# Thread: Relay supply

1. ## Relay supply

Just need someone to check my homework on this supply. The 21vDC is tapped from a 6800uf cap.

The relay is:
http://www.mouser.com/Search/m_Produ...DA6e0NZw%3D%3D

Datasheet states about 20ma of coil current.

Do the resistor, zener, cap all fit value-wise? I found with my basic calculations that they will work but would like another set of eyes on it.

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2. Try it a different way.

The relay is a 12v coil? And it wants 20ma current? use Ohm's Law. 21vDC supply, less 12v leaves 9v to drop across a resistor. 20ma dropping 9v across a resistor? R = V/I = 9/0.02 = 450 ohms. 470 is the nearest standard value. SO just put a 470 ohm resistor in series with your coil and run it on 21v. None of the other stuff needed.

However you power it, put a reverse diode across the coil.

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3. Thanks Enzo. Hey that's great I can save on space, pcb, and extra parts. I guess relays are fairly forgiving with coil voltage tolerance. Awesome.

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4. Shall I pick a diode that can withstand about twice 2x 12v forward and same for reverse breakdown?

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5. A plain old 1N4148 is usually what they use, but if you don't have one, a 1N4007 is fine too.

As to 12v, I doubt you can find many diodes with less that 24v ratings either way.

Look at the relay circuits in a 5150 for example.

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6. As a side note:
I guess relays are fairly forgiving with coil voltage tolerance.
Not really, but relay coil will never see 21V at all.
With switch open: zero current so voltage drop across coil ends.
With switch closed: resistor drops 9V, relay drops remaining 12V ; resistor is always in series.
Diode across coil also sees 12V reverse voltage and passes 20mA for a millisecond after switch opens.

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7. So I'm considering the 24v relay

http://www.mouser.com/Search/m_Produ...OC8r3WaA%3d%3d

From the datasheet it says the relay will work fine and still have some room for voltage tolerance if I just run it off 21vDC. Reason being one less component...the current limiting voltage divider resistor.

The datasheet says 70% or more of specified voltage and not more than 10% OVER. 70% of 21vDC is about 16v. I highly doubt any mains voltage will drop that far.

Any errors in my thinking here? I like the simplicity. A relay and a flyback diode. Nothing else.

Is there any reason to have the relay decoupled from the main 21v rails? (They are the supply for a LM3886)

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8. Oh, it will work fine with 21V.

But are those 21V always availble? If that´s a raw supply rail for the LM3886, it might easily drop up to 20% under full power, 10% for certain.

If that´s still enough, go ahead.

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9. Without some isolation, there's also a higher chance for relay switching noise appearing at the output.

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10. Thanks guys. Both great reasons to go 470ohm (or a bit less for supply sag) and 12v relay.

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