Page 46 - Troubleshooting Analog Circuits

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When Is a Resistor Not Just a Resistor? 33
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Figure 3.4. If you use a good voltmeter to measure Vref and V, and take the ratio, you can resolve the Rx
a lot better than in the OHMS mode.

don’t go to low ohms or to a short circuit. The accuracy or stability of a high-value
resistor ( lo8 to lo1* a) can be badly degraded if dirt or fingerprints touch its body.
Careful handling and cleaning are important for these high-value resistors and high-
impedance circuits.
One problem that occurs with all resistors is related to the Seebeck effect: the
production of an EMF in a circuit composed of two dissimilar metals when their two
junctions are at different temperatures. In precision circuits, you should avoid
thermal gradients that could cause a large temperature difference across a critical
resistor. For example, don’t stand a precision resistor on end, as in an old transistor
radio-if it has any dissipation, it might get a lot hotter on one end than the other.
Many precision wirewound and film resistors have low Seebeck coefficients in the
range 0.3 to 1.5 kV/“C. But avoid tin oxide resistors, which can have a thermocouple
effect as large as 100 kV/OC. If you are going to specify a resistor for a critical appli-
cation where thermocouple enors could degrade circuit performance, check with the
manufacturer.
So, you ought to know that resistors can present challenging troubleshooting prob-
lems. Rather than re-inventing the wheel every time, try to lem from people with
experience.

When Is a Resistor Not Just a Resistor?
When it’s a fuse. Obviously, when a low-value resistor is fed too much current and
fails “open,” that is sometimes a useful function, and the multi-million-dollar fuse
industry thus serves to protect us from trouble. But the fuses themselves can cause a
little trouble. They don’t always blow exactly when we wish they would. As Ian
Sinclair put it in his book Passive Component- User’s Guide.* “If you thought

2. Mr. Sinclair’s book has a lot of good information in it, about all kinds of passive components. and I
thoroughly recommend it-see Ref. 1 for more information.