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Old 24th April 2013, 02:12 PM   #1
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Default relay coil that heats.

Hi everyone!

I just finished a DC protection for speaker and everything works as expected.
But I have a question for the relay coil heater.
My question is simple: It is normal that the heater coil?

Thank you!
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Old 24th April 2013, 02:41 PM   #2
jaycee is offline jaycee  United Kingdom
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No, the coil should not get hot. If it is, then there is too much current flowing through it. Are you using a higher voltage than the coil is rated for ?
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Old 24th April 2013, 03:34 PM   #3
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Here is the diagram, the original and the change I made was with the 12v relay.

Regards!
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Old 24th April 2013, 03:46 PM   #4
jaycee is offline jaycee  United Kingdom
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The zener will not do what you think. The diode is there to catch the EMF kickback the relay coil produces when it is de-energised. You have 24V across a 12V relay, which is why the coil is overheating.

The easiest solution here is to add a resistor in series with the relay. Measure the resistance of your relay's coil, then add a resistor that will drop the required power using the Voltage Divider formula. For example, if your relay's coil is 400 ohms, you would do R = ((24 * 400)/12) -400.
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Old 24th April 2013, 03:50 PM   #5
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jaycee thank you, I will modify and test!

Regards!
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Old 24th April 2013, 04:20 PM   #6
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by jaycee View Post
The easiest solution here is to add a resistor in series with the relay.
Even easier, since he's already got the 12V Zener - just put the Zener in series with the relay coil. That will drop the 24V down to 12V.
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Old 24th April 2013, 04:22 PM   #7
AndrewT is offline AndrewT  Scotland
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Godfrey,
that makes the relay pull in very susceptible to variation in mains supply voltage.
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regards Andrew T.
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Old 24th April 2013, 06:50 PM   #8
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Quote:
Originally Posted by AndrewT View Post
Godfrey,
that makes the relay pull in very susceptible to variation in mains supply voltage.
In principle yes, but I don't think this will be a problem, as most relays pull in at approx. 85% and, once activated, remain in the on position down to even lower voltages.

I would worry more about the Zener diode passing approx. 100mA while dropping 12V, making for 1.2W dissipation. Not every small Zener diode will take that in free air.

Rundmaus
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Old 24th April 2013, 07:18 PM   #9
jaycee is offline jaycee  United Kingdom
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I did consider putting the zener in line but a resistor is far better for the job, and a 1W resistor should be adequate
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Old 24th April 2013, 07:25 PM   #10
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Quote:
Originally Posted by jaycee View Post
a 1W resistor should be adequate
Doubt that.

At least, the nominal relay coil current should be looked up and the actual dissipation in the series resistor should be calculated. If my rough estimate of 1.2W is right, I would at least use a 2W or even 3W resistor to avoid excessive surface temperatures.

Rundmaus
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