories Supplied
Dimension3
OPERATION
Operating Control3
Preliminary Set Up
Operation
I
II
Xesl3t3ncs
Speed
III
CIRCUIT DESCRIPTION
Input Circuit
AC Amplifier
Demodulator Circuit
DC Amplifier
Loop Description
Power Supply
MAINTENANCE
General
Trouble Shooting Procedure
Replaceable Part3 List
Voltage Resistance Diagram
Circuit Schematic
151
Iv
0763
SECTION 1 - INTRODUCTION
The Keithley Model 151 Null Detector is a stable, low-level DC
amolimwith high-~<~utesistance and low noise.
fiitering,
and gu&di.nk permit floating operation, as in a bridge nuli-
Careful shielding.
detector, with excellent rejection to extraneous voltages.
With a power sensitivity of lo-l7
second, lack of overshoot and a zero-centered meter, the
watt, a re3pon3e speed of ""3
151
may be used
,;n applications Inhere a suspension galvanometer might be employed, or
in other applicaticns where a galvanometer is not sufficiently sensitive,
fast or rugged.
Five non-linear ranges are provided which have the same center-
scale sensitiv'ty as the linear ranges,
logarithmi: way to three decade3 on each range.
but are compressed in a quasi-
The3.e range3 permit
speedier null-searching th3.n the l~inear scales, without sncrlfizing
sensitivity at null.
Two zero controls for opera 2.r.d short-circuit inputs ai~low prop3r
zeroin,- for any Source resistance even in the presence of Bridge thermal
emfts eliminating the need for disconnecting bri~di=e voltage cc set zerr).
Although de?-:gned for nu?l detector applications, the Model
151
is
also useful as a dc voltmeter with 3 maximum full-scale sensi~isi~y ?f
100 microvolt3 and as an ,ammeter with a maxlm::m full-s,x1e ren:ii~tivL::.
of 10 micro-microamperes.
The 151 has i full-scale ouL.out of 10 volts at up to one mlliiamperc.
Thiv is sufficient Lo drive one milliar5~ere reco!.ders 'ii .well as ~iervorebalance recorder3 and oscilloscopes.
0763
I
I-l
MODEL 151 ~fICROVOLTMETER
RANGE:
Linear:
ranges in lx and 3x steps.
Non Linear:
3 decades.
ACCURACY:
Linear ranges: +3% of full scale exclusive OE noise and drift.
Non-linear ranges:
ZERO DRIFT:
INPUT NOISE (with input shorted):
INPUT RESISTANCE:
LINK FREQUENCY REJECTION:
COMMON MODE REJECTION:
500,000:1.
ISOLATION:
microfarad.
chassis ground.
0.1 millivolt full scale to 10 volts on zer*-center meter.
Five ranges of 0.001, 0.01, 0.1, 1.0 and 10 volts full scal.c, eaci1 covering
?lO% of input exclusive of noise and drift
Less than 10 microvolts per day after 30-minute warm-up.
Less than 2% of fui.1 scale on ~11 r;,,r::cs
10 megohms on all ranges.
2,OOO:l
With l-megohm source resistance. dc:5,000,000:1. l.i~,le rriqut,ncv:
Circuit ground to chassis ground:
Circuit ground may be floated up to ?500 volts dc or ~p~!ak :<i!:il rcspcc: KC,
Approximately 109 ohins ijllillted 5'; ll.i!L
ILL overLappin,:
RISE TIME (10% to 90%):
OVERVOLTAGE:
ZERO ADJUST:
RECORDER OUTPUT:
Output:
Resistance:
Gain on linear ranges:
LINE STABILITY:
shift on all ranges.
CONNECTORS:
POWER:
DIMENSIONS, WEIGHT:
11-l/2 pounds.
ACCESSORIES SUPPLIED:
LOS-125 or 210-250 volts, 60 cps, 50 watts.
200 volts steady,
For both open and short-circuit inputs.
?lO volts dc at up to 1 milliampere for full-scale meter deflection.
Less than 50 ohms within the amplifier pass band.
A 10% change in line voltage will cause less than a 2,X of full-scale
Input:
Binding posts.
10 inches high x 6-l/2 inches wide x 8-l/2 inches deep; net wcighc,
Approximately L second on all ranges.
