Why is my PWM signal going negative?

Thread Starter

robotDR

Joined Mar 17, 2020
93
I have an EMC2101 driving an N channel mosfet that sends a pwm signal to the pwm pin of a fan. See the schematic below:
Also see the waveforms below. The low side I would exepct to be 0V but it is going about -0.5V when the duty cycle is high and as the duty cycle drops, the negative voltage gets closer to 0.

This is measured on the drain of the mosfet which is connected to the pwm pin of a fan that has an internal pull up to 5v.

The offset on the scope/probe is 0V.
 

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dendad

Joined Feb 20, 2016
4,493
Just a thought, is your CRO probe GND right at the FET and not over a length of wire that will be carrying current?
 

MrChips

Joined Oct 2, 2009
30,924
There are many possibilities making it an artefact on the oscilloscope.
Do you have the oscilloscope probe set to x1 or x10 attenuation?
As Dick says, the frequency compensation on the probe is not properly adjusted.
 

Thread Starter

robotDR

Joined Mar 17, 2020
93
There are many possibilities making it an artefact on the oscilloscope.
Do you have the oscilloscope probe set to x1 or x10 attenuation?
As Dick says, the frequency compensation on the probe is not properly adjusted.
It is set to 10x. I’ll be looking into adjusting the frequency compensation today. Thanks!
 

MrChips

Joined Oct 2, 2009
30,924
It is set to 10x. I’ll be looking into adjusting the frequency compensation today. Thanks!
It is not that difficult. There is a probe compensation test signal at the right bottom corner of the oscilloscope.
Connect your probe to this signal. Trim the probe compensation on the probe for the flattest top and bottom of the signal trace on the screen.
(Do this for each probe. It is best to use one probe for each channel, i.e. don't move the probes around. This way the compensation is done with the probe on that channel. Use the coloured rings on the probes to match the probe with the channel.)

1710862868582.png

You want the scope trace to look like the last one on the right.
 
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