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Old 24th April 2013, 09:32 PM   #21
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Sure, but it will need to dissipate more than L versions handle and is just a more complex and expensive resistor needing additional parts in this role.
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Last edited by Ian Finch; 24th April 2013 at 09:37 PM.
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Old 25th April 2013, 01:41 PM   #22
sregor is offline sregor  United States
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One comment: Fairly standard practice is to run the protection circuit directly from the transformer to its own rectifiers and diodes with a smaller filter cap and appropriate dropping resistor. This allows it to drop out as soon as power is removed, rather than waiting for supply to drop down. Would only add two diodes and capacitor (typical is 50 to 100uf) to supply power to the circuit, but add resistance based on coil resistance ad voltage. My 2 cents.
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Old 25th April 2013, 02:20 PM   #23
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This is an easy solution to an easy circuit. I have run 5V relays from 12V sources plenty of times. I also like to slightly under-volt my relays to keep them living longer.

First thing to do is measure the DC resistance of the relay coil, to help you determine your resistor value needed! Since in your case you need half the supply voltage, use a resistor that closely matches your relay coil, to functionally divide the voltage. Slightly higher resistor is good if you want to run the relay cooler.

A method I like to use is a series resistor with a 220uf capacitor in parallel with it. This allows the relay to get a stronger magnetizing current at startup, then once the capacitor charges, the voltage drops across the resistor to run the relay at the desired lower value.
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Old 26th April 2013, 05:40 PM   #24
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Thank you all for your answers.

I have another question.

The solution with the series resistor works well but imagining that my power supply is 30v of example and not of 24v, will it be necessary to adjust other parts of the circuit or only the value of this resistance?

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Old 27th April 2013, 09:19 AM   #25
AndrewT is offline AndrewT  Scotland
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just the resistance.
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Old 27th April 2013, 09:37 AM   #26
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AndrewT, many thank's!
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Old 28th April 2013, 04:39 AM   #27
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Make sure the voltage of the "startup" capacitor is higher than your power supply voltage.

Use a 35V or 50V part in this case.
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Old 28th April 2013, 12:37 PM   #28
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EWorkshop1708, it is worth mentioning that.

Thank you!
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Old 28th April 2013, 01:17 PM   #29
JMFahey is offline JMFahey  Argentina
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Quote:
I also like to slightly under-volt my relays to keep them living longer.
Well, it's quite the opposite.
What kills relays is not "worn coils" or something (never saw that ) but pitted contacts.
Applying *less* pressure than designed worsens contact and helps faster pitting.
Just sayin'.
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Old 30th April 2013, 09:14 AM   #30
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Hello everyone!

I drew a circuit protection (one with regulator and one with resistance adjustment (R7)).
Here the circuit with the resistance and if some are interested I'll post the necessary files.

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