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Need a resistor to operate a 12 volt fan off 24 volt supply in an amplifier.

post #1 of 19
Thread Starter 
I want to use 12 volt fans, such as the Noctua NF-R8, which I find provides the best air flow / noise ratio over actual 24 volt fans that would make a direct replacement for my power amplifiers. SVC specified the fan to have 31CFM at 17dBA.

The stock fan in the QSC PLX Series amps, push 39.6CFM, at 32dB noise. (I don't know the specs on the "Superred CHA8024EBN-K" fans in my Crest CC Series amp.) So I feel the Noctua NF-r8 would provide the best of both worlds, in a controlled environment.

What kind of resistor figure do I need for the Noctua fan? I know under normal computer operations you can use resistors for quieter operation in computer cases, I see no reason why it can't work in this case for a safe operation in a power amp. thumb.gif
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post #2 of 19
don't use resistor, use at least an LM7812 instead.

just like your attenuator (or pot) on your pre-amp, the output voltage of the resistor may vary as you change your fan.
since the impedance may vary for each fans
post #3 of 19
I think I have got this right:

Current of fan = 110mA
Voltage to supply to fan = 12V
Supply voltage = 24V

so for ohms law you use the voltage you are trying to dissapate across the resistor

R = V / I
R= 12 / 0.11 = 109 Ohms

I would use an 120 ohm resistor

Now for the power calculation (this is very important)

P = V x I
P = 12 x 0.11
=1.32W

The resistor you need is 120 ohm power rated at 2W, that is the most common value I would use.

Someone please feel free to correct me if I am wrong.
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post #4 of 19
Quote:
Originally Posted by inVain View Post

don't use resistor, use at least an LM7812 instead.

just like your attenuator (or pot) on your pre-amp, the output voltage of the resistor may vary as you change your fan.
since the impedance may vary for each fans

yes a voltage regulator would be better but I think you can go for a resistor you may not get exactly 12V to supply the fan but will be close enough seen as its only a fan that you are supplying the voltage to.
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post #5 of 19
Thread Starter 
Quote:
Originally Posted by inVain View Post

don't use resistor, use at least an LM7812 instead.

just like your attenuator (or pot) on your pre-amp, the output voltage of the resistor may vary as you change your fan.
since the impedance may vary for each fans

Well I intend to keep using the Noctua fan as the main choice for the amp. That regulator IS a good option, I have to admit. Thank You for the suggestion on it, as I may consider that. Though the only downfall is it will keep the fan from being variable when the amp will typically varies the fan speed on it's own.

Quote:
Originally Posted by Carlosmans View Post

I think I have got this right:

Current of fan = 110mA
Voltage to supply to fan = 12V
Supply voltage = 24V

so for ohms law you use the voltage you are trying to dissapate across the resistor

R = V / I
R= 12 / 0.11 = 109 Ohms

I would use an 120 ohm resistor

Now for the power calculation (this is very important)

P = V x I
P = 12 x 0.11
=1.32W

The resistor you need is 120 ohm power rated at 2W, that is the most common value I would use.

Someone please feel free to correct me if I am wrong.

So is this the typical formula used for calculating the needed resistance of a resistor? (Someone tried telling me a 3 ohm resistor would work. headscratch.gif By multiplying the voltage by the current draw. But I was unsure, which is why I came here seeking for more information. Best to be safe than sorry, I say. thumb.gif)

Okay, so pretty well doubling the resistance of the fan would half the voltage? As if hooking two fans in series?
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post #6 of 19
i wouldn't mess around with it till you've gotten the specs of the fan you are replacing... some of those 24v fans are selected for high static pressure reasons so you may be jeopardizing your amp by replacing them. also the components you add may also introduce line noise into the system which may degrade the performance of your amp.
post #7 of 19
Thread Starter 
Quote:
Originally Posted by psyclum View Post

i wouldn't mess around with it till you've gotten the specs of the fan you are replacing... some of those 24v fans are selected for high static pressure reasons so you may be jeopardizing your amp by replacing them. also the components you add may also introduce line noise into the system which may degrade the performance of your amp.

Stock fan in the QSC has this rating for Static Pressure: 0.149 in H2O (37.1 Pa)
Noctua NF-R8 fan, as per this site's listing: Static Pressure 1,41 mm H2O (Which I think is "0.0555118" in inches.)

But, while that may be true on the design being based off the high static pressure, this was so due to designs being used for live sound. Driven hard in harsher climates. While my room system is in a well climate controlled environment, likely to never be driven to point of thermal protection. So I don't think it will be a problem, since fan mods, for slower speeds, such as this model are popular that I seen for the QSC amplifiers for quiet operations. The static pressure of that fan I just linked, is super close to what the Noctua NF-R8 is rated for. Except that the Noctua is rated for a higher CFM at a lower noise than the fan I just linked as well. thumb.gif
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post #8 of 19
Quote:
Originally Posted by Hoodcom View Post

Well I intend to keep using the Noctua fan as the main choice for the amp. That regulator IS a good option, I have to admit. Thank You for the suggestion on it, as I may consider that. Though the only downfall is it will keep the fan from being variable when the amp will typically varies the fan speed on it's own.
So is this the typical formula used for calculating the needed resistance of a resistor? (Someone tried telling me a 3 ohm resistor would work. headscratch.gif By multiplying the voltage by the current draw. But I was unsure, which is why I came here seeking for more information. Best to be safe than sorry, I say. thumb.gif)

Okay, so pretty well doubling the resistance of the fan would half the voltage? As if hooking two fans in series?

I think i know what you mean, if you had two of the noctua fans in series you wouldnt need a resistor but if you wanted to run the fan at say 6v then the voltage to be dissapated would be 18v therefore you would need more resistance and a bigger wattage resistor.

You said that the amp has its own fan controller dependent on the load if that is the case is the 24v the maximum voltage or is that the minimum, if the maximum voltage is 24v then with the resistor it will control the fan speed dependent of the load which i think will work quite well.

I have not really got experience in this kind of mod.
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post #9 of 19
Thread Starter 
Quote:
Originally Posted by Carlosmans View Post

I think i know what you mean, if you had two of the noctua fans in series you wouldnt need a resistor but if you wanted to run the fan at say 6v then the voltage to be dissapated would be 18v therefore you would need more resistance and a bigger wattage resistor.

You said that the amp has its own fan controller dependent on the load if that is the case is the 24v the maximum voltage or is that the minimum, if the maximum voltage is 24v then with the resistor it will control the fan speed dependent of the load which i think will work quite well.

I have not really got experience in this kind of mod.

Yeah, that's what I was meaning. thumb.gif

Yes, 24 volts should be the max, since that's what the existing fans are rated for, and I was thinking that too. As long as the resistor halves the power at maximum power, I think all should be well, as well. wink.gif
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post #10 of 19
I second the recommendation to run a LM7812 voltage regulator for this project. You're going to be wasting a lot of power in that resistor just to drop the voltage from 24V to 12V for your fan. The circuit that powers the fan in your amplifier might not be rated for the additional current draw.

LM7812 is super cheap, super simple to hook up. The thing only has three pins. You put your 24V across pin 1 (positive) and 3 (ground), and pull the 12V off of pin 2 (+12V Positive) and 3 (ground). Screw the LM7812 to a small heatsink, or you can attach it to a piece of metal inside the amplifier that is grounded.

Here's a link to a datasheet for the LM7812. http://www.fairchildsemi.com/ds/LM/LM7812.pdf

Greg
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