Automotive Power Systems

Brent Brown says:

I use a regular 7805 in a 12 or 24V automotive environment. My design has a 1N4007 series diode, 10ohm 1W series resistor, 33V 1.3W zener to ground, and 1000uF 35V cap to ground on the input side of the regulator. Adding the resistor here is the key to getting the most from a simple design:-

1. The resistor limits the peak currents into your zener (during spikes) helping it do its job and ensuring that it doesn't snuff it on the first spike.
2. The resistor and capacitor make a low pass filter that helps reduce noise into (and out of) your circuit.
3. Zeners are a little slow perhaps? - then the low pass filter helps here too.

I suggest using a 16V zener for a 12V only application. I also noted that most 7805's have a 35V absolute maximum input voltage rating, but better than some other regs that have been suggested.

My circuit draws about 40-50mA which makes the regulator get warm on a 24V supply so I have a small heatsink on it.

The 1N4007 diode in series with the input prevents damage from reverse supply connection. It also stops the 1000uF cap from discharging if the supply voltage falls (eg. engine cranking), so if your circuit doesn't draw too much you have a short term UPS!

See also:

• Hawker Energy 100% deep cycle and cranking power

Questions:

• Stephen Carr of Scarr asks:

  Hi I have a project that is going on a car and I need to filter the supply can someone give me an idiots guide to a general purpose automotive DC filter (12-24v) 1.5amp max. from what I can see I need a choke and two caps (one for high and one for low freq) but what values? and configuration? E.g. 0.2uf cap on the input, into a 10uh choke, and a 200uf electrolytic cap on the other side. Thanks P.S. I'm a newbie so please take it easy on me :O)

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