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Need a resistor to operate a 12 volt fan off 24 volt supply in an amplifier. - Page 2

post #11 of 19
Quote:
Originally Posted by hammong View Post

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

I do agree that a regulator would be better.

but

hmm I have been thinking about the additional current darw comment, both the 24V fan and 12V noctua fan draw pretty much the same amount of current and the resistor would only be dropping the voltage by turning the extra unwanted voltage to heat therefore I don't believe there will be any additional current to be drawn from the circuit. bear in mind that the resistor will get hot.

The only real benifit in this application to use the regulator is that it will give you a much more stable power output and you know you will be getting the exact power you want.


Let us know what you decide to do and how it went thumb.gif
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post #12 of 19
lol not a resistor, a voltage regulator, / step down transformer
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post #13 of 19
24v to 12v is common for lorry drivers using 12v accessories. Ebay has many 24v-12v adapters/converters too
post #14 of 19
Quote:
Originally Posted by Hoodcom View Post

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
Amplifiers are extremely cramped areas so it's not wise to use a weak fan like that
Quote:
Originally Posted by Carlosmans View Post

I do agree that a regulator would be better.

but

hmm I have been thinking about the additional current darw comment, both the 24V fan and 12V noctua fan draw pretty much the same amount of current and the resistor would only be dropping the voltage by turning the extra unwanted voltage to heat therefore I don't believe there will be any additional current to be drawn from the circuit. bear in mind that the resistor will get hot.

The only real benifit in this application to use the regulator is that it will give you a much more stable power output and you know you will be getting the exact power you want.


Let us know what you decide to do and how it went thumb.gif
It's actually working as a current limiter, as the voltage goes down (Which the amp does control using voltage) current will go down and fans aren't exactly linear so you won't get linear response and it might overheat at some points


A simple option if you really really want to use a noctua for an application it should NEVER do.
http://www.ebay.com/itm/LM2596-AC-DC-Buck-Converter-Step-Down-Module-Power-Supply-24V-AC-DC-to-DC-12V-5V-/261125230909?pt=US_Surveillance_Cables_Adapters_Connectors&hash=item3ccc46d13d
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post #15 of 19
Thread Starter 
Quote:
Originally Posted by DaveLT View Post

Amplifiers are extremely cramped areas so it's not wise to use a weak fan like that
It's actually working as a current limiter, as the voltage goes down (Which the amp does control using voltage) current will go down and fans aren't exactly linear so you won't get linear response and it might overheat at some points


A simple option if you really really want to use a noctua for an application it should NEVER do.
http://www.ebay.com/itm/LM2596-AC-DC-Buck-Converter-Step-Down-Module-Power-Supply-24V-AC-DC-to-DC-12V-5V-/261125230909?pt=US_Surveillance_Cables_Adapters_Connectors&hash=item3ccc46d13d

What would make the Noctua not worthy of an application that this P14751-ND at Digikey, which is a substitute of the P9739-ND (That they no longer keep instock,) that does a fine job in keeping two QSC PLX3402 and two QSC PLX2402 quiet, cool in stable condition for this guy for a long time, with no reported issue in his home?

Comparing specs by specs the Noctua seemed to be a nice solution. Unless I am missing something here between the fan I linked and it. headscratch.gif

By the way, here's an inside of how a PLX model basically is like, does static pressure amount needed, vary by heatsink design? I know CPU coolers are different than amplifier heatsinks here. Pictures here: http://www.avsforum.com/t/1036690/is-there-a-qsc-plx-3402-fan-mod#post_21224705
Edited by Hoodcom - 5/16/13 at 5:56am
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post #16 of 19
Definitely varies, for sure. Computer aftermarket heatsinks are not so cramped, the fan operating there is going to pull a hell of a long distance, with the sort of static pressure the FLX has airflow will collapse very quickly
Longer heatsink = more pressure needed or else CFM will collapse very quickly
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post #17 of 19
Quote:
Originally Posted by Hoodcom View Post

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.)
And DaveLT's reply:
Quote:
Amplifiers are extremely cramped areas so it's not wise to use a weak fan like that

.0555" H2O versus 149" H2O pressure... no comparison.

Noctua may work but it's a long shot. 1/38 the static pressure means it probably won't move much air. Remember static pressure rating is the pressure it reached when fan quits moving air.. static = no moving = no air movement..
post #18 of 19
Thread Starter 
Well, if you guys know of any quiet 80 x 80 x 25mm fans with good static pressure, feel free to let me know, I just basically found Noctua to be the best option I have compared to other quiet 24v fans I found on Digikey so far.
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post #19 of 19
Quote:
Originally Posted by Carlosmans View Post

turning the extra unwanted voltage to heat therefore I don't believe there will be any additional current to be drawn from the circuit. bear in mind that the resistor will get hot.

Yeah, I guess you're right. LOL. If the 12V fan would have run say 8W at 24V overdriven, it would only run 4W at half voltage, the other 4W lost as heat in the resistor. Same power, just split up differently.

I'd still use a regulator. =)

Greg
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