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I have a n channel MOSFET controlling a high current LED panel. The FET is driven by a FET driver, which is itself driven by a MCU pin. I am using PWM to control the LED brightness.

I want to double check the PWM with my scope, to ensure the duty cycle is correct. I can check all along the circuit, and I get proper readings from my scope. If I check the outputs from my FET, I get a garbage reading.

This is because, I believe, when the FET is off, the ground clip is disconnected from the ground reference, and it is left floating.

How can I check the FET output with my scope, since whenever the pulse is low, the ground reference is no longer connected?

EDIT: PWM should be 0-12vdc square wave, 5khz, 10% duty cycle. Also, when I use with the LED, it dims properly as would be expected with PWM, so I'm almost positive it is outputting correctly.

img 1 Scope probe is at 12v, grd clip is at OUT_LED_GND

img 2 Scope output

img 3 Scope connections, output LED

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  • \$\begingroup\$ What point are you probing, and where is the ground clip? \$\endgroup\$ – Phil G Nov 13 '18 at 20:47
  • \$\begingroup\$ @PhilG added to original post \$\endgroup\$ – Kyle Hunter Nov 13 '18 at 21:00
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    \$\begingroup\$ A scope isn't made to see its ground changing - it's not a handheld battery powered voltmeter. Instead, use the probe to see the LED's cathode change relative to the power supply ground shared with the scope. You'll see what you want to see, it's just that the waveform will be upside down from what you expect. A modern scope may be able to flip it. Or you can get out the other probe, put it on the LED anode, and select the subtract function. But most people are fine with seeing the cathode voltage as a negative pulse rather than seeing the voltage across the LED as a positive one. \$\endgroup\$ – Chris Stratton Nov 13 '18 at 22:54
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    \$\begingroup\$ (You will need something connected across the LED output terminals - either the LEDs or the resistor or both) \$\endgroup\$ – Chris Stratton Nov 13 '18 at 22:58
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    \$\begingroup\$ @ChrisStratton that worked perfectly. Thank you! I'd accept your answer if you posted it as one! \$\endgroup\$ – Kyle Hunter Nov 13 '18 at 23:52
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For the record, your LED (D2) is connected backwards. But...

  1. Place a resistor to Vdd instead of the LED. That would provide you with a +12V square signal driven by the FET.
  2. Connect the ground of your oscilloscope to the ground of your circuit (it is likely to already be connected, BTW).

I believe you already have a resistor in that position, so the most likely reason for the waveform you are seeing, is that the oscilloscope is shorting the FET. That is you are connecting one side to ground, when the oscilloscope itself is connected to ground. If the oscilloscope was floating, you should be able to see the signal on the resistor.

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  • \$\begingroup\$ Well D2 is a flyback diode for inductive spikes, OUT_LED_GND is what goes to my LED panel. But with it being a lowside switch, the gnd from the FET is what is switching, not the 12v. That's the problem I'm having \$\endgroup\$ – Kyle Hunter Nov 13 '18 at 21:26
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    \$\begingroup\$ If your oscilloscope is grounded (they normally are) your FET is not switching anything as its drain is already grounded. \$\endgroup\$ – Edgar Brown Nov 13 '18 at 21:49
  • \$\begingroup\$ I can buy that. But even with a bigger load the same thing happens. The issue is that when the FET is off, the ground clip is attached to the drain, which isn't ground anymore, and is left floating? \$\endgroup\$ – Kyle Hunter Nov 13 '18 at 22:39
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    \$\begingroup\$ Point 2 is key here. The scope ground goes to the circuit's supply ground. The scope probe should go to the FET's drain aka OUT_LED1_GND where the ground clip was mistakenly put. A resistor in place of the LEDs probably won't be necessary but in the unlikely event that the probed signal isn't going high with the FET off one could put a high value pullup resistor in parallel with the LEDs \$\endgroup\$ – Chris Stratton Nov 13 '18 at 22:48

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