Automotive Circuit Protection

[Jeff Webb]

Nic van der Walt wrote:
> 5KP is plenty.

Great.

> You need a reverse polarity diode after the fuse, before the TVS. Even
> a 1N4007 would suffice, although I prefer SMS2100's.

I can see why the reverse polarity diode is a good idea, but I was
hesitant to add another diode drop in the power supply.  Under cranking
conditions, the battery voltage might drop pretty low.  Since the 2940
is an LDO regulator, things will probably be fine for a 5V supply.  I
need an 8V supply for one application, and that might prove to be a
problem.  I think you're right, though, so I'll put the diode in there. 
I can't find any information (or sources for) the SMS2100.  What's
better about it?  Any links or equivalent part numbers?
 
> The cap before the regulator should be at least 100uF 63V Electrolytic.
> It's as much part of the protection as the TVS.

I just found some application notes from a TVS manufacturer.  They have
one specifically about automotive applications.

http://www.gensemi.com/product/AppNotes/appnotes.htm

They don't show a cap, but do have a "GP30" reverse diode.  I can
probably make room for a 100uF 63V electrolytic if you think it's a good
idea, but the high temp versions of these caps are pretty huge.  If they
keep my boards from blowing up, it will be worth it though ;)

Here's the GP30 diode I mentioned:

http://www.gensemi.com/Pdfs/gp30a.pdf
 
> Don't forget to calculate the dissipation in the regulator. 9V drop at 1A
> gives you 9W to dissipate. Things heat up rapidily. 

Yes.  According to the datasheet, the regulator will have to be well
heat-sinked.  I don't think the total current will come to more than a
few hundred mA, so that should cut down the wattage a bit and make
things feasable.  Still, at high temperatures, there could be a problem
if the heat sink isn't very good.  The fact that I've seen the 2940 used
in several ECUs gives me a *little* confidence.  Of course, that doesn't
make it right.

> I prefer switch mode
> power supplies, look at something like the LM2596.

I'd like to try a switching supply, but that will probably have to wait
for the next revision.  I've got too many unknowns as it is now and need
to get this thing cranked out.  One thing I worry about with a switcher
is the effect on analog circuits.  Some people I know have had problems
with this and went to a linear regulator for the analog stuff and a
switcher for the rest.  I don't know the details.  What are your
experiences?

Thanks for the input, Nic.

Jeff Webb
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