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

--
Posted via a free Usenet account from http://www.teranews.com

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.

Scanning the available devices I see that the highest input voltage spec is
60 volts (LM 2575HVN-12, a 1 Amp device).
This makes me wonder what would happen if you wired several of these in
series across your 74volt input to provide 3 separate 1 amp supplies. They
would all have different ground potentials so you couldn't wire the outputs
in parallel to get 3 amps into one load but you may not care if you just
want to be able to charge a few small batteries at the same time. Just a
thought.



"Emory Lehman" wrote in message
7...
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

--
Posted via a free Usenet account from http://www.teranews.com

On 2006-12-31, Emory Lehman wrote:

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.

google for "DC-DC converter"




--

Bye.
Jasen