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12v Relay with selectable 12 or 24v DC supply
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ajb:
That would be 130% *of* the rated voltage, IE 12V*1.3 = 15.6V, not 130% *above* rated voltage.  But even so that's the maximum rating, and in general you want to leave yourself as much margin as you reasonably can on the design to reduce typical stress on components and leave headroom for things like variations in supply voltage, plus margin for transients or other outside interference your device may experience. 
Ditch:
Brilliant ajb, Thanks for your help :-+

You mention above EMC implications using PWM. I'd imagine switching the relay coil using PWM like you suggest could possibly be problematic for emissions testing?

What would be the best approach for reducing this?
Zero999:
PWM can be implemented with discrete components too, which has the advantage of it varying with the voltage, without having to worry about the software.

How about running the MCU of a buck regulator and using a 5V relay? That would also avoid the need for a separate regulator, or SEPIC.
SmallCog:
I'd just go with Solid state relays rather than traditional ones

I've got some some nice DIN rail ones here that trigger off 1.5-24VDC
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