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Old 27th June 2011, 10:21 AM   #1
Billyo is offline Billyo  Australia
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Default using pc fan for cooling meanwell in case

I want to use a cheap plastic tupperware style enclosure to house my HifimeDIY TK2050 and Meanwell 27 V PSU.

I can drill a few holes for cables and air, but I figure that heat buildup would still be a problem.

Would it be possible to power an ordinary PC fan via the Meanwell? The fan would be at the back of the case and ensure a constant flow of cool air through the holes I would drill.

My back of the envelope calculation using the power and amperage from a random PC fan of P = 1.8W and amps of .15 A suggests (using ohms law) that the effective resistance of the fan is 80 ohms.

If the fan is a 5V fan, then would putting a 350 ohm resistor in series with the fan successfully drop the voltage down to 5V across the fan and successfully run it - or are my calcs or concepts of what a fan needs to run completely wrong and the fan would vaporize?
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Old 29th June 2011, 12:47 AM   #2
! is offline !  United States
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I would use a metal case for shielding, but it helps that the PSU has a metal case and the amp board has a ground plane, yes?

Yes you can power an ordinary DC fan, but get one that is at least 24V rated, then current limit it with a resistor down to an acceptably low noise operational RPM.

These figures you have of 1.8W and 0.15A, is this what is on the fan label? Fan labels tend to cite the max momentary current not average, keep in mind that 0.15A at 5V is not 1.8W, it's 0.15*5= 0.75W

I suggest that the power calculation from a random fan is only useful if you are going to use that particular fan. The problem is that it is average power while a fan uses it in pulses, you will probably have voltage spikes well exceeding the capability of the fan's active circuit components. Even when you use a fan rated higher than the peak voltage in a circuit it usually takes experimentation to arrive at the correct resistor value to use for any particular average current or reduction of peak voltage level.

It is possible the fan is a modularized design, to the extent that the voltage sensitive parts inside are designed to handle the highest voltage rated member of that fan family and only the coil and drive current are changed for lower voltage operation, but to be on the safe side I would buy a 24V fan, or at least a 12V fan from a model line that contains a 24V version to address the possibility mentioned above that it "might" have components still operating within their safe zone.

Regardless of what I wrote, you could still try the 5V fan, just don't leave the amp unattended till you figure out if the fan is going to survive, then if it doesn't survive you needed to get a new fan anyway... so then pick one rated for 24V if not 36V (36V DC axial fans almost always have no trouble at 27V input, and "Most" fans rated for 24V can be ran at slightly higher voltage too, they are just spec'd at that as a common voltage that equipment uses so the RPM can be spec'd... but only within a little margin, not 22V difference.
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