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?
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.
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
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.
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
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
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.
It's not vibrating. Ir has a marking of a solid line under which is a dotted
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
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!!!!!
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.
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!
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>:
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.
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!
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.
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.
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