# Keeping 12v on automotive circuit

#### victorduchateau

Joined Jun 26, 2019
8
I feel kind of dumb asking this question but I would like some input.

I'm designing a circuit for automotive purposes where I'll be using signals from the BCM.
Because automotive electronics are very noisy and voltages can differ, I'd like to know what protection I could use best.

I'd like to keep a steady 12v in the circuit, even when the vehicle is on and voltage increases to 14v.

Thanks

#### Delta Prime

Joined Nov 15, 2019
1,150
I feel kind of dumb asking this question but I would like some input.
Hello there I know exactly how you feel... What's a BCM?

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,090
I know exactly how you feel... What's a BCM?
Body Control Module

It controls the lights, for example.

There is a good Littlefuse paper about the automotive electrical system. There is another artical, but I'd start here: https://m.littelfuse.com/~/media/el...utomotive_tvs_diodes_application_note.pdf.pdf

You have to worry about large positive and negative transients from the alernator. A reverse polarity jump. So you have -50 to =200V transients to deal with for starters.

Analog devices makes a "surge stopper" device which could be useful.

What you did forget, is when the car is being started, the system night be 8V or so. Your basically dealing with 8-18 V with some pretty nasty spikes including polarity reversal.

Your basically dealing with 3.3 or 5V logic systems.

Food for thought.

#### shortbus

Joined Sep 30, 2009
8,817
Don't know if it still holds true, but in earlier days of ECM's BCM's they sank rather than sourced voltages and currents. This was due to using Nmosfets and the need to control by the easiest method, with no bootstrapping or other components.

#### LowQCab

Joined Nov 6, 2012
1,110
Here are several Circuits that will protect against anything .......
If you don't really need "tight" Voltage Regulation or adjustable Current Limiting,
the use the single FET "Voltage Pre-Regulator", it's super simple.
Just remember that if you are dropping very many Volts, at a given Amperage,
multiply the dropped Volts times the maximum Current, this will give you the number
of Watts of Heat that will have to be removed from The FETs with a Heat Sink.
Do not try to use these circuits without any Heat Sink, at the very least mount them to
an aluminum box with insulating hardware.