RC Low-Pass Filter for Blower Fan

SnowCrash

@wilriker happy you got your fan working quietly (though perhaps not in the way you were hoping to achieve this )

Anyway, at another discussion, @dc42 insisted that an inductor-capacitor circuit is necessary for preventing a non-PWM fan from potentially being damaged while being run with a high frequency PWM signal, and just I started looking into the idea of making a little add-on module that would provide this circuitry for all 3 PWM outputs on the Duet (plus the ability to choose between 12V and 24V for each fan which is the main reason for the module).

So hopefully @dc42 or someone else familiar with this subject would chime in and clarify this point about the inductor's value.

dc42

@snowcrash said in RC Low-Pass Filter for Blower Fan:

Thanks @wilriker!

But that's HUGE just for a fan that draws maybe 0.15mA, no? (ok, the fan will draw more current at startup but still...). Is there some calculation behind this value?

Yes 1mH. The fan draws 0.15A (150mA), not 0.15mA.

SnowCrash

Thanks for the confirmation, @dc42! But what's the calculation behind it?

dc42

The calculation is L = V * t/I where V is the voltage across the inductor, I is the maximum ripple current current you want to allow, and t is the time involved. Or, turning it around, I = V * t/L. If we take the case of a 24V fan running at 50% PWM and 50kHz PWM frequency, then for L=1mH and 12V across the fan, we have I = 12V * 10us/1mh = 120mA. This is around the same as the fan current, so a capacitor is needed after the inductor to reduce the ripple current in the fan. For the capacitor, C=I*t/V where V is the allowed peak ripple voltage. So V=I * t/C and 1uF will give 120mA * 10us/1uF = 1.2V. This is 10% of the 12V average voltage at 50% PWM, so it is likely to be low enough.

fma

I recently added a Berd air pump for cooling the part, and I noticed that increasing the PWM frequency increased the efficiency: for the same PWM ratio, the higher was the frequency, the higher was the rotation speed. I went up to 100kHz.

Is it normal? Could such low -pass filter improve things?

dc42

That doesn't necessarily mean that the efficiency was increased. The increased rotation speed may have been caused by a EMI suppression capacitor within the motor having a smoothing effect.

fma

Yes, efficiency was not the appropriate word (I didn't measure the current).

What about the low-pass filter? Could it improve things?

BTW, I also noticed that adding the flyback diode increases the speed for the same PWM ratio.

dc42

if you are driving the brushed DC motor from a heater output, then a flyback diode is essential. Without it, energy from the back EMF of the motor inductance is dissipated in the mosfet by avalanche breakdown, leading to reduced efficiency and maybe eventually a blown mosfet.

fma

I was aware of potentially blowing the FET, but I didn't know it changes the efficiency. Thanks for the explanation!

vfjr

I know this is a very old topic, but it is the first one that comes up on Google when searching for fixing fans that do not work with PWM, so I hope to be able to share a solution with people who need help.

I found a good and cheap solution and I made a new topic about it here.