Electronics > Beginners

Is it a bad idea to underpower a DC fan?

<< < (3/5) > >>

Circlotron:

--- Quote from: SparkyFX on December 02, 2019, 06:25:49 am ---I also run some on a PWM output on a fan controller, which makes them occasionally emit some noise. An electrolytic cap parallel to the fan kind of solved the problem.

--- End quote ---
A largish cap across a load that is being PWM'd is going to eventually break either the cap or the switching device. Large currents will flow at the start of every switching cycle. Much better to leave the cap in place and feed the combination via a series inductor and have a reverse biased catch diode between the switching device and + volts.

SparkyFX:

--- Quote from: Circlotron on December 03, 2019, 06:43:46 am ---A largish cap across a load that is being PWM'd is going to eventually break either the cap or the switching device. Large currents will flow at the start of every switching cycle. Much better to leave the cap in place and feed the combination via a series inductor and have a reverse biased catch diode between the switching device and + volts.
--- End quote ---
So far it worked out. I figured given that the fan controller needs to be able to handle a blocked rotor, starting currents and such it might be a smaller problem. When in operation the running rotor limits the current as well.

Might be different once the control is somehow closely matched to a certain fan (load), but it would be a very fragile system, should the fan once end up being clogged.

Zero999:
He's right. Putting a capacitor on a PWM circuit is a bad idea. It might have worked for you, but it doesn't mean it's right and won't gain you anything efficiency wise over a linear supply. Every time the switch turns on, a large current flows, causing a lot of power to be dissipated. If the capacitor is large enough to smooth the ripple to a negligible amount, the efficiency will be exactly the same as a linear circuit, although the power will be dissipated in huge surges, rather than continuously in the linear circuit.

james_s:
If you put a resistor before the capacitor then you have a low pass filter, and that will convert a PWM signal to a variable voltage. Probably not gonna be of any advantage when powering a fan though.

PC motherboards drive the fan directly with PWM and it works fine. I have had a few fans that squeal at very low PWM values but no other issues.

Zero999:
Actually if you add a capacitor, it will try to keep the voltage close to the full supply voltage, because the R component of the low pass filter depends on the impedance of the switch which is very high, when it's off and extremely low, when it's on.

I've done a quick LTSpice simulation. The switch is modelled as a resistor which oscillates between 1M and 50m Ohms, at 50% duty cycle, 1kHz. Look at the huge current spikes.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod