Page 31 - Troubleshooting Analog Circuits

18 2. Choosing the Right Equipment

Figure 2.5. You can vary the output voltage of this DC power supply from 3 to 30 V by adjusting R,, &,
should be between 3 and IO0 the short-circuit current is equal to about 20 mA + 600 mV/ kc

don’t let you continuously sweep the voltage up and down while you monitor the
scope and watch for trends. In cases when the power supply’s output capacitor causes
problems, you may want a power supply whose output circuit, like that of an op amp,
includes no output capacitor. You can buy such a supply, or you can make it with an
op amp and a few transistors. The advantage of the supply shown in Figure 2.5 is that
you can design it to slew fast when you want it to.
(For speed, use a quick LF356 rather than a slow LM741). Also, if a circuit latches
and pulls its power supply down, the circuit won’t destroy itself by discharging a big
capacitor.
While we are on the topic of power, another useful troubleshooting tool is a set of
batteries. You can use a stack of one, two, or four 9-V batteries, ni-cads, gel cells, or
whatever is suitable and convenient. Batteries are useful as an alternate power supply
for low-noise preamplifiers: If the preamp’s output doesn’t get quieter when the
batteries are substituted for the ordinary power supply, don’t blame your circuit trou-
bles on the power supply. You can also use these batteries to power low-noise cir-
cuits, such as those sealed in a metal box, without contaminating their signals with
any external noise sources.

8. A few RC substitution boxes. You can purchase the VIZ Model WC-412A, which I
refer to affectionately as a “Twiddle-box” (Figure 2.6) from R & D Electronics, 1432
South Main Street, Milpitas CA 95035, (408) 262-7 144. Or, inquire at VIZ, 175
Commerce Drive, Fort Washington, PA 19034, (800) 523-3696. You can set the unit
in the following modes: R, C, series RC, parallel RC, open circuit, or short circuit.
They are invaluable for running various “tests” that can lead to the right answer.
You may need component values beyond what the twiddle boxes offer; in our labs,
we built a couple of home-brew versions (Figure 2.7). The circuit shown in Figure
2.7a provides variable low values of capacitance and is useful for fooling around with
the damping of op amps and other delicate circuits. You can make your own calibra-
tion labels to mark the setting of the capacitance and resistance values. The circuit
shown in Figure 2.7b provides high capacitances of various types, for testing power
supplies and damping various regulators.

9. An isolation transformer. If you are working on a line-operated switching regulator,
this transformer helps you avoid lethal and illegal voltages on your test setup and on