[GreenKeys] Building my own power supply

[Ethan Blanton]

Windows XD wrote:
> I'm trying to build my own power supply for my Teletype model 19, it needs
> 120-130VDC of power @ 830Ma. I'm working on a circuit for it, but I don't
> exactly know what I am doing because I've never done this before. Would you
> let me know if I am on the right track to building a switching power
> supply?

This looks like a linear supply to me, but you cannot rectify the
incoming AC to DC _before_ passing it through a transformer, you'll
just burn up your transformer that way, with no voltage coming out the
output except the instant when you plug it in and the instant when the
primary burns through.  ;-)

A linear supply is very reasonable for this, but you'll need a
step-down transformer to something like 85-90 VAC.  You then put your
bridge rectifier and filter capacitor on the _output_ of the
transformer instead of the input, and voila -- you're done.  This is
exactly what I use for my iron horse perforator.

> P.S. I know it needs a snubber circuit somewhere and none of the actual
> switching circuitry is there, I don't know enough to know what goes where.
> [image: circuit.png]

In theory you do, but in practice it probably doesn't matter.  The
snubber prevents a high reverse voltage from kicking back out of the
punch solenoid coil when the supply is cut, but since there are no
other devices on the loop (and in particular no semiconductors that
cannot tolerate the reverse voltage), I doubt you'll find you need it.
If you did, you'd just put a diode antiparallel to the filter
capacitor.

Ethan