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400mv spikes on lm35 output when fan is running

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Inspired by Daves,  constant current source. I decided to build my own, and as the heatsink gets quite warm, i decided to have a temperature controlled fan on it, and that works well. Apart from those spikes that show up, i have a 0.1 uf on the output of the lm35. The 2n2222, that runs the fan, has its base driven by a 358 op amp.

when the fan is not on there is no noise at all.

I have a feeling that for this use i can ignore the problem, I'm just curious what could be done about such a problem.


Maybe the fan introduces spikes to the line ? (Or , unfiltered ground that is sent back to the pcb , might happen , who knows ?)
Or EMI Interference from the fan ?

Are the spikes still there if you connect both the probe tip and the ground clip to ground? Does reducing the loop area of the ground lead or using a shorter ground lead change the shape/amplitude of the spikes? It may just be a common mode issue, or inductive coupling into the probe ground lead.

Put a capacitor of 100uF 25V directly across the fan power leads, and a series resistor of around 10R in the supply to the fan before the resistor. This will reduce the noise reflected back to the supply. Twist the leads of the fan together all the way to the control board Connect the heatsink to circuit common with a dedicated short wire, this should reduce it a lot.

Thanks everyone,

I see the spikes if i probe the fan on its own, interesting, its a fan i pulled off a nvidia graphics card, so its behavior surprises me.

Sean, you mean a resistor before the 100uF capacitor right? :)

update: I just tested this and all those spikes are gone, so thanks a bunch.



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