Recently, Somebody Somewhere wrote these words
>
> This thing always sits in the truck logging cab compartment, so it
> is not exposed to anything too terribly awful. I have decided to
> definitely add a choke and capacitor to the front end. I am
> curious about your suggestion for transient protection. Are you
> thinking of one of the little transient suppression diodes? If so,
> what value would be appropriate? I think vehicle DC power is 13.8
> volts nominal when the alternator is fired up.
You need to be aware of a few phenomena
1. Spikes as mentioned in my previous post up to 1000V when the
alternator brushes get an open spot between sections on the commutator.
2. Load dumps. These are slow risingand decaying high voltage
pulses (I got one when an automatic slipped out of gear that blew the
radio). The figure I have (Worst case) is 100V/0.5 seconds long. That is
a paranoid figure. I have seen 200V zeners blown on a 24V truck,
however and you have to ask why?
3. Capacitative effects of these on the best intentioned of
regulators. Notably a sudden rise in VC capacitatively coupled to the
base _internally_ always effects the output.
4. Another paranoid figure of 70V on a dry 12V battery.
Given this it is best to allow things to try to tolerate as much voltage
as possible, and then protect above that. Trucks here are all 24V, btw,
and the cars are 12V. I feel sure they must use 24V. Figure the amps
starting a truck with a 12V battery!
I would look at 13.8V -->8V then 8V --> 5V; then 5V -->12V. Plenty of 8V
regulators have load dump specs.
--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
INET: [EMAIL PROTECTED]
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