Easy way to get 12DC from 78 VDC ?

I've got a project that has a 84VDC unregulated power supply on it. I need 12VDC,3A for another part of the project.
Is there a simple way to get 12VDC from that supply ? Regulated ? Like maybe a PWM circuit driving a chopper transistor with a capacitor to smooth out the waveform ? I say regulated because the 84VDC supply is heavily loaded and it droops.
Thanks.
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me wrote:

The usual solution is a SMPSU configured as a "buck" http://www.maxim-ic.com/cookbook/powersupply/pdfs/CB100.pdf http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3939
You will need to derive a low current, lower voltage supply for the chip itself.
--
Sue


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On Wed, 09 May 2007 21:25:04 +0000, Palindrome wrote:

That circuit has a lot of components. Is there a simpler way ?
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me wrote:

There's a simpler way - buy a separate power supply. But setting that possibility aside: Does your 84 VDC supply have the "guts" (current capability) to provide an additional 36+ watts beyond what it is providing now? If it doesn't, you need a separate supply.
And if it has lots and lots of "guts" you could go simple with a linear, but it would waste a *lot* of power. As Sue recommended, and assuming the existing supply can provide an additional 36+ watts, a buck regulator is the way to go, even though you don't find it simple.
Ed
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Try a switch mode power supply that has a wide input voltage range of nominally 85-264 VAC. These have a full wave bridge input and will often operate on lower voltages. You will need one rated for more than the 3A because the input diodes are rated for the input current for normal operation in which the diodes carry current only half of the time.
There are also solar system controllers that convert higher voltages down to 12 or 24 volts for battery charging. Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA snipped-for-privacy@pvri-removethis.biz
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Bill Kaszeta / Photovoltaic Resources wrote:

Bill, you just gotta read the thread. I'm not the op - I'm not searching for the solution. You should post to him, not me.
Also, his supply provides 84 volts, not 85, and it droops under load, so a supply with an input range of 85 - 264 vac is a non-starter.
Ed
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On Thu, 10 May 2007 05:48:34 +0000, ehsjr wrote:

Yes, it has the guts.

How do you make a simple linear PS that has an input of 84V ? I redesigned things and now I need about 50 ma instead of 3A. Is that getting into Zener diode territory ? All I need to do now is power a single sided op amp.

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me wrote:

Yep, a series resistor + zener will do it. However, you really need to allow at least double the off load zener current than the load current, to ensure regulation. 100mA at 84v is a 10W resistor.. and a 3W zener.
Better to take the zener voltage to the base of a series pass traansistor with a suitable voltage rating. You then only need a couple of mA through the resistor and zener. The transistor collector goes to the supply and the emitter to the load.
-- Sue
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