Automotive Circuit Protection

[Nic van der Walt]

>to be hard to source.  Digikey has 5KP's available.  What amount of

5KP is plenty. 

>BATT --[ FUSE ]--+-------+-------[ LM2940 ]--+---------+--------+---- 5V
>OUT
>                 |       |           |       |         |        |
>                 |       |           |       |         |        |
>               ____/   -----         |     -----     -----    -----
>              / /\     ----- 0.47 uF |     -----     -----    -----
>         TVS   /  \      |   Ceramic |       | 0.1 uF  | 22 uF  | 220 uF
>               ----      |    50 V   |       | Ceramic | Tant.  |
>Electrolytic
>                 |       |           |       |         |        |
>                 |       |           |       |         |        |
>                GND     GND         GND     GND       GND      GND
>

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

The cap before the regulator should be at least 100uF 63V Electrolytic.
It's as much part of the protection as the TVS.

Don't forget to calculate the dissipation in the regulator. 9V drop at 1A
gives you 9W to dissipate. Things heat up rapidily. I prefer switch mode 
power supplies, look at something like the LM2596.

N.
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