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> Use AC Relay on DC Circuit?
sentinel
Posted: May 19, 2006 10:18 pm
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Hi

First off I want to say that I have some background knowledge in electronics basics and theory, but I have some blanks that need to be filled in. wink.gif

What I'm wondering is this:

I picked up a small device which controls a device plugged into a 120V AC outlet with an RF remote (on/off). Kind of like a 3-pronged pass-through thing. Looks like this.

I was wondering if it would be possible to use it on a DC circuit to power on a computer or coffee maker remotely. (Forgetting for a minute that a computer needs a momentary switch.)

I cracked it open and the switching seems to be controlled by an "801H-1A-C" SPNO Relay. (PDF Datasheet I found) {it also has the following written on it: 24VDC, 10A 250VAC, 15A 125VAC} Unfortunately I don't know a lot about relays.

Would it be possible to adapt such a device to control a simple DC circuit?

If I draw power from an external source (PC's DC Power Supply, DC batteries, or indepentdant AC line if need be), could it just act as a simple switch to open/close a DC circuit?

If so, how would I wire that?

Any help would be appreciated! smile.gif

This post has been edited by sentinel on May 20, 2006 12:17 am
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Jimthecopierwrench
Posted: May 20, 2006 12:17 am
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I certainly don't see why not. I mean obviously it won't live long if you're trying for remote swiching of an automotive starter motor, but yeah, for the most part mechanical relays aren't going to be too fussy about what they're switching.

QUOTE
how would I wire that?


Well, input and output...

The "output" is a set of contacts, not unlike ones in a regular switch. They're labelled the same way too - by number of poles, throws, and rest state.

The "input" is a coil - solenoid if you will - that does the mechanical work of the switch throw. It'll be DC for sure and low voltage. No current limiting required, but driving with a semiconductor will require a protection diode. The coil is electrically isolated from the output.

Unless it's a holding relay (the datasheet one isn't) it IS technically momentary. remove the coil current and the relay will return to it's "normal" state.

Wiring diagrams on the bottom of page two for SPDT (1C), SPST-NC (1B), and SPST-NO (1A) Note that the SPDT is functionally identical to either of the other two.



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sentinel
Posted: May 20, 2006 02:29 am
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That is so amazingly helpful! Thank you so much!!


Just a couple quick questions about this part:
QUOTE
The "input" is a coil - solenoid if you will - that does the mechanical work of the switch throw. It'll be DC for sure and low voltage. No current limiting required, but driving with a semiconductor will require a protection diode. The coil is electrically isolated from the output.
So does that mean that I could put any sort of DC voltage across the solenoid and it would throw the switch. Like just a regular 5V (or 12) from a PC's power supply connector (or USB cable) would work? Regardless of current?

Looking at the datasheet it says that the coil voltage on mine is 24Vdc, and the rated current is ~15mA. Does that mean that I would need to apply that V&I to the relay before it would switch?

QUOTE
No current limiting required, but driving with a semiconductor will require a protection diode.
So are you saying that I would need these to do this, or is that just another option?

Thx again.
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Radio Man
Posted: May 20, 2006 04:23 am
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You can try any voltage less than the rated voltage (24v DC), it hay or may not work, but will not hurt the coil. It may however not hold the contacts as hard and increase the contact resistance. Trial and error thing...

QUOTE
I was wondering if it would be possible to use it on a DC circuit to power on a computer or coffee maker remotely. (Forgetting for a minute that a computer needs a momentary switch.)

Are you planing on just using the relay?
If you are planing on using the entire device on DC you will need to power it. There is some type of DC power supply built in that get power from the wall plug. So you could use it on DC with out a big problem, but will need t mod it.

As for switching DC v. AC:
DC switches require larger contact throw distance than AC. This is why most relays ratings are lower for DC capacity. This is due to arching.

Another idea that would not require you changing anything. Get a wall transformer (you know the big black things you have plugged in all over the place) and plug it in to the “remote switch” thing you have and use it to power any relay yo want! This way you can switch anything! thumbsup.gif


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sentinel
Posted: May 20, 2006 04:42 am
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cool!

Yeah I want to use the whole thing, (specifically the RF part of it)

I could post pics of the PCB if that would clear things up at all...
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sentinel
Posted: May 21, 2006 04:46 am
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Ok I've got some pix if someone doesn't mind taking a look and helping a bit more.

1st of all:
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And this is kind of how I figured that it's hooked up - in a VERY simplified sense.
Diagram
I dunno if that would make sense to anyone but me. wink.gif

That whole big, scary cap/resistor/diode bank has got me a bit confused. Will I need all that if I'm just feeding it DC?

Does any of this make any sense? dunno.gif
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Jimthecopierwrench
Posted: May 21, 2006 05:49 pm
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Okay, not to be an arse, but lets keep in mind that a little knowledge can be a dangerous thing.

Your diagram is baically correct (outside of the R/C/D) box in the sense that the neutral (white) wire is continuous and the hot (black) is switched via the relay.

Although I'll be damned if I can see it from the pics (then again, I'm drunk and it's 4:14AM) the hot should also enter your "R/C/D" box to power the low voltage circuitry that's doing the RF thing and switching. As there's no stepdown transformer, the low voltage will be derived from capacitive reactance. (google) this also means that there is no mains isolation and that you should be careful with this circuit.

The rest of what's immediatly obvious is: The caps, resistors, and Zener on the board top derive the low voltage. The small vertically mounted circuit SHOULD be the RF module, and the IC should be the decoder. the components R4 (base current limiting) Q? (can't see the number - relay driver tranistor) and D2 (back EMF clamp diode) actually drive the relay.

So if you want to use the existing circuit to switch something else, you'll have to isolate the traces coming from the relay output (just above and below D2). With these totally isolated from all other connections - ie cut the traces - then those two connections can be brought out from the relay and you'll have a remote controlled "switch"





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