Re: Getting Parallel port to run relay

--- In CAD_CAM_EDM_DRO@yahoogroups.com, codeSuidae <codesuidae@c...>
wrote:

> 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?

Coil current 89.3mA resistance 56 ohms

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

Had diode hooked up externally

>
> >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).

I thought I understood that Pin 1 on serial port was direct path to
comp. power

>
> Your transistor circuit is the correct way to handle the relay,

Is NPN proper style of xsistor??

but

> you'll need to connect the coil to an external power supply.

I assume that external and internal grounds will have to be common
right?

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

Shouldn't the circuit have fired with 5v power source going directly
to Base? Or does it have to be current reduced with resistor?