Re: [CAD_CAM_EDM_DRO] Getting Parallel port to run relay

bertmantx wrote:

>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.
>
>

What are the voltage and current requirements of your relay?

Does the relay coil have a built-in snubber diode?

>All was well for about 20 cuts and then it just stayed on.
>

This would probably indicate damage to the parallel port drivers,
they're tough, but they aren't rated to sink the kind of current that
most relays require.

>Any thoughts on damage to computer?
>
>

Try connecting a couple of 1k resistors in series to the parallel port
pin and connect it to ground. Toggle the port pin high and low and
check the voltage relative to ground in each state. Repeat with the
resistor connected to +5v. The port should be able to produce the
correct logic level in each case. If it can do that, you should be able
to drive a transistor to run your relay.

I would not recommend trying to run the relay on power from the ports,
they aren't intended to supply that kind of current. Instead use an
external power source (a 5ish volt battery is fine for testing, or grab
a 5v supply line from inside the computer).

Your transistor circuit is the correct way to handle the relay, but
you'll need to connect the coil to an external power supply. Verify
that the relay has a snubber diode built in, otherwise the inductive
kickback from the coil will eventually blow your driver. If it does
not, just put a diode across the coil.

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

There are a few, but the market tends to be rather small, so they can be
hard to find. Most any electronics hobbiest (including myself) could
whip up a reliable circuit board to do this for you in an evening for a
few dollars in parts.

Hope this helps,
CS