Re: [CAD_CAM_EDM_DRO] Getting Parallel port to run relay

HI Scott,

Perhaps the easiest thing to do to protect the PC is to use
opto-isolation. Commercial drivers like Geckos have these already.

A series resistor will limit the current, but also drop the voltage. A
resistor is generally used in the base lead of a transistor driver.
Also useful in small values to limit ringing of bus lines.

What are you driving? If you need a relay, use a relay driver circuit
(transistor or IC), or a SSS (Solid State Switch).

If you're driving a stepper driver, then you might want to use a
"Parallel Line Extender", like Bravo sells. This provides driver ICs to
help isolate the PC, and also provides additional driver current for
long lines.

Another good way to protect the PC is to use a "plug-able" parallel port
card.

The best way would be to use an "industrial interface card". (expensive).

Haven't seen any fuses that would protect a port line. Keyboard, yes.

A choke would be an inductive load, and would actually MAKE a spike! A
diode would only allow current in one direction. Would work a little
like a diode across the coil of a relay (inductor!).

HTH

Alan KM6VV


Scott L Golden wrote:

> I may be way out on a limb here but can't one put fuses in line? I admit low
> amp fuses but wouldn't that help with spikes?
>
> Now go easy on me... I am not too deep into electronics.
>
> Or isnt there a one way choke or something to limit the flow of power
> backwards?
>
> Sorry for the lack of knowledge but....my thoughts anyway. lol
>
> Scott L Golden
> Never Give Up
>
>
>
> ----- Original Message -----
> From: "Andy Wander" <awander@...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Friday, January 28, 2005 3:27 PM
> Subject: RE: [CAD_CAM_EDM_DRO] Getting Parallel port to run relay
>
>
>
>>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
>>
>>
>>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.
>>>
>>
>>
>>
>>
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