Re: [CAD_CAM_EDM_DRO] Getting Parallel port to run relay

HI Andy,

That's true! The spike will kill transistors. I was just pondering the
load of the coil. If the coil doesn't saturate anyway, then there
wouldn't be much of a spike. And, as you've mentioned, there is a diode.

When I was in H.S., I loved to get RS5D relays, often from surplus
Radiosone (sp) weather probes. These small relays would power from
solar cells! And thus would be directly drive-able from a parallel port
pin. Not much current capability in the contacts, 'tho.

These days, I just use a SSS (Solid State Switch) to do my switching.
Small relays can be driven with a 2N2222 transistor and a 1K ohm or so
resistor, which I think has been mentioned.

Alan KM6VV


Andy Wander wrote:

> Alan:
>
> I'm talking about the huge inductive "spike" that occurs when the "control",
> whether it is a switch, a parallel port pin, or whatever, opens the circuit.
> This causes a voltage spike that WILL fry transistors!
>
> But the OP has indicated that he DID have a suppression diode installed
> across the coil, so that is probably not the problem.
>
>
> And it is very possible to control relays with the parallel port directly;
> I'm controlling some small 5V relays directly with my parallel port. You
> just nned to make sure that the relays source less current than the port can
> sink(low-active is recommended).
>
>
>
> Andy Wander
> Verrex Corporation
>
> -----Original Message-----
> From: Alan Marconett [mailto:KM6VV@...]
> Sent: Friday, January 28, 2005 3:02 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: 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
>