# How to suppress voltage spikes caused by USB fan on ATX 5V rail?

I bought a cheap largish USB fan and plugged it in to the 5V rail on my adapted ATX bench power supply. Immediately, the ATMega328P that is present on the adapted ATX supply started to act erratically. The '328 is there to measure voltages and currents and to drive 7-segment displays and it is powered out of the same 5V rail. (picture and schematics are at the end of the post).

I checked the 5V rail with the oscilloscope (screenshot below) and, to my surprise, the fan was causing the rail voltage to spike up to 6.3V at 160Hz. That's way past the poor '328 absolute maximum 5.5V supply voltage.

I know what is causing me problems, but I don't know why or how to fix it. But I thought I should learn something from this. My questions then are:

1. What is likely the cause of those spikes?

My guess is that the problem comes from the inductance of the motor coils. When the commutators loose contact with the brushes, the current through the coils is suddenly interrupted, and according to the inductance formulae $V = L \dfrac{di}{dt}$, there's an increase in voltage to counter it. I hazard a guess that the frequency I measured (160Hz) is double the fan RPM (I'm guessing 4,800RPM).

Am I in the right direction? The only thing that doesn't add up is that I think the spike should be negative (same signal as $di$), not positive.

2. Based only on my oscilloscope readings, can I consider that the fan design is flawed? If so, what circuitry is likely missing on the fan, that would otherwise prevent those voltage spikes from happening?

My guess is that the voltage spike is something that all brushed DC motors produce by design, but additional circuitry (possibly filters) are put in place to suppress its effects. But I don't know what these filters would be.

3. How can I improve the design of my adapted ATX bench power supply to protect it from such voltage surges?

Is this a case in which a flyback diode should be applied?

The fan is 1.5W (2.5W start up power). I have two other smallish USB fans (1W) that do not produce any spikes like that.

ATX power supply adapter schematics:

My adapter plugged to the ATX power supply in the background (they are actually two separate units), and the fans.

• Possibly the effect of power supply pumping. .. You may try connect the fan to the 5V thru a diode and some larger (2200uF) capacitor. The cap will capture the pumped voltage and the diode will prevent it from flowing backward into yr PSU. Mar 8, 2016 at 1:19

$$f_c = \dfrac{1}{2 \pi \sqrt{LC}}$$