400 volts transient
10 volts
Range setting in volts
output: Amphenol 80-PC2F.
50-cps models available.
I
Mating output connector.
0166R
II-1
SECTION III -- OPBlATION
OPERATING CONTROLS
A.
The operating controls of the Model 151 are listed below:
ON - OFF SWITCH;
RANGE SWITCH: Selects both the linear and the logarithmic ranges.
SHORT CIRCUIT ZERO:
Sets the meter zero ,with the input shorted.
This control compensates for thermal EMF's in
the connected circuit and the amplifier input.
OPEN CIRCUIT ZERO: Sets the meter zero with open or high resistance
input. This control compensates for an:, leakage
currents present in the external circuit.
B. PRELIMINARY SET UP
--
Connectto the power line. Unless otherwise marked, the unit is wired Co.117 v.,
60
cps power.
consult MAINTENANCE section.
grounds the cabinet.
To convert to 220 volt and/or 50 cps operation,
A three-wire line
cord
is furnished which
If a three-wire receptacle is not available, use
the two pin adapter furnished, and ground the third lead to an external
ground for the best operation. If open circuit unbalance is encountered,
reverse line cord.
Set the range switch to the 10 volt linear range. Xrn on the power.
In about
.1 millivolt range. Zero the meter with the SHORT CIXCUIT ZERO.
the input
30 minutes of operation some zero drift may occur and it may
30
seconds the meter will zero.
and
zero the meter with the OPEN CIRCUIT ZERO. In the first
Short the
input and
turn to the
Open
be
desirable
to reset the zeros.
USE OF ZERO CONTROLS
C.
----
The stability of the Model 151 is such that, after
30 minutes of operation,
only infrequent attention need be given to either zero co::trol except
occasionally on the most sensitive ranges.
The twc zero controls, however,
allow the user to maintain a constant zero with varying input resistances.
The short circuit zero
by thermal EMF's.
balancing any current generated by
practice, if zeroing is required, the SHOR
However,
in the case of a critical null balance application where it is
bucks
ou,t low impedance voltages such as generated
The open circuit zero is a current buckout control for
chemical
effects or leakage. In normal
T CIRCUIT ZERO is sufficient.
necessaw to eliminate zero shifi; over a range of inuut re4istances, this
can be assured by balancing the null detectcr using i,ne SHORT CIRCIJIT
ZiTiO at the lowest impedance involved and the OPEN CIHCIIIT ZERO at. the
hiphest impedance involved.
0763
III - 1
INPUT RFSISTAlsCE AND SOURCE IMPEDANCE RIWTRICTIONS
D.
-
The input resistance of the Model 151 is 10 megohms, within 5% on the
most sensitive range and within 1% on all ether ranges. Other than the
consideration of circL.'.t loading and speed of respcnse (See F.), there
are no source impedance restrictions.
CURRENT MEASUREMENT
E.
Since the input resistance is a constant 10 megohms, ibe Model 151
bt- used '.?
full scale.
sensitive range,
measure current with a maximum sensitivity of lo-11 amperes
Since the tolerance cn input resistance is 5% on the mcst
an accuracy of about 7% can be expected there. On
may
all other ranpes an accuracy of k% is possible.
F. SPEED OF RESPONSE
--
The specification in Section II is for low impedance input. With an
open-circuit input, speed is about 5 seconds to 90% of final value.
LINE FREQUENCY REJECTION
G.
--
The specification for 60 cps rejection are given in Section II. The
rejection is high enough so that usually no precautions are necessary
with regard to 60 cps pick-up. While the input filter is specially
peaked at 60 cps for maximum rejection,
there is sufficient rejection
at other harmonics to make pickup troubles unlikely.
H. FLOATING OPERATION
The common mode rejection to DC as well as 60 cps is extremely high in
the Model151 as spwified in Section II.
with no difficulty up to 500 volts from pround.
The instrument will operate
For floating cperation,
remove the shorting link between the LO and G terminals at the front of
the instrument.
I. RECORDING
The output at the recorder terminals for ful; scale is 10 ~clts at up to
1 milliampere.
Since the output has a common ground with the input, when
recording, either the Model 151 must be grounded or the recorder input
capable of being floated.
COMected
to the output, the system rejection will be no better than that
It shoud be remembered that, with a recorder
of the recorder.
III - 2
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