🚀 go-pugleaf

In article <4674@ptsfa.PacBell.COM> dmt@ptsfa.PacBell.COM (Dave Turner) writes:
>In article <7485@ihlpf.ATT.COM> warren@ihlpf.ATT.COM (Montgomery) writes:
>>I am looking for a stunt box that will switch my stereo receiver on
>>according to whether or not my turntable is running.  This strikes
>
>Both Radio-Shack and Heathkit sell them.
>I don't have Heath's catalog handy but theirs is listed in both the
>stereo and ham sections.
>
>Check RS's Auto-Power Controller 26-1396 $69.95.

The Heath version is GD-1495 for $59.95 in the winter '88 catalogue.
It's a kit.

The Heath box is not available in Canada (something about not passing
CSA) and I can't find the Radio Shack one in their catalogue either.

A couple of years ago, I decided I needed a box like this, and with
no apparent commercial sources of them (the Heath didn't appear until
later), I designed one for myself:

The heart of the box is a commercial solid-state relay.  The input
is 2-32V DC across about a 1 Kohm load, and is optically isolated
from the output circuit.  The output can be rated at 10, 25, or more
amps, depending on your budget, and can handle subtantial surges
above that - good for me, since the main switched device was to be
an audio power amplifier with big input capacitors.  It needs a heat
sink, but the metal mounting plate is isolated from both input and
output circuits.
 
I put a full-wave bridge rectifier in series with the "control" outlet
on the box.  The DC outputs of the bridge are connected to the solid-state
relay inputs.  Four 6-amp ordinary rectifier diodes are connected in
a series string, and the string is connected across the SSR input so it
will be forward-biased.

Whenever the "control" device (in my case the preamp) is turned on,
it draws current through the bridge, which supplies current and voltage
to the SSR input.  The voltage across the SSR rises to about 2.4 V,
and then the diode chain conducts, carrying almost all of the load current.
I believe there is also a capacitor across the SSR input to provide
reliable SSR triggering even when the "control" load is not resistive.
There is a 1A fuse in series with all this, so excessively large loads
will eventually blow the fuse.

The voltage "lost" across the bridge and diode string is about 4 or 5 V,
not enough to bother most loads.  The bridge must be rated to block
the full AC line peak reverse voltage plus some extra, and must carry
the full "control" load current.  The voltage-limiting diodes are never
reverse-biased, so their voltage rating is unimportant, but they need
to be rated for the full control load current with a good margin of
safety.  These diodes are protecting the input of the SSR (the only
expensive part), and in case of a current surge you would like the
bridge to fail before the voltage-limiting string.  That's why I used
4 forward-biased rectifier diodes instead of a single Zener - their
surge current rating is something like 400A, much higher than an
equivalent Zener.

The advantage of using this sort of circuit directly in series with the
control output is that it is very sensitive - it takes only a few mA of
load current to trigger the box.  The disadvantage is that the full
control load current goes through these diodes, so they have to be
heavy, and you can't use a really large load on the control outlet,
so the box isn't as general-purpose as I'd like.  There's also the
voltage drop across the bridge.

You could also sense the control load by using a step-up transformer
made with 14-gauge wire for the primary.  However, I have no experience
in transformer design, and I'm not sure how you would go about designing
it to work reliably over the 10 mA - 15 A range of load currents while
driving the SSR directly.  And adding amplification means power supplies,
another headache.

Does anyone know how the Heath or Radio Shack units sense the load
current?