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using a relay to isolate automotive spikes

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Build-A-Burger

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Since the most destructive spikes and surges in the 12vdc circuit in vehicles are when the alternator switches on and off, if I know when the alternator is going to switch on or off, can I use a relay to isolate a sensor I want to use to measure something like battery voltage? Or can the bad stuff come through the ground?
 

If your idea is to turn on/off the relay at the speed of the alternator, I would think it is impossible for a mechanical relay to do this. It will also introduce more noise on the power supply lines due to the relay coil and it's inductive nature.

There are several overvoltage protection components to use for the analog inputs. For a simple voltage monitoring you can also add low pass filters to the input signal, to further protect the input from high frequency spikes.
 

When I say "alternator switches on and off" wouldn't it do that only when the battery gets too low and too high (resp.)? Just assume for the sake of the question that I know before hand when the alternator is going to start or stop charging the battery.
 

The alternator is always on, except when you turn the ignition key off.
 

My bad, I was thinking of the ignition system and the distributor, due to the question about noise.

Regarding measuring the battery voltage while the motor is running, you will mostly see the alternator charging output, and not the actual condition of the battery itself.
 

I was just toying with the idea of using my micro to act as the regulator, which on this model ('77 ford) is external. All the regulator does is turn the alternator off when the battery gets over something like 14.5 volts. And then back on again when it gets below a certain point.

- - - Updated - - -

Actually, the regulator pulses the field circuit according to the battery voltage. The pulses are longer when the battery voltage is lower.
 

Here is an easy way to filter out noise on the power supply. It will create a smooth supply for your measuring device. Install a resistor and filter capacitor.

The effect is to isolate your 'quiet' circuit from the electrical noise.

 
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