Automotive Circuit Protection

[Nic van der Walt]

This is probably my last posting on this matter....

"Peter D. Hipson" wrote:
> If you are worried about voltage drop on the diode then wire the diode
> across the input *after* teh fuse (or better, circuit breaker). THen a
> cross connect will blow the fuse instead.

That completely misses the point of having the diode. One of our main
competitors
made a habit of trusting things like "reverse polarity protected" etc...
They aren't competitors anymore. Lack of a 20c diode had them recall thousands
of units at $50 to $100 per unit. Fatal. Engineers that gamble have short
careers.

> I'd like to be able to put this under the hood.  I've been specing parts
> good from -40 to +125C.

I have been avoiding underhood like the plague. Problem is that you can't get
electronics to survive more than 125C. Underhood can go above this easily. All 
the units I have seen relied on some form of cooling to ensure operation in 
the 60C range. In my experience any electronics running above 80C is dicey 
in the long run. If you can't touch it it's to hot...

Underhood does not allways equate really high temps. I have a couple of
sensors 
in setup in my A3. Some spots in the engine compartment never goes above 60
- 70C.
Problem is to be sure that its allways true... Gambling again.
 
> Okay, I'm looking into it.  The main thing is I'll need to find caps and
> inductors good to 125C.  If you have any suggestions, let me know.  The
> Nichicon PJ's look like they are good to +105C.  They have a BT series
> that's good to +125C, but I haven't checked the ESR, etc. to see if they
> will work with the switching supply.

BT's are OK. Just use 50% more capacitance. ( Never used them though.
Double check. )

Once again, I don't trust caps at such high temps. It's easy to test for a
while,
but how will they hold up to the temperature cycling in the long run? Look
at the 
rating called "Load life". 5000 hours at 105C. Is 5000 hours enough?

> Thanks, I might take you up on that.

Anytime.

N.
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