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I'm doing this project, where i control the speed of a motor with a 0-3.3 V PWM signal from a microcontroller. In the test setup i use a function generator (Digilent Analog Discovery). I would like the PWM to run at 50 kHz, but for better measurements, I have lowered the frequency to 15 kHz, 50 % duty cycle. My problem is, that there seems to be some sort of capacitance in my circuit, lowering the FET's rise time, when i test it.

The schematic: enter image description here

  • "Opdrifts_PSoC-Blæser_signal" is the microcontroller. It is here i supply the - PWM in my test setup.
  • "R6" is where the motor will be. In the test i use a 10 kOhm resistor.
  • "Scope" is where i measure relative to ground.

The measurement:

  • Output signal enter image description here Rise time is measured to about 13 us.

Hope somebody here can help me out.

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  • \$\begingroup\$ Is Opdrifts_PSoC-Blæser_signal a push-pull output from the microcontroller? Or open drain? \$\endgroup\$ – Steve G Nov 21 '16 at 17:07
  • \$\begingroup\$ Woops, forgot to write that i use a function generator as input in the test. I use a Digilent Analog Discovery. \$\endgroup\$ – Mads Stockfisch Lintrup Nov 21 '16 at 17:11
  • \$\begingroup\$ Whats your problem? What is your question? electronics.stackexchange.com/help/how-to-ask \$\endgroup\$ – Voltage Spike Nov 21 '16 at 17:13
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    \$\begingroup\$ Are you sure you put the "scope" node in the right place on the schematic? If that voltage changes at all, it's because your battery isn't doing a very good job supplying current to the "motor". \$\endgroup\$ – The Photon Nov 21 '16 at 17:20
  • \$\begingroup\$ Solved it now. It was the "motor". See answer below for details. Thanks! \$\endgroup\$ – Mads Stockfisch Lintrup Nov 21 '16 at 17:29
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The MOSFET drain has a lot of capacitance. The diode adds a bit, as does your 'scope probe. You should use a much lower resistance than 10K to simulate the motor.

Also, the 1N4007 is unsuitable for 50kHz, the reverse recovery time is of the order of 5-10usec (unspecified on datasheets). Use a Schottky diode such as 1N5819 for a low voltage motor (for higher voltages than about 30V-50V you can use an ultrafast silicon rectifier rather than a higher voltage Schottky) .

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    \$\begingroup\$ Oh god, I'm an idiot.. I just fetched a 10 kOhm pull up resistor from a logic level converter i had on the same breadboard. I've tested it with a 25 Ohm now, and it works. Rise time is down to 35 ns. Also good point about the diode. Haven't thought about that before. Thanks! \$\endgroup\$ – Mads Stockfisch Lintrup Nov 21 '16 at 17:27
  • \$\begingroup\$ Good to hear. The diode thing won't bite you badly until you have an inductive load on there. \$\endgroup\$ – Spehro Pefhany Nov 21 '16 at 17:32

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