[GreenKeys] LTSpice

[Jeffrey D Angus]

On 7/21/2013 9:23 PM, Jim Haynes wrote:
> What if you put a 2K resistors in series with the diode?
It cleans up the fall time of the selector magnet. Cutting the spacing
distortion down to 2 mS or so, but the result is a very nasty fast spike
across the contacts up to 213 volts. 93 v above the 120 v supply.

Removing the diode (and) resistor from across the selector magnet
and putting a pair of 120 v zeners back to back (in series) across the
switch results in the second wave form. Not much spacing distortion
and it does clamp the kickback voltage to very close to the 120 v
loop supply

The zeners I used in the simulation are a pair of 1N5173 diodes. 60v
each. So a total of four in series two each facing in opposite directions.

By the way, the whole idea behind putting snubbers across the contacts
(or keying semiconductor) is that each one protects the keying circuit.
You can put the same snubber circuit across the tape reader, keyboard,
loop keyer etc. Since only ONE device at a time is keying the loop, only
one snubber circuit is active at a time. But every keyer is protected.

That being said, my simulations have been done with the assumption
that the selector magnet coils require a 60 mA loop current and they
have a series resistance of 200 ohms and 2 henry inductance.

If someone wants to measure their selector magnets, I'll be happy to
run a "custom" simulation for you and derive an optimized snubber.

"Where were these spiffy tools back in the '70s when I was heavily into
green keys with real iron?"

Jeff-1.0
wa6fwi


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