Re: [CAD_CAM_EDM_DRO] Getting Parallel port to run relay

Hi Bert, Andy, list,

After thinking about it, I wonder if simply connecting a parallel port
output pin to a relay coil would damage it. It wouldn't work, mind you,
but relay coil will still offer some resistance, and the current flow
will be limited. If the other end of the coil is grounded, then a HI on
the pin will typically have minimal source capability. If the other end
of the relay is connected to +5, then the pin will try to source
probably 20-30 ma. What was the voltage/current rating of the relay
coil? Provided the coil was not not left "energized" for a long
period, no damage may have been done. Give it a check with an LED and a
220 ohm resistor!

Alan KM6VV


Andy Wander wrote:

> 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.
>