I have a refrigerator on my boat, and it takes a fair bit of current from the 12V supply. I would like to make a circuit such that it was only switched on when the battery was charging, say over 13.2V.

I think it would be best to use a LM311 comparator (datasheet here), but I'm not sure about the ancillary components.

One issue to consider is that when the relay fires, the load of the fridge will cause a dip in the supply rail. If there is not some form of damping, this will cause the circuit to switch off, then the voltage will rise, and the circuit will switch on ...

Any suggestions?

 

Run your sensing circuit from the 12 volts supply rail via a diode (1N4001) .

The sensing circuit will run at the accu voltage minus 0.6 volts.

If you sensing circuit has a big capacitor at it's input supply rail, the voltage dip is blocked by the diode and the sensing circuit takes the supply from the capacitior during the dip and is unaffected.

Try 4700 uF 35 Volts to begin with and see how it goes.

Regards, Raymond

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This will do the job.

What turn-off voltage do you require?

I've assumed about 12V.

Attached Images

Battery Charge Comparator.GIF (3.4 KB, 20 views)

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Many thanks for that.

I hadn't considered that the turn-on and turn-off voltages could be different, and I'm not sure I want them to be. However, I guess it gets round the problem of oscillation.

I will do a test and see what the voltage drops to when the fridge turns on, and for how long...