Putting a series resistor/zener to a DC fan to reduce voltage and/or RPM - Page 1
Forum section: electronics / beginners

author: hp3310a
author_rank: Regular Contributor
timestamp: October 03, 2025, 05:45:55 pm
content: I have a "new" toy named 3245a that comes with a 15V fan. Due to the noise it produces that one had to go and a replacement from the same manufacturer (EDM-Papst) was even louder. So I put a 12V fan there with a suitable series resistor to bring the voltage down. The fan in question is a Sunon MF60251V2.The voltage drop is dependent on the current draw and how it behaves is unknown (to me) especially when it is turned on. So I looked at the voltage the fan gets using a scope because I wanted to make sure it never exceeds the 13.8V maximum voltage it is rated for.This is what I found:Contrary to my assumption, the current draw (and hence the voltage drop) isn't constant at all. Though the voltage stays withing allowed limits, the fan voltage varies by 3V at a frequency of about 230Hz.So I put an electrolytic capacitor in parallel to stabilize the voltage, I tried 22µF, 47µF and 100µF. At 100µF the fan only sees a 73mV RMS 0.2Vpp AC component (down from of >550mV RMS 3Vpp).22µF47µF47µF Detail100µF DetailLooks like a capacitor is really necessary for proper operation of the fan. It complicates things with respect to how the series resistor is going to be integrated into the circuit, but I think it's better to do it. I can hear the RPM of the fan vary if there's no cap in place.Maybe this is something that is completely obvious to everyone but me. Since the series resistor thing is done often to silence the fan I thought I put it out there anyway.

author: Seekonk
author_rank: Super Contributor
timestamp: October 03, 2025, 06:18:14 pm
content: A series of diodes in series would be more predictable. Regardless, you don't get a lot of range by just dropping voltage.

author: nfmax
author_rank: Super Contributor
timestamp: October 03, 2025, 06:20:27 pm
content: I have used a 4.7 V zener as a simple speed reducer, with a relay contact across it as a dual speed control

author: mikerj
author_rank: Super Contributor
timestamp: October 03, 2025, 06:35:13 pm
content: The commutation of the fan motors mean that will draw pulses of current as each stator winding is energised. This is also why trying to control regular two/three wire fans with PWM often doesn't work very well.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 03, 2025, 06:59:47 pm
content: +1 for zener or diode string.

author: hp3310a
author_rank: Regular Contributor
timestamp: October 04, 2025, 08:29:09 am
content: +1 for zener from me tooThanks a lot for the suggestions!As luck would have it I had some 4,5V zeners lying around, so I tried again:There is still some ripple that I'm not sure where it is coming from (I was careful to eliminate wire and connection resistances as much as possible):Overall much better and much cleaner (AC 34mV RMS , 433mV Vpp) so I'm happy but I would like to understand the voltage spikes.Is the changing zener voltage due to changing current flow?Edit:That's probably not it, as the spikes' Vpp only increases by 40mV going from 20mA current to 80mA current through the diode. I guess it's something the motor/motor control board does.

author: CaptDon
author_rank: Super Contributor
timestamp: October 04, 2025, 01:37:04 pm
content: Probably could have just paralleled a smoothing capacitor also, perhaps 470uf@25vdc

author: hp3310a
author_rank: Regular Contributor
timestamp: October 05, 2025, 08:46:15 am
content: Quote from: CaptDon on October 04, 2025, 01:37:04 pmI did and it works, see above. However having two components complicates integrating the stuff into the unit so I'd rather have just one.What is puzzling me is: Why is the zener voltage not constant? It varies at the same rate as the fan voltage. On one hand it is unsurprising since the 15V supply is completely stable and the voltage fluctuations at the fan just have to cause opposing fluctuations at the zener (or the other way round). On the other hand, should the zener voltage not be stable or more stable?Yellow=zener green=fan

author: nfmax
author_rank: Super Contributor
timestamp: October 05, 2025, 09:20:22 am
content: Zener diodes, particularly low voltage zener diodes, do not have an abrupt characteristic. As the current drawn by the fan changes, for example as the active windings are switched, the voltage dropped across the Zener changes too. Take a look at the data sheet for a low power Zener and compare the characteristic curves of low and high voltage parts.

author: hp3310a
author_rank: Regular Contributor
timestamp: October 05, 2025, 10:22:11 am
content: Quote from: nfmax on October 05, 2025, 09:20:22 amThanks that explains it. For completeness, a snippet from the data sheet of the Vishay BZX (I believe that's what I have):

author: hp3310a
author_rank: Regular Contributor
timestamp: October 05, 2025, 12:09:22 pm
content: Actually I don't have a BZX55 (chart above) but a BZX85 (chart below). The fan draws around 65mA and that's in the knee area of the curve (4.7V) where the zener voltage is much more current dependent.That of course begs the question: Are there low voltage zeners with a power rating of at least 500mW that have a much sharper bend?

author: ISO_mukund
author_rank: Newbie
timestamp: October 05, 2025, 04:22:46 pm
content: I suggest using an IC LM317 on a heat sink to precisely vary the DC voltage given to the fan.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 05, 2025, 04:25:25 pm
content: Just use regulator IC if you need sharper than that, e.g. 7812.