View Full Version : super easy question

02-10-2007, 03:01 AM
I am doing a project for my car and want to know if
the 7805 can handle the 13v from the car ?


02-10-2007, 03:05 AM
Yes it can, but car eletrical systems are very noisy. And the regulator will not remove all of the noise.
You will probably need some additional filtering to get a stable +5V.


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02-10-2007, 03:13 AM
how about if I threw a 7808 (8v) ·in front of the 7805 ?

sounds silly .. but ... I can get em for like a buck and you would

think that the 7808 would lower and clean up just enough.



Mike Green
02-10-2007, 03:36 AM
Truthfully, I think you'd do better with an inductance/capacitance filter ahead of whatever regulator you use. The main problem is electrical noise (transients, spikes) and a filter works much better than a regulator for that. There might be other reasons for adding an 8V regulator, like spreading out the power dissipation if you're drawing a significant amount of regulated current.

Chris Savage
02-10-2007, 03:37 AM

Depending on the age of the car and many other factors noise may or may not be an issue. Daisy-chaining regulators is not the answer though. Noise is handled through capacitors and/or chokes (inductors). But you may simply have to try it and see what happens. Just be aware if you have strange problems that they may be noise related. On the other hand if you have a scope you could verify noise and/or ripple on the power supply. I hope this helps. Take care.

Chris Savage
Parallax Tech Support

02-10-2007, 05:30 PM
I have a 34 ply streetrod.

I put a A500 computer controlled overdrive tranny in it with lockup converter

The car doesnt have a computer to tell the tranny when to
go into overdrive and lockup the converter.

Just have to know how fast I am going and if the brake pedal is
pushed and maybe a couple of other "conditions"

I do have a scope .. but .. my filter knowledge sucks .. so
I have to do some digging ..

but thanks for the reply guys

02-10-2007, 06:57 PM

is a link to automotive filters - I have used similar - if you are running electronic ignition and NOT standard points the noise will be less of an issue. I had a system in a 1940 ford flathead - was running points - even with condensing caps on the 'dizzy' - it was noisy - use filter before reg. - should be fine - alternator/dynamo another big noise source.

02-11-2007, 04:47 AM
here ya go

Beau Schwabe
02-11-2007, 05:03 AM
I have to agree with Mike and others, a "inductance/capacitance filter ahead of whatever regulator you use"·is going to be your best bet.

Here is a design similar to what 'japer' posted but it uses a diode/capacitor filter on the front as well as the·back end.

Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.