? for you electrical gurus

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Magic Static

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Oct 12, 2010
I have a DVR with a noisy 40mm 12V case fan. Not very loud but it's always been that way. I have a few fans that size from a HDD cooler. But those fans spin too fast and make even more noise than the faulty one. I had some resistors in a harness to reduce speeds on a motherboard fan setup But I can't find those right now. So I got to thinking would any of you electrical wizards be able to suggest a value of resistor to use on a 12V 0.06a DC fan motor to cut the speed in half?
What I think I'll do is set up a 12v breadboard with a potentiometer and fan. Dial in the desired speed/airflow and measure the potentiometer for the value and select a resistor and add inline on the fan wires. Cheap and easy.
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Make a voltage divider with a 10K resistor and a 10K POT. Dial in what you want and then replace the POT with another resistor keeping the other 10K in circuit. That would be a better way to do it.
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If the fan spins too slow, the DVR might not have enough cooling.
I have run a 12Volt fan slowly on a USB port for a PVR fan that comes on only when it powers itself on. This PVR did not have a fan to start off with that I did this with.

(a 200 ohm resistor inline should cut the voltage in half)
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I just use a 24v fan in a 12v circuit, turns fast enough to move enough air, and slow enough to not make noise. Might not work in all cases where you need a lot of cooling though. Same idea as the 12v fan in a 5v USB supply as I use in a raspberry pi case. Better than using a resistor, but if you have to, use the circuit as KE4EST provided above, it's more stable.
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Make a voltage divider with a 10K resistor and a 10K POT. Dial in what you want and then replace the POT with another resistor keeping the other 10K in circuit. That would be a better way to do it.
View attachment 128348

Ok, I gotta ask about the diagram. Removing the R2 line/resistor and making R1 of sufficient size would keep the voltage but reduce power out?

I used to know all this stuff.....
Just doing that with an inductive load like that would work. The impedance would be kind of like resistance. However, the voltage will fluctuate constantly.
I chose 10K because they are the most used. So very easy to find one. :)
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Not the best solution, but if you don't mind removing the cover and letting the DVR sit open on top of a shelf or stand you can get away with removing the fan.

There is also a website out there where they sell quieter versions of fans.

I used to remove the covers on my Dish DVR's all the time. If the hard drive went bad I would RMA the unit under warranty.
Voltage divider can not be of any value, as the fan needs some current, namely 0.06 Amp (or, 60 miliAmps).
Using 10 kOhm resistor would limit current to 1.2 miliAmp.
The fan's coil is the lower part in voltage divider, thus just one resistor (upper part) is needed. Like Larry1 suggested, 200 Ohms.
The substitute resistor for fan's coil is just around 200 Ohm, (12 V divided by 0.06 A).
Small bulbs (from old flashlight, or, from old Christmas tree lights) could be tried, in series or parallel (assuming they are still available...).
Probably lowering the voltage to 8- 9 V would reduce noise enough, so, maybe 100 Ohm resistor or similar would do the trick.
6 silicon rectifying diodes, connected in series, would cause voltage drop about 4 Volts (6 x 0.7 V = 4.2 V).
Cheers, polgyver
Great catch polgyver! Where was my brain. You are correct! The fan would not even spin up with 10K's!!!
Yes, use a couple of 100 ohm resistors. Or maybe just a single POT as a voltage divider.
I like your idea of diodes also. A hand full of 1N4001s and he can keep adding until it is where he likes it.
Well this has been a fun learning experience. After all is said and done, the three fans I have That I thought I could rig up to work are indeed cheap junk. First one I tried was junk to start with. The second one ran quietly about 30 hours and the bearing gave up. I did find out that it would not move as much air as the original and my mod just reduced that so it looks like I'll have to go with Claude's recommendation. I found a 40mm x 20mm fan which moves 20% more air and spins at almost half the RPM and is only 19 decibels. $4.95 cheap :)
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