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How to use the same RELAY on 12V & 24V vehicles

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soldar:
Look at circuit #4 at http://www.4qdtec.com/csm.html

Psi:
It's been a while,
but from memory automotive electronics are supposed to handle double the battery voltage for 60 minutes without damage. I don't think it has to actually be functional at double the voltage though.
So for a relay you might monitor input voltage and have MCU choose not to energize the relay if the input voltage is way to high. You'd probably be monitoring input voltage anyway to choose which PWM duty to select.

soldar:
A quick and dirty solution that should work but the main transistor will dissipate some heat at 24 V. Emitter resistor should be calculated for the specific current desired.



David Hess:
I would either use a current mirror to multiply the current from a logic output or include a separate 12 volt regulator for the relay.  Both solutions have to dissipate 12 volts time the coil current however.

Pulse width modulation avoids having to dissipate power but is more complex.  This suggests using a simple switching drive circuit like a blocking oscillator which applies power to the relay and then modulates its switching to maintain a constant current.

james_s:
How about use an efficient switching regulator to step 12/24V down to 5V and use a 5V relay, then use a small linear regulator to drop 5V down to 3.3V for the micro?

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