RE: [CAD_CAM_EDM_DRO] Getting Parallel port to run relay

If you hooked a relay up to your parallel port, and you didn't install a
diode or other suppression device in parallel with the relay coil, then you
may have fried your printer port pin. When a coil has a field set up in
it(by having current flow through it), opening a contact(your port pin)
stops the current flow and causes the field to collapse, generating a large
reverse current through(and hence a reverse voltage across) the coil. If not
shorted through some sort of snubber circuit(I use a reverse-biased diode)
this can cause switch contacts to arc, and can also blow out transistor
circuitry.

This is the same principle used to generate high spark plug voltages using a
DC battery in your car(ok, I know the new cars may use a different method,
but this is the traditional methed)

Andy Wander
Verrex Corporation


-----Original Message-----
From: bertmantx [mailto:bertmantx@...]
Sent: Thursday, January 27, 2005 10:06 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Getting Parallel port to run relay



Looking for any advice an how to get parallel port to activate relay
to start plasma torch. Inatalled PC relay (which I now know to mean
Printed Circuit) and tried running it direct from parallel out put.
All was well for about 20 cuts and then it just stayed on. If it is
off when hooked up will stay off until turned on then stays on. Have
tried hooking NPN xsistor with Game port 1 to coil, coil to
collector and emitter to ground. 1K resistor between parallel signal
and Base. No response when hot signal (verified +5.9v) touched to
resistor or direct to base of xsistor. If shorted from collector to
base will trip 20 or so times then nothing. Remaining Para ports are
running motors fine.

Any thoughts on damage to computer?

Any pointers on assessing computer damage?

Hard to believe there isn't a plug and play device just for this,
but have had no luck finding anything.

First cuts looked surprisingly good and had High hopes. Depression
is now setting in quickly.

Any advice greatly appreciated.