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Author Topic: Regulators and inadequate supply.  (Read 1010 times)

Hak Foo

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Regulators and inadequate supply.
« on: November 11, 2005, 01:32:47 PM »

I noticed my AV8 lets me software-adjust the system fan voltage.  Down to 8.0 only, and it's still too loud.  (It's a 120mm 'Power Logic' rated 290ma)

So I have a handful of 7808s.  No real help if I can only get 12v to 8v with one.  What would happen if  I fed 8 volts into a 7808?  I know with 12 volts input, I get 8 volts output, but I think the output is no guarantees below 11v input.
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linear

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Re: Regulators and inadequate supply.
« Reply #1 on: November 11, 2005, 01:52:20 PM »

You suspect correctly.

A regulator is doing things for you--it maintains Vout within some tolerance over a range of load conditions. In order to do its work, it does use power internally, and more to the point, it drops voltage, so that Vout < Vin - 1.4V or so. Look at the 1.4V loss as the tax you pay to get the regulation.

SO what are you trying to do, just drop some ovltage to the fans? I'd suggest some rectifier diodes (1N400x) in series with the fan if you need to go a little lower than what the software gives you. Each 10 cent diode will drop 0.7V, and there's no regulation occuring, but the results will be pretty predictable.
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