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• posted on August 23, 2008, 4:21 pm

I am connecting a 12V power source to a relay (12V input).
I looked at http://home.howstuffworks.com/relay1.htm and in place of the lower battery I have my 12v transformer. Whe the switch is On is it not short circuited? It's just copper wire coiled. Does it have some resistance?

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<%-name%>
• posted on August 23, 2008, 4:48 pm

I don't know what you are doing, but I first have to suggest that there are two kinds of 12 volts. It can be DC or one or another flavor of AC. They work very different under some conditions. Using the wrong one can damage equipment.
As for your apparent question, the coil in a transformer creates a magnetic field using the coil. That field then generates a flow of electrons (electricity) in the secondary coil, generally at a different voltage (like 120V vs 12V) The design of the coils determine the difference.

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<%-name%>
• posted on August 23, 2008, 5:11 pm

Yes, coils have resistance. If the relay is designed for 12 volts, as you say above, the coil is designed so that the resistance is sufficient to limit the current to what the wire is capable of carrying. So the wire doen't melt, doesn't even get more than slightly hot. So the relay coil is not a short circuit.
Does your transformer put out AC current? I haven't used AC to close a relay, but it might cause the relay armature to vibrate. Is that what's happening? Is that what's bothering you? If the armature vibrates, it's probably not connecting to the load well. In the diagram the load is the lightbulb. I don't think anyone ever uses AC current to power a relay coil, because of this.
Are you using one of those black boxes, adapters, that plug into the wall? If so it will have a label, and the label will say if the output is 12 volts DC or 12 volts AC.
Or are you using only a transformer, with the windings visible and a metal frame around the middle? If so, that puts out AC, unless you have added a diode in the circuit where the diagram shows only a battery.
Do you have a multi-meter, a volt-ohmhmeter, They make things much much easier and an 18 dollar meter from Radio Shack or even Home Depot, or a 5 dollar one from Harbor Freight is good to have. (The HF one won't have an audible continuity tester.)
You probably need only one diode and Radio Shack sells two for a dollar, but a diode will lower the voltage from say, if it's 12 volts AC to about 6 volts DC, so you may want to use a transformer with a higher voltage. Or you can buy 4 diodes, or a bridge rectifier (which is 4 diodes in one package), and if you connect that right, well I forget what voltage you'll get then, maybe 12, maybe 9 or 10.
A 12 volt relay might work fine with only 9 volts on the coil. It's not a lot of money so just try.
They sell bridge rectifiers at radio shack also, for 2 or 3 dollars I think. They come in more than one size, but for just a small relay coil, the smallest is probably big enough, but if it's only a dollar more, buy the next one up. You may want to take apart this thing some day and use the relay somewhere else.
I think I've left things out so feel free to post back.

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<%-name%>
• posted on August 23, 2008, 6:02 pm
wrote:

A transformer is an AC device. The things don't work on DC. You need the proper circuitry (usually 1, 2, or 4 diodes and a capacitor) to convert AC to DC).
Also, there are solid state relays, which are easier to use (but are more expensive, and it still matters whether you have AC or DC).

There are different relays for AC and DC. 12VAC relays and 12VDC relays aren't the same thing.

They do. I've seen plenty of relays used on AC. They need to be made for it.

Most (but not all) seem to be DC. They contain a transformer, and simple (non-regulated) AC-to-DC conversion circuitry.

If you're going to draw more than a little current it's probably going to be 4 diodes (full-wave bridge) or 2 diodes (with a center-tapped transformer) and a filter capacitor.

A diode will lower the voltage by an amount depending on the material it's made of. (Silicon) power diodes usually drop about .7V. The bridge rectifier you mentioned will always have 2 diodes in series, so the voltage drop will be 1.4V. The output of this power supply (one that's not regulated, and there's no need for that with a relay) should be about 4V above stated voltage (with no load) but will drop when loaded.

I have a large connection of DC wall warts, and find the 9V ones work OK for 12VDC relays. Note that the relay needs the full voltage only to close (it'll stay closed on less) and 9V supplies put out over 12V initially (no load).

At least they used to. 25 years ago, I lived near a Radio Shack store, and frequently bought stuff like that.

According to Radio Shack's website, they have several bridge rectifiers for \$1.59 to \$3.29. I searched for "rectifier"
Any of these should be suitable for use with 12VDC relays.

--
Mark Lloyd
http://notstupid.laughingsquid.com

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<%-name%>
• posted on August 23, 2008, 7:35 pm

It's not vibrating. Ir has a marking of a solid line under which is a dotted line. 300mA.
The coil does warm up somewhat. Feels warm.
I have another similar setting which is rated for 24V. The coil gets hot when I connect it to a 24V transformer with 1.5A. Should I put a resistor to limit the amperes?

