Re: MRC Ampack upgrade finished

David Starr wrote:

As drawn your circuit looks good to me. I see no shock hazard. Here in
North America, connecting metal casework to the third (green) ground
wire is good practice. Should a wiring fault short the case to power,
the ground will pull enough current to pop the branch circuit breaker. I
know nothing about overseas electrical practices, they may be quite
different.
Your transistor is running cool enough so long as you can touch it
with the back of your finger for the count of ten. Worst case heating
will occur around half throttle. At full throttle the transistor
doesn't drop any voltage across it, at min throttle it doesn't pass much
current. Back of finger (or hand) is considerably more heat sensitive
than the palm. If it's running hotter than you like, make the heat sink
bigger. The heat sink is more effective when the collector is directly
fastened to it. The insulating washers are insulators of heat as well
as of electricity. The heat sink is also somewhat more effective when
the transistor is torqued down good and tight, tight as you can go short
of stripping the screws.

Does they circuit develop a full 12 volts at the track at full
throttle (and full wave) ? Or is it a volt and a half low from the
double diode drop in the Darlington transistor? Just curious, doesn't
matter much, especially as many HO locomotives will do 200 scale miles
per hour at 12 volts.

DS:

Some of the discussion is leading me to consider insulating the
collector.
At the moment, there's no pressing need. For one thing, I wired the
basement and tested it myself, and for another thing, all my 120v
receptacles
down there are GFCI protected. However, when and if I do insulate it,
I
will most likely insulate the heat sink from the case, and leave Q1
right
against the aluminum.

The circuit does develop full 12V on the 10 ohm load, and about 15v
when unloaded (well, loaded with the voltmeter's internal resistance,
to be fair). It might be a little off 12v, since I was reading from
the
0-50V scale on my cheapo analog multimeter, my good digital one
being currently AWOL. It's probably within .5 V, anyway.

I measured close to 18 VAC at the transformer, so this seems about
right - one silicon diode plus two more drops in Q1 would bring the
output down near 15 V.

Cordially yours:
Gerard P.
President, a box of track and a solid-state Ampack.

.

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