Page 166 - Troubleshooting Analog Circuits

Systems and Circuits I53

These people have to be able to communicate the early signs of trouble, so the leader
can get things fixed. (Well, OK, he or she.. . .)

How to Trim without Trimming Potentiometers

Speaking of keeping circuits well trimmed, some people like to use trimming poten-
tiometers to get a circuit trimmed “just right.” Other people hate to, because the po-
tentiometers are expensive or unreliable or drifty. Worst of all. if a circuit can be
trimmed, it can also be mis-trimmed; some person may absentmindedly or misguid-
edly turn the potentiometer to one end of its range or to the wrong setting. How long
will it take before that error is corrected?
For just this reason, some people prefer fixed-voltage regulators because they
always have a valid output (f5%) and can never get goofed up by a trimming poten-
tiometer. Other people need a tighter tolerance yet are nervous about the trimming
potentiometer. You will find the solution in the snip-trim network in Figure 12.4.
(Ref. 5). This scheme will let you trim a regulator well within 196 without trimming
potentiometers. Note that you could also use this technique to set the gain of integra-
tors and the offset of amplifiers. It’s not always easy to engineer the correct values
for these trims, but it is possible. And, nobody’s going to go back and tweak the
potentiometer and cause trouble if there’s no potentiometer there to tweak.
A pet gripe of mine concerns engineers who design a circuit with an adjust range
that’s so wide that damage can occur. For example, Figure 12.5a is a bad idea for a
regulator for a 5-V logic supply because the TTL parts would be damaged if someone
tweaks the pot to one end of its range. Figure 12.5b is better.

What about Solderless Breadboards
Here’s a chunk of late-breaking floobydust-the topic is those solderless
breadboards, which consist of a number of metal strips and solderless connectors
hidden underneath a plastic panel with lots of holes in it. Schools often use them to
introduce students to the joys of breadboarding because you can easily connect things
by just stuffing wires and components into the holes. The problems begin with capac-
itance. The breadboards usually have 2,3, or 5 pF between adjacent strips. On a good
day, only a wise engineer could plan a layout that all the capacitors, sprinkled
throughout the circuit, wouldn’t ruin.
The next serious problem with solderless breadboards is the long leads, which
make adding effective power-supply bypass capacitors close to a chip difficult.
Next, I suspect that some of these panels, although they are not inexpensive. use
cheap plastics such as nylon. On a warm, humid day, cheap plastics do not offer high
insulation resistance. Nobody wants to talk about what kind of plastic the
breadboards are made of.
Finally, Mr. Scott Bowman of Dublin, CA, points out that after you insert enough
wires into any given hole, the solderless connector will scrape sufficient solder off
the wire so that the scraps of solder will pile up and start to intermittently short out to
an adjacent strip. Further, the adhesive that holds on the back panel tends to hold the
solder scraps in place, so you can’t clean the scraps out with a solvent or a blast of air.
I didn’t even think about these solderless breadboards when I wrote my series
because I see them so rarely at work. They just have too many disadvantages to be
good for any serious work. So. if you insist on using these slabs of trouble, you can’t
say I didn’t warn you.