[GreenKeys] WU Modeal 2-A "Iron Horse" doesn't strike

[Ethan Blanton]

Ethan Blanton wrote:
> Jim Haynes wrote:
> > If you don't mind living a bit dangerously you could just put a bridge
> > rectifier across the AC line and use the output to run the punch.  To
> > make it a little less dangerous use a 3-wire power cord with the frame of
> > the unit connected to the green wire ground.
> 
> I was considering doing that with a variac in line.  I have an
> appropriate variac.  It's even fused.
> 
> Sounds like it's time to invest in an isolation transformer.

OK.  I built a power supply from an isolation transformer, a variac,
and some junk box parts.  It's a reasonably safe setup[*].  The best
part is -- it works great!  I was able to punch several tapes at high
speed with no trouble.

The power supply is filtered with only an 0.33 uF capacitor, which is
essentially no filtering at 750 mA; it works fine.  I don't know if
that's because the input impedance of the solenoid at switching time
is sufficient to let the capacitor filter, or because it needs little
or no filtering to begin with.  The 0.33 uF capacitor was the largest
HV capacitor I had on shelf, and I figured I'd just order a bigger one
if it didn't work.  Looks like there's no need.

Thanks, all!

Ethan

[*] The only unsafe part is the existence of a non-isolated line cord
    rectifier, but it is clearly labeled.