On 31 Dec 2006 03:36:23 GMT, Emory Lehman
wrote:

I'm planning to work on a new project for use in my work. On the console
of a locomotive there is a 74VDC plug and would like to step it down to
12VDC for use of other electronic devices that would plug into a car
lighter plug. I though of using a 7812k but I see the max input voltage is
only 30VDC. I then though of using a step down transformer, but I really
don't want it to be to big and bulky since I have to carry it in my work
bag with me all the time.

What does anyone recommend? The current draw should not be too much like 3
amps at the most. This would be used to power cell chargers, PSP chargers,
etc.

Emory Lehman

Want the truth, or can I lie to you? bg

Say 74 VDC, 3 amps, 12VDC output. That's a 61 volt difference, at 3
amps, or about 183 watts of power (HEAT!) to get rid of. Major work,
fans, all kinds of stuff like that. Fans, big heat sinks, the works!
Not small enough to slip into the pocket.

You would be much better off with a switching regulator (more complex)
with this kind of difference in voltage levels. Would be much smaller,
more efficient, etc.

It is perhaps possible that a computer powersupply could be reworked
to make it work at that voltage... You would proably have to rework
the input circuits (so it takes DC, and at a lower voltage) to do that
however.