24v relay

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Old 12-15-11, 01:46 PM
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24v relay

I've got a question for the experts - I have a relay with a 24 vdc rating for the operating coil. Can I run it with 24 vac instead?

Thanks, Steve
 
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Old 12-15-11, 02:10 PM
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Short answer is no. You could use a rectifier diode but the voltage would then be incorrect and also a pulsing DC voltage. Best is to just buy an AC relay.
 
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Old 12-16-11, 12:55 PM
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A "full-wave bridge rectifier" should work ; $2 Radio Shack item.
 
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Old 12-16-11, 01:26 PM
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Wouldn't you also need to throw a pair of capacitors in there too with the bridge rectifier to even out the DC, otherwise the relay would be trying to turn on and off 120 times per second... and would probably burn itself out quickly.
 
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Old 12-17-11, 08:10 AM
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Caps wouldn't hurt, but DC ripple isn't as big an issue with a simple coil as it is with electronics. The coil isn't fast enough to respond to the rise & fall of the ripple. OTOH, DC power supplies are cheap. Even the smallest will do that job, since the coil draws only a few tens of milliamps.
 
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Old 12-17-11, 08:20 AM
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But to repeat Furd buying an AC relay would be the simpler solution and probably cost not much more unless this is some special high amperage relay. After all 24 volt coil AC relays are common as dirt.
 
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Old 12-17-11, 06:59 PM
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It is NOT as simple as just using a full wave rectifier, either with or without filtering capacitors. DC coils are wound differently than are AC coils and they have differing voltage and current requirements. Yes, if it is only being energized intermittently you might be able to get away with a rectifier but if the relay coil is energized for a prolonged time it WILL burn out.
 
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Old 12-18-11, 01:37 PM
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If the "input" voltage to a full-wave bridge rectifier is 24 V AC , what is the voltage at the Positive & Negitive "output" terminals?.
 
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Old 12-18-11, 08:22 PM
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Interesting comment Furd. I think you're right.

24v AC is 24v RMS = 34v peak
24v DC is 24v peak

So that 24v AC would probably overheat or burn out the relay.

(This is all based on textbook knowledge... haven't actually tried it)
 
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Old 12-19-11, 12:36 AM
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Ignoring ripple the output of a single phase full-wave rectifier is about 63% of peak applied voltage. With a 24 volt nominal RMS AC input this is about 21-1/2 volts pulsing DC output. Maybe this is within the voltage parameters of the particular relay, maybe not. The other factor is magnetic saturation of the core and the resultant heat build up that ends up cooking the coil, reducing the resistance, allowing a larger current flow, etc.
 
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Old 12-19-11, 05:42 AM
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Theory is one thing, but in the real world here's what I've seen that seems to be consistent across brands of AC transformers and DC wall-wart (unregulated) power supplies:
-- A 300 to 1000 milliwatt-rated AC 120v -> 24v transformer with no load will read between 28 and 32 volts. With a load (including the 40 to 80 ma of a typical coil) the voltage drops, but it will rarely drop to 24 volts. Usually it will hover around 26 volts. Most devices are tolerant of +/-10%.
-- A 300 to 750 ma unregulated DC wall wart has a similar overvoltage with no load, but rarely exceeds 28vdc. With a load it drops to within tolerance. With a full load it can drop to under 24vdc, sometimes to as low as 18. When its output is viewed on an o-scope there is significant ripple.

The company I work for has installed thousands of AC and DC relays over the years with such a low failure rate that it's not worth mentioning. Some have been in constant operation for more than 20 years. I know of at least two DC relays that had their power supplies replaced with AC wall-warts by a customer. They continued to operate for weeks. (How did we discover this? The customer called to complain that something was buzzing ...)

In any case, the original answer to the OP's question (buy an AC relay) is correct.
 
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