Tech Forum
accuracy anywhere close to that,
which your six digit DVM is capable
of, check out the circuit with a
100,000 megohm resistor on his
website. Since you have a highly
sensitive voltmeter, you may omit
the OP-07 which he uses to drive
a conventional meter. Remove the
OP-07, 47K, 4.7M, and 0.1uF
components at pin 2 of OP-07.
Connect your Keithley 181 to R1
the left hand end (not the FET
gate end) of the 100,000
megohm resistor with the Q1
common lead on the wiper of R2
the 1K ohm potentiometer.
Since the 100,000 Fig 3
megohm resistor is a hard-to-find part, you may want to start
out with some of his other circuits,
R3
12V +
Q2 Q3
M
Q4 NC
R4
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Q1, Q3 = MSPW45A Q2, Q3 = MSPA64
R1, R3 = 10K R2, R4 = 2.2K
which will be
less accurate
due to being M+
"open loop." 12V
That means Q1
that the MSPW45A output of the Fig 4
amplifier is not fed back to the
amplifier input on these simple
circuits.
One of the less accurate circuits
with obtainable parts is shown in
Figure 3, drawn a bit condensed from
Wenzel's. The transistors are
Darlington pairs contained within
normal three-pin packages. The
virtue of this circuit is that it is a
bridge circuit with the NC (No
Connect) half balancing the active
half with ambient temperature
changes.
Half of this circuit appeared in the
January issue of Nuts & Volts, Tech
Forum. Connect your voltmeter in
place of the current meter shown in
that issue. If that is overly sensitive,
replace the 1 mA current meter with a
wire, and measure the voltage drop
across the 2.2K ohm resister.
Finally, Wenzel shows a simple
circuit using a MPSW45A, and
2n2203 driving a 10 megohm
voltmeter. Since you have a more
sensitive 1,000 megohm Keithley 181
(I looked up the spec), I propose
Figure 4 based on that circuit less the
2n2203. Once again, we use a high
gain Darlington pair to amplify the
current, hopefully to a level detectable
by your meter.
Note that the connection from
the neon lamp to the transistor base
is a high impedance connection,
subject to leakage. You may build all
of these circuits on a perf-board,
except for that one connection. It
must be air-wired or supported by a
clean glass or Teflon insulator, or
SEPTEMBER 2005
