Page 74 - Troubleshooting Analog Circuits

When Is a Connector Not a Connector? 61

high-speed component. At first, I was aghast. But, after I thought about it and real-
ized that the socket would add barely 1 pF of capacitance, I said, “Well, OK. it may
not do much harm.”
The addition of the socket led to the realization that the speed problem was criti-
cally correlated with that component, and the problem was soon solved. The socket
that might have caused terrible strays actually caused almost no harm and. in fact.
facilitated the real troubleshooting process. If nothing you do leads in an encouraging
direction, and you have a half-baked notion to tell your technician to install a socket,
that may be the best idea you have all day. The socket may do very little harm and
could lead to many experiments, which might give you the vital clue that puts you on
the track of the real culprit.

When io a Connector Not a Connector?
When it’s a relay. A relay is an electrically-controlled connector, and though relays
are not as popular as they used to be, there are many times and many ways to use a
relay to get a job done exactly right. Conversely, you can use a relay to get things
done wrong, and you don’t want to do that. Let’s discuss.
Some relays are made with gold-plated or other precious-metal contacts, for the
highest reliability in low-level circuits. What is the definition of low-level? As with
every relay application, you have to refer to the manufacturer’s data sheets. Very
important. Because if you try to use a low-level relay for a high-power use, the con-
tacts can erode, wear, or maybe even weld together. Conversely, if you try to use a
heavy-duty relay, with its special metallurgy and tungsten contacts, they can run dry
and refuse to contact at all in a high-impedance circuit. Gold is a wonderful (if expen-
sive) material, and it can prevent contacts from running “dry,” but it won’t be found
in high-current contacts.
Reed relays are hermetically sealed, and have good proven reliability if properly
applied. I evaluated some recently that had less than 5 fA of leakage, when I guarded
them care full^.^ I was impressed. Also they can be engineered to have low thermo-
couples and fast response; but they need a lot of ampere-turns as there is no iron in
their magnetic circuit, so they are not normally considered low-power devices.
Oh, yes, I must add in one more comment on reed relays. If you install a well-built
coil around a clean glass reed, you can get those low leakages. But if you BUY a reed
relay, complete with coil, all packaged in a neat package, I bet I can tell you what the
package is made out of: nylon. At room temperature, nylon is a fair insulator, but at
35 degrees, under conditions of high humidity, nylon is a LOUSY insulator. You
can’t even get lo9 0 of leakage. So, if you want decent low leakage, you may well
have to “roll your own” reed relays.
All mechanical relays have contact bounce. This may last 2 or 20 ms, and the dura-
tion can vary. When you want relays or switches to talk to digital logic circuits, anti-
bounce circuits are de rigeur. Also, when there is even a moderate amount of voltage
or current, the manufacturer will usually spell out the need for some series RC net-
work to put across the contacts, to help minimize arcing or “burning.” You gorru read
the data sheet.
But, these days, not all relays are mechanical. First of all, there are mercury-wetted
relays which are credited with giving no contact bounce. Most of these must be held
upright or they do not work. There are mercury-wetted reed relays that do not bounce

3. Cot0 Type 1240-12-2104, Cot0 Corporation, 55 Dupont Drive, Providence. Rhode Island 02907
(401) 943-2686.