Page 47 - Troubleshooting Analog Circuits

34 3. Getting Down to the Component Level

that a 1 A fuse would blow when the current exceeded 1 A, then you have not been
heavily involved in choosing fuses.” (Ref. 1) Fuses are generally guaranteed to carry
100% of their rated current indefinitely, and most will carry 120% for several hours.
Even the fast-blow ones cannot open up much faster than 10 ms if overloaded by
1OX their rated load, or 100 ms if overloaded by 2X. You may be able to get faster
response than that if you shop for the new semiconductor-rated fuses with very fast
blow time. If somebody in your organization-a components engineer or an old-
timer-can help you find the right information in a catalog of fuses, he can save you
a lot of time. Without that kind of help, you will probably not be able to find a catalog
from a fuse maker, or to figure it out when you get it. The curves of various ratings
are a little ohscure until you get used to them.
You may not use fuses much-modem solid-state circuits have such good current-
limiters and thermal limiters that you may not see fuses every day. So when you do
see fuses, you may be surprised. The low-current ones act pretty soft-resistive.
Some fuses just happen to fail unprovoked. The one in my clothes-dryer fails every 3
or 4 years, leaving my wife perplexed. Finally I wrote down the list of symptoms, so
any time the fuse goes out and there is no heat, we at least save time by recognizing
the symptoms. When my microwave oven quit working recently, I was a little con-
cerned because the label on its back said, “No user-serviceable components inside.”
When I opened it up, there was a fuse clip with a blown fuse. After shopping unsuc-
cessfully at several electrical supply houses, I finally went in to a Radio Shack. They
had them and, I realized, that was thefirst place I should have gone. I replaced the
fuse and turned on the power-would the fuse blow for a good reason, or had the old
fuse just fatigued out? The new fuse has held for several months, so it was just a
fatigue failure.
Most fuses are fully rated for 115 or 230 VAC, but not more than 32 V of DC.
That’s because the alternating current flow gives time for an arc to be extinguished,
which would not happen with DC. So for high-voltage DC, the answers aren’t so
simple. Some circuit breakers are rated for as much as 65 VDC, but often that’s not
enough. There is a CD Series that is good up to 125 VDC, and a larger GJ Series that
is rated up to 150 V, available from Heinemanm3
Another approach in circuits with rectified power is to put the sensing coil in the
DC circuit, but connect the breaker into the AC circuit. That’s no help if you just
have a 120-V battery supply.
These days, high-powered MOSFETs can be used to make such a good high-
voltage high-current switch that you can build your own fast-turn-off switch, acti-
vated by over-current-an electronic equivalent of a fuse. I built one of these-and it
didn’t work very well, the first time. It blew out the ET. Twice. I haven’t really
given up on it, and when I get some time after I put this book to bed, I’ll go back and
get it running. When I get it running, I’ll publish it somewhere where you can all see
it: The equivalent of a solid-state fuse that can handle as much as 200 V of DC.
Meanwhile, when you need some fuse protection on a DC power supply, just put a
fuse in the secondary of the transformer, so it sees AC current flow and AC voltage,
rather than DC.

Inductors and Transformers Aren’t So Simple

Inductors and transformers are more complicated than resistors-nonlinearity is rife.
Their cores come in many different shapes and sizes, from toroids to pot cores and
from rods to stacks of laminations. Core materials range from air to iron to any of the

3. Heinernann Electric, P. 0. Box 6800, Lawrenceville, NJ 08648. (609) 882-4800.