How can I easily step-down a DC voltage with out too much power dissipation?

Question

I need to step down +45v DC to +12v DC, with as little amps dissipated as possible. I was thinking about using a simple voltage divider circuit, but this dissipates too much heat (thus too much mA). I also looked into some VRegs from Linear and National, however the +12v does not need to be superbly regulated and using a VReg would waste power, and needs too many external parts.

+42vDC IN to +12vDC OUT, with little Amps lost, it needs to ouput around 2.5 Amps. Hope that helps anyones confusion.

Answer 1

You'd probably want a switching power supply like this "Simple Switcher" from National Semiconductor. (You can get them at DigiKey)
Using resistors for a voltage divider is a very inefficient way to go about the voltage reduction. If you are dropping 33 volts at 2.5 amps, you are losing 82.5 watts to heat. Switching regulators use an inductor instead of a resistor and will probably be better than 75% efficient (losing 20 watts or less).

Answer 2

I cannot think of any very efficient way. You can try using transistors. Every transistor requires aroud 0.7 volts to turn on, so you put about 45 of them in series and you get the voltage drop, but that is not very practicle either.

You could use a voltage divider with huge resistors. Eg 1 10meg and 1 30meg resistor in series gives close to 12VDV across the 10meg, with only 45/50meg amps. approx 1 micro amp

Answer 3

You didn't indicate how much output current you needed at 12 VDC output. I assumed you were looking for 12 VDC output with about 200 mA ouput. Once quick solution, Datel DC to DC convertors. A 2" x 1" module, part number TWR-5/1000-12/210-D48A, accepts up 36 to 72 V inputs, and provides 8 to 11 watts of power with a +/-12V output up to 210 mA (+/-5V also available to 1A). An input capaictor, 10uF, 100V, and output capacitor 22uF, 20V low ESR all needed for external parts.

Just saw your 2.5A rqmt. Suggest DC-DC convertor - look at second link for sources, select Power.

Answer 4

You keep using the word power. Once you used amps. Electricity is broken down into parts such as Amps(heat), Watts(resistance), voltage(current) etc. Combined they produce different strengths of power. if it is amps you're trying to maintain, Have you considered a step down resister to bring down the voltage and a coil to build the Amps back up.? I really don't know much about it.

Answer 5

oldman says that "Electricity is broken down into parts such as Amps(heat), Watts(resistance), voltage(current) etc."

Not in any of the textbooks which I read it isn't.

There is electrical voltage (like pressure), measured in volts, abbreviated to V.

There is electrical current, measuresd in amperes, abbreviated to A.

There is resistance to current flow, measured in ohms, abbreviated to the Greek lower-case omega.

There is electrical power, measured in watts, abbreviated to W.

For a bit more on the convention of abbreviations see http://www.davidbridgen.com/conv.htm

Answer 6

You want a DC/DC converter.
You need to find one with the input and output voltages you want at whatever powerlevel you need.
Here is one place to start looking.
http://www.73.com/a/0194.shtml
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Andre' B.