Page 75 - Troubleshooting Analog Circuits

62 5. Preventing Material and Assembly Problems

and can be used in any position. But even they will not work at temperatures colder
than -38 “C, where mercury freezes.
Then there are the solid-state relays. Some of them can switch many amperes-but
they often use SCRs, and thus have a loss of more than a volt. You can’t use that for a
little signal. Others have low-ohm MOSFETs, and can handle a few amperes with
low losses and low offset voltage. But the big ones have a lot of leakage and capaci-
tance (which is not always mentioned). The little ones are nice and delicate, for preci-
sion switching, but cannot carry many milliamperes.
So, for high reliability, you have to be pretty knowledgeable and thoughtful, and
selective, when you choose a relay, or you’ll pick one that’s inappropriate for your
application, and you’ll be embarrassed when some of them provide poor performance,
or fail sooner than expected.

When Is a Relay Not a Relay?

When it is just a switch, mechanically operated by hand. But when you choose a
switch, the contacts have almost exactly the same limitations as the relay’s contacts.
There are high-current ones, there are delicate ones, there are hermetically-sealed
ones. So in the same way, be thoughtful when you make your choice. You do have
the advantage here that if you try to wear out a relay, a few million operations can
cause failures in just a few weeks; but most people couldn’t wear out a switch by
hand fast enough to get in trouble! As with relays, if you aren’t sure what the data
sheet is trying to tell you, talk to the manufacturer’s good people for advice and inter-
pretation. They may have a switch in their “back room” that is just what you need.

Weird Wired World

Now, I’ll add a few pithy comments about wire and cable. Not all wire is the same.
For example, when I first got a job in electronics, I was having a lot of trouble with
Teflon-insulated wire. The wires would often break right at the point where the
solder stopped. After several engineers assured me that all wire was the same and
suggested that I was just imagining things, I was ready to scream.
Finally, I found an engineer who admitted that cable manufacturers couldn’t put
individually tinned wires into a Teflon insulator, as they do with plastic-insulated
wire. At the temperatures at which the Teflon is extruded, the solder would all flow
together, thus making the stranded wire a solid wire. Instead, cable manufacturers use
silver-plated wire strands for Teflon-insulated wire. With this type of wire, solder
tends to wick up into the strands, thus making the wire quite brittle. Once I under-
stood the wire’s structure, I was able to solve my problems by adding strain relief for
any bends or pulling stresses.
As I mentioned in Chapter 2, the ordinary plastic-insulated single-conductor wire
that is used in telephones has just the right stiffness to make good twisted-pair wire
for making capacitors with values of 1,2.1, or 4.95 pF. The wire doesn’t have a
Teflon dielectric, but it’s good enough for most applications.

Consider Your Wire Type
Shielded or coaxial cables, such as RG-58U, RG-174, shielded twisted pairs, and
other special flat cables, all have their place in the job of getting signals from here to
there without undue attenuation or crosstalk. When you have a large number of wires
mindlessly bundled together and you don’t have any bad crosstalk, you’re witnessing