Why PWM waveform has negative oscillation between pulses when should be 0V

• Why PWM waveform has negative oscillation between pulses when should be 0V
  Posted by alanvernon on 01 Jul, 2022 09:23
• I am new to oscilloscopes
  
  I just bought a Hantek DSO2D15 to try and resolve an issue with a laser driver board. The laser driver is controlled via a 24V PWM fan control supply from an Ender 3 Pro.  My first step was looking at the PWM supply.
  
  The PWM seem to be working correctly, in that, adjusting the fan speed changes the size of the pulse.
  The problem is between the pulses instead of going to 0V a sinusoidal shape wave of primarily negative voltage occurs. See Photo 1.
  
  What is causing this?
  
  If I connect the oscilloscope in reverse I actually get the correct shape with no sinusoidal shape wave between pulses. See Photo 2.
  
  Why the difference when connected in reverse?
  
  Any help will be appreciated
  
  Alan Vernon
  
  Why the difference when connected in reverse?
  
  Any help will be appreciated
  
  Alan Vernon

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• #1 Reply
  Posted by Terry Bites on 01 Jul, 2022 12:31
• That looks like it might Ground Bounce. The ground impedance may be too high either in your sytem or in your probe.
  Check your scope probe configuration. Use the barrel probe tip https://www.electronicspecifier.com/products/power/oscilloscope-probing-techniques-for-measuring-power-supply-ripple
  
• #2 Reply
  Posted by alanvernon on 01 Jul, 2022 21:14
• Hi Terry
  
  Thank you for the feedback. I read the article and shortened the ground loop to tip and barrel but sorry to say it made no difference in amplitude or wavelength of  the unwanted oscillations.
  One other test I had done previously was to drop the 24V down to 5V using resistors and a Zener diode. This worked perfectly for the pulses however the amplitude and wave length of these unwanted oscillations never changed.
  
  Appreciate any other suggestions.
  
  Thanks
  
  Alan Vernon
• #3 Reply
  Posted by bdunham7 on 01 Jul, 2022 21:34
• Try using the math function (CH1 - CH2) with two probes and no ground clips. 
• #4 Reply
  Posted by alanvernon on 02 Jul, 2022 00:41
• Hi
  
  Thank you.
  
  I did investigate the math function for the first time and connected  using two probes as suggested. I did see some interesting displays but they did not make sense I would have thought - (minus) would have canceled out but didn't. I am probably not using correctly however I did some more checking and found this oscillation is mostly likely something to do with leakage/noise from mains (power supply) as it is 50HZ (New Zealand AC Power  Frequency)  See attached image. The image was taken with the probe connected to the +/_ of the Fan PWM output of an Ender 3 Pro. The fan was set to 0 (Zero) so the output should be 0V.
  
  How does one get rid of this leakage/noise?
  
  This also raises another question about PWM. I though normally fan control was in the khz range not in the low hz range. Am I missing something?
  
  
  Alan Vernon
  

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• #5 Reply
  Posted by Krisby on 04 Aug, 2022 03:34
• Hello Alan,
  
  I probably know a lot less than you, but I would try out the 'scope on a small DC to DC PWM module, such as they sell from China for about $5, or any similar PWM module on some other device.  That way you could determine if the problem is with your 'scope, your set-up of the 'scope, or the PWM on your Laser Drive Board.
  
  Krisby.
• #6 Reply
  Posted by alanvernon on 04 Aug, 2022 05:08
• Hi
  
  Thank you for the reply. Have been on holiday but intent to get back to the problem within a few days. Based on an internet search it look like the issue of the low frequency of the PWM is software related. I have to figure out and compile my own version of Marlin that will work on the Ender 3 Pro printer board.
  
  This still leaves me this strange issue of the noise from the power. I notice that even if I drop the voltage to 5V ttl levels the noise level stays the same.  I may follow your idea of getting a separate board for test purposes to verify the PWM and see if any noise that could be related to oscilloscope or its leads.
  
  Thanks
  
  Alan Vernon