Car Power Supply question

John Couture

When building a PCB to be fitted into a car with 12VDC:

1) Let's say I tap the cigarette lighter jack and put a fuse on the line
2) I use a 7805 IC to drop the power down with·a 100uf·25V cap on·either side to mellow out the voltage
3) I use a 1N4004 from out to in (anode connected·to in)

Does this provide reliable·power for a SX.· In other words, car power systems are typically pretty noisy and prone to ups and downs.· Will the 7805 and the two caps mellow out the power to be reliable for a sensitive SX circuit?

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John J. Couture

San Diego Miramar College

Mike Green

The 1N4004 connected with anode to IN and cathode to OUT will bypass the regulator and burn out your equipment.

1) I would suggest using one of the more modern regulators designed for automobile use like the LM2940. It doesn't need the diode and is protected against reverse polarity input and input voltages to 26V (with transients to about twice that). It does REQUIRE an output capacitor, preferably a 22uF (or higher) 6V Tantalum capacitor optionally with an electrolytic capacitor of higher value in parallel with it. You should have a 0.47uF input capacitor with a rating of about 30-50V

2) Cars are very electrically noisy environments. The regulator will help keep the supply voltage constant, but noise will get through. You can add a choke on the input as well as additional input capacitance (both small like 0.1uF and large values like 100uF with a 30-50V rating). Keep in mind that noise can travel in on other wires like sensors and even on output wires like servo control signals. Shielding helps. Avoid ground loops.

CCraig

When I build mine, I dropped it down to 8 volts first, then down to 5 volts. I was trying to off load the drop so the LM2940 didn't have to do all of the work. I would have used 9 volts, but all my 9 volt regs. were low current.

HTH, Chris

JonnyMac

Car electrical systems can be very noisy -- I'd be inclined to use more filtering, upstream and downstream of the regulator, and with 0.1 uF bypass caps near any IC in your circuit.

John Couture

Thanks guys!

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John J. Couture

San Diego Miramar College

John Couture

Hey a followup question:

The Prof Dev Board uses 10uf caps on either side of the voltage regulator.

Jon's NV Apr 07 article uses (point) .47 on the input and 47 to grn and .01 to grn on the output.

Jon's NV May 07 article uses 47 on the input and 47 to grn and .01 to grn on the output.

The SX28 Proto Board uses 47 on both sides

The SX48 Proto Board uses 470 on both sides

Am I justified in being confused? (grin)

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John J. Couture

San Diego Miramar College

JonnyMac

That's probably why the book is called, "The Art of Electronics".... Even electronics design, at times, can be subjective.

John Couture

(grin) Ok.

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John J. Couture

San Diego Miramar College

T'Saavik

another thing to remember about cars:

1. engine off: you are likely to see ~12v
2. engine on: you are likely to see ~14v
3. engine start: you are likely to see dips down to 8v or more!

So, your poor device goes from 12v to 8v to 14v. Not pretty.
A turn-on delay to get around the engine start may be preferable depending on your situation.