[dsavitsk]

To power a NOS DAC, I am using a PS that is about half way between a Tread and a welborne ps1 (essentially a lm317 based PS.) It supplies 11V at about .5A. Because the DAC has 8 stacked chips, I would like to cool it with a fan.

I happen to have an extra REG102GA-5. It seems that it will regulate a voltage down to 5V, and that since I don't care about noise (it is just powering a DC fan) I can skip all of the caps (though one on the input seems recommended.)

Data sheet is here: http://rocky.digikey.com/WebLib/Texa...ata/REG102.pdf

The questions I have, then, are:

(1) is running this at 11V in going to cause a problem? The datasheet, I think, says that 10V is the max. The fan says it is .05A, so I don't think a lot of heat is going to be generated. Should I put a resistor in series to drop the voltage a bit, and if so, how do I figure the voltage drop across the resistor?


(2) will this cause noise in the rest of the PS circuit?

(3) it appears that the enable pin must have a voltage to it for the regulator to work. Do I just connect it to the input?

(4) I don't need a precise voltage for the fan. Am I better off using a resistor based voltage divider?

 

[guzzler]

1) it's never sensible to go over the maximum stated values, a resistor is best (if you go this way).

R = V/I where V is the voltage drop you want, and I is the expected current

P = VI for power rating

2) no

3) yes

4) The REG102s are very good LDO, low noise regulators. Personally, I'd find a 7805 in the parts box, and just stick that in. A voltage divider would work as well

 

[dsavitsk]

Quote:

Originally Posted by guzzler 1) it's never sensible to go over the maximum stated values, a resistor is best (if you go this way). R = V/I where V is the voltage drop you want, and I is the expected current

I, of course, know (or should know) this, but I always think that it must be too simple. Where are the integrals?

Anyway, it seems like a 120R/.5W resistor in series with the fan is the easiest solution of all here.

Quote:

4) The REG102s are very good LDO, low noise regulators. Personally, I'd find a 7805 in the parts box, and just stick that in. A voltage divider would work as well

I have these left over from ordering the wrong package, so I don't see them being used for anything else anytime soon.

 

[mono]

While I don't know how much it would isolate fan electrical noise, I'd consider just using a resistor for the fan, no 2nd regulator. The resistor value varies per fan but with a typical 12V fan a value of around 68-120 Ohm, 1-2W, with typical lower-RPM fan models generally being able to use the upper (approaching 120 Ohm if not a little higher) value.

 

[guzzler]

Quote:

Originally Posted by dsavitsk Where are the integrals?

We're not dealing with any varying quantities here at all, so no need to resort to calculus

 

[Jazper]

scuse me for butting in, but it's a fan we're talking about here, not a high priced dac chip or opamp.

Running 1v over stock won't hurt it all that much, you may lose a little bit of life as a result, but fans (especially small dc fans) are usually subject to unregulated power supplies and manage fine. Not to mention they're usually pretty cheap unless you go something exotic. Youll get at least 20,000 hours or so usually before the fan dies even with 1v over.This is all provided it is a reasonable quality fan and not a cheap chinese piece of crap.

it's when you start talking 5-10v over that it's more of an issue.

(I know all this from many years of tinkering with dc fans in a pc setup)

edit: that said, the ideal would be chucking in a cheap LM7807 or LM7808 for near silent operation.

 

[guzzler]

Quote:

Originally Posted by Jazper scuse me for butting in, but it's a fan we're talking about here, not a high priced dac chip or opamp. Running 1v over stock won't hurt it all that much, you may lose a little bit of life as a result, but fans (especially small dc fans) are usually subject to unregulated power supplies and manage fine. Not to mention they're usually pretty cheap unless you go something exotic. Youll get at least 20,000 hours or so usually before the fan dies even with 1v over.This is all provided it is a reasonable quality fan and not a cheap chinese piece of crap. it's when you start talking 5-10v over that it's more of an issue. (I know all this from many years of tinkering with dc fans in a pc setup) edit: that said, the ideal would be chucking in a cheap LM7807 or LM7808 for near silent operation.

We're talking about the regulator, not the fan. The REG102 is quite a low voltage device. dsavitsk wants to drop the 11V input to 5V at the fan, presumably to cut down on noise, or it may even be a low voltage fan. I still think the REG102 is overkill, but....

 

[beerguy0]

I would definitely give the fan it's own regulator. DC fans have a switcher in the motor, and generate enormous amounts of electrical noise. I added a +12V fan to a Class A headphone amp I built, and running it off the +12V supply made the amp unlistenable. I substituted a +5V fan, and ran it off a separate regulator. Noise problem solved.

I'd probably just throw in a LM7805 and be done with it. The ripple rejection of the regulator will take care of the noise.

Cheers,

bg