Page 66 - Troubleshooting Analog Circuits

Leakage Can Be a Problem 53

DIGITAL VOLTMETER
LOW-
NOISE
”S
(VARIABLE)

*
I
NOTES:
St -RESISlOR SENSE SELECTOR
RI-DVM INPUT IMPEDANCE. FOR SOME WMS, RI IS (OM
IN SOME MNGES (CONSULT USERS MANUAL)
Figure 5.3. The DVM approach is an alternative to the approach illustrated in Figure 5. I for testing
leakage. You can calculate the leakage current from Ohm’s Law: Vs = I, X Rs, or IL = V,/R,.

There are basically two ways to measure leakage current. The approach I have
used for many years is to connect a couple of transistors as a wide-range. logarithmic,
current-to-voltage detector across the feedback path of a low-bias-current op amp.
These days, I’m not using vacuum tubes-I have graduated to an LMC660, as shown
in the circuit of Figure 5.1. I calibrated the meter with a hand-drawn scale to sense
currents ranging from +1 pA to +1 mA and -1 pA to -1 mA (Figure 5.2). As long as
the air conditioning doesn’t break down, I know my calibration won’t drift much
more than 10 or 20%, which is adequate to tell me which decade of current I am
working in. (The V,, does have some temperature sensitivity, but not enough to
bother this circuit very much.) Because these transistors are, of course, quite non-
linear as current-to-voltage sensors, you do have to shield the summingpoint away
from AC noises (60 Hz, 120 Hz, 1 MHz. etc.) to prevent rectification and false read-
ings. So the whole test circuit and the unknown impedance are best located in a
shallow metal box, grounded, with an optional metal cover.
It’s true that the DVM approach shown in Figure 5.3 has a little more accuracy and
perhaps more resolution; but it too is easily fooled by noise, and the digital readout
doesn’t show trends well. And, if you want to cover a wide range of currents, you
have to switch in different resistors or wait for the DVM to autorange, which is not
my idea of fun. On the other hand, you can find a DVM almost anywhere, so this
approach is easy to implement.
In either case, if you put 15 V across 1,OOO,OOO MO and measure 15 PA, that is at
least 50,000 X higher resolution than most meters that can only measure up to 20
Ma. Whichever detector you use, apply a reasonable voltage across the unknown
impedance and see where the leakage gets interesting. This method can also be used
for diodes and transistor junctions. The op-amp circuit is not especially
recommended for measuring the leakage of large-value capacitors; neither is the
DVM approach because of the slow charging of the large capacitance, and because of
soakage, or dielectric absorption, effects. But, if you’re desperate and start out with a
low value of R,,,,, you can eventually get some approximate measurements.
Recently, a customer had a problem with a simple basic design using an LM3 17
regulator in which the circuit’s impedances were fairly low-just a few hundred
ohms. (The same basic circuit as in chapter 14, Figure 14.3.) After just a few minutes
of operation, the output of the LM3 17 would start drifting badly. The cause turned