software. I use Raptor at college (
ITT-Technical Institute) and it is available
at http://raptor.martincarlisle.com.
Best of all, it’s free and really small.
— Joseph Fulton
Dear Russell: Re: Soldering Iron
Timer, March ‘ 10, Page 28.
— Anonymous
Dear Russell: Re: Mystery
Component, July ‘ 10, Page 25.
Response: Thanks for the
feedback, Warren.
In your column, a person asked
about a mystery component in a
microwave oven. The component
inside the transformer is a thermal
fuse. It blows under two conditions:
over current and over temperature.
If this fuse is blown, the transformer
is considered bad. I used to repair
microwave ovens and the only way
to get this transformer is to buy a
new timer unit. Putting any part in
there will compromise the UL safety.
Response: Thanks for the
feedback, Randy. I would forward
your comments to Jon Carter if I
could find the original email!
Dear Russell: Re: Power Supply,
July ‘ 10, Page 24.
According to the Hammond
transformer website —
www.hammondmfg.com
/pdf/5c007.pdf — IDC output = . 62 x
IAC for a FWB with cap input power
supply. That means that their 3. 8
amp transformer will only produce
an output of 2. 3 amps DC when
used in your circuit.
— Mike Eck
Response: Thanks for the
feedback, Mike. You are right, that
transformer will overheat if loaded
to three amps. Your comment
brought to my mind the question:
How much inductance — in series
with the rectifiers — will produce a
continuous current such that full VA
rating applies? Trial and error
simulation indicates that 5 mH will
do it but I can’t find an inductor
rated 5 mH at three amps. One could
connect seven 680 uH inductors in
■ FIGURE A
QUESTIONS & ANSWERS
series (Mouser PN 542-2322-V-RC) if
you want to do it.
Dear Russell: Timer, July ‘ 10,
Page 25.
Thank you for your suggestion
for building a two-hour timer. I am
enclosing some pictures which I took
of the finished units (Figures A and
B); two were made. Your circuit was
used almost exactly as you
presented it except that I omitted the
trimpot. I used a larger series
resistor (560K) paralleled with one
determined using a decade box.
The units as originally built
controlled the AC line using a 4015L2
thyristor/MOC3020 solid-state relay
combination. However, I found that
the power was not being turned off
with the very small load of a small
“wall-wart.” The voltage dropped to
only 65 volts! Only then did I think
to check the specs of the 4015L2; the
minimum current is listed at 0.8A!
One of those obscure facts you learn
with experience. As you can see in
the pictures, a relay was substituted.
Since only two were required,
the circuit was assembled on a Vero
board rather than an etched one. The
timing capacitor was metallized
polyester rather than an electrolytic
for long-term reliability. I thought
that you might like to see how one
of your “babies” turned out. Once
again, thanks.
■ FIGURE B
November 2010 27