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<%-name%>
• posted on August 23, 2008, 5:48 pm
John Smith wrote:

Hi, 12V power source? Is it DC or AC? Relay with 12V input? Is it DC relay or AC relay? What is the rating? 12V at what current rating? Say if the current draw is 0.5 amp. then it has a resistance of 24 Ohms or so. Coil is not just a short piece of wire. Coil means many turns of winding on metal core(amature, Pole, etc.) which will have resistance. DC resistance is measured by straight Ohms. AC is little different because it involves reactance by sum of inductance and capacitance. It's called impedance, still measured by Ohms. BE CAREFUL, electricity can kill you!!!!!

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<%-name%>
• posted on August 23, 2008, 7:05 pm

If you are worried about a relay always drawing power in its on mode then use a latching relay instead. Given a short pulse of power the coil energizes, toggles the switch state which stays "latched" until the next pulse which toggles it back again.
The coil has an impedance (or resistance) so its not a direct short but does have a constant current draw to keep the switch engaged.

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<%-name%>
• posted on August 23, 2008, 7:34 pm
John Smith wrote:

Right.
Sure.

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<%-name%>
• posted on August 23, 2008, 10:30 pm
John Smith wrote:

If you're using a relay designed for AC, you're probably ok. It has impedance (the AC analog of resistance), as well as some DC resistance.
You don't want to use a DC relay on AC (or vice versa).
--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form snipped-for-privacy@prodigy.net.

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<%-name%>
• posted on August 24, 2008, 2:11 am

Certainly not the first, but what IS the problem using an AC relay with DC current?

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<%-name%>
• posted on August 24, 2008, 2:31 am

It won't let any current flow.

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<%-name%>
• posted on August 24, 2008, 3:17 pm

An ac relay used on DC might draw excessive current, plus the ratings aren't equivalent. 12V DC is not equivalent to 12 V ac. So you have to use math to get equivalent voltages to use. 12V ac = 12 x 1.414 peak-peak, then get mean voltage, .606 or something like that, etc.. Don't trust my numbers; been a long, long time!

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<%-name%>
• posted on August 24, 2008, 3:25 pm
TWayne wrote: ...

...
That would be 0.707 for rms... (sqrt(2)/2)
--

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<%-name%>
• posted on August 24, 2008, 4:09 pm

Agreed; but for peak to peak to effective DC isn't it "mean"? For DC you lose more of the peaks (0.707 each peak, + and -) to get to the effective value? rms isn't actually the same as DC value; it's effective value, right? Maybe I'm wrong, not sure, but ... <g>. Like I said, been a long time. 0.707 doesn't remove the intervening no-voltage points, right? Ouch! My brain aches! I don't think it was .606 though; probably more like 0.638? Ahhh, hell, now I AM confused!
AHA! Here's what I was thinking of; that was going to bother me for a long time so I had to look<G>:
http://www.tpub.com/content/doe/h1011v3/css/h1011v3_25.htm Excerpt: <quote> There are six basic equations that are used to convert a value of AC voltage or current to another value, as listed below.
*Average value = peak value x 0.637* (7-1)
Effective value (RMS) = peak value x 0.707 (7-2)
Peak value = average value x *1.57* (7-3)
Effective value (RMS) = average value x 1.11 (7-4)
Peak value = effective value (RMS) x 1.414 (7-5)
Average value = effective (RMS) x *0.9* (7-6)
The values of current (I) and voltage (E) that are normally encountered are assumed to be RMS values; therefore, no subscript is used. </quote>
You'll have to read the page to see the definitions of the terms; they always used to drive me nuts, too.
Thank heavens for the 'net!
Cheers,
Twayne

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<%-name%>
• posted on August 24, 2008, 5:02 pm

You're right. I was thinking of DC through a transformer. I'm not sure I'd trust ever using a DC relay w/ AC. I'd rectify and filter it first, either with a half wave circuit (simple diode) or a bridge rectifier plus a cap.

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<%-name%>
• posted on August 25, 2008, 4:32 pm

Very often a RS power brick is cheap and can be found that produces what the specs of the relays want. Inadvisable to add rectifiers on one's own: against code and insurance regs.

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<%-name%>
• posted on August 25, 2008, 5:03 pm

How would it be any problem at low voltage.

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<%-name%>
• posted on August 26, 2008, 2:38 pm

Because what you are calling "low voltage" doesn't have anything to do with the IR abilities to start a fire and/or get very hot. There are still safety issues related to fire.

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<%-name%>
• posted on August 26, 2008, 2:55 pm

If you're only dealing with a few hundred milliamps, it can't generate more than a few watts of heat and beside, it would be inside an electrical box.

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<%-name%>
• posted on August 26, 2008, 4:08 pm

OK; go for it then. But fault current on the secondary of a transformer is much more than a few hundred milliamps as is the resultant primary side draw. Safety issues don't bother you, so ... don't look up UL or CSA requirements or anything; just do it.