38 Vdc to 12V dc @ 600 ma :(

You are looking at 26V X 0.6A = almost 16W dissipation in the regulator if you use a linear approach. I'd seriously consider a switcher.

If you do decide to go linear, an LM317 derivative would allow you to accommodate that sort of input voltage, as they only see the input-output differential. Refer to the data sheet for more info.

Everyone has their favourite switcher. You'd need to select carefully though, as many have an upper input limit of 40V and your +38V could be ???

38V -o------o---------- | | [1.0k] | | c o-----b 2N3055 | e | | [2.2k] '-----------> to 7812 regulator Vin | Gnd -o------------------> to 7812 regulator Gnd

The 1k resistor must be rated for 1W. The 2N3055 should be on a heat sink, since it'll be dissipating 10 to 15 watts.

You don't need any smoothing capacitors, since the input is DC. You can use a 0.33uF cap before the regulator if it is any distance from the

2N3055.

A small DC-DC converter would be a better choice, since it will waste far less power (and thus stay much cooler).

-- Regards, Bob Monsen

Science without religion is lame, religion without science is blind. Albert Einstein, "Science, Philosophy and Religion: a Symposium", 1941

I wonder. Does that audio amplifier not have any +/-15V circuitry in it? Many amps have got high-voltage rails for the output stage but also have low-voltage rails for control circuitry, preamp stages, etc; often those low-voltage rails are powered from a separate tap, or even separate winding, on the power transformer, rather than being stepped down from the high-v rails. If so, perhaps you've already got the supply you need.

If not, take a look at switchers, as others suggested. You might still need to drop a couple volts to get into the input V range of the switcher - a lot are rated for 18V to 36V. But dropping 4V or so at 500mA is a lot easier than dropping 22V at 600mA.

is a good source for switchers in small quantities in the US. In Canada, maybe Futurlec, I dunno.

Nat.Semi LM2576HV-12 (-HV version gives more input headroom, up to 60V). You may want to put an LC filter on the input to minimise noise coupling, and maybe use a toroidal inductor in the switcher.

38V - 12V is only 26V difference. A 30V mosfet should be able to handle it. For additional safety, you can drop the input with a 12V auto light bulb, which would also be a current indicator as well.

Thanks for the replies. Yes, the amplifier does have a 15 v supply for the pre-amp stage etc. I actually already tried this. I actually need a + 5 volt and a +12 volt supply. I tried to source both of these off the 15V but and it all measured fine as far as voltage goes, but when I tried to power the LCD the screen would blink, flicker and fade in a out when I tried to actually run it. I assumed there was to much of a current draw and the 15v source could not handle it. So, I was now trying to just leave the small 5v for the LCD functioning to draw from the 15v and make a new independent source for the 12volt.

I have never heard of a "switcher" before....interesting...

As a side note, I just posted another post on this issue with a different subject line. I thought no one replied as I was not getting the responses until just now (only 3). So, I am not sure what is up with my newsgroup/app/server :(

Jerry

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I was just doing some reseach and educating myself on switchers such as LM2576HV-12. Before I go ahead, will the circuit (fixed output voltage version) on page 11/12 on the below link be adequate for the 12V version?

Jerry

The voltage difference is not the problem. It's the power. A linear regulator would dissipate 15 watts, which is a lot of heat, whether it's in a mosfet of a bulb.

Yes.

If the load is fixed and the source are fixed, you might get away with a 55 timer driving a power transistor so the average current is OK. An LED will not obvject to 2x rated current when pulsed like that. Just a thought.

Of course, if the input goes up/down, then this will not work too well, but is at least worth giving a bit of thought to.

--
Dave K

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