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I'm trying to drive a motor for an academic project, using the circuit in the image. The STM32F4 Discovery board that I'm using in this phase of the project produces a 30Khz PWM at 3V, that is correct (checked with an oscilloscope) but the motor does not turn on. Anyone can suggest me some motivation for this behaviour? Thanks, Daniel

enter image description here

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  • \$\begingroup\$ Is that really how you have wired it up or just your intentions? and equally this STM32F4 board, does it share the same 0V \$\endgroup\$ Commented Oct 26, 2014 at 18:04
  • \$\begingroup\$ The circuit is that I already have, given from an old project by the professor (I have to integrate it with a microcontroller). In this phase I'm checking the circuit but does not work, the circuit consist in 4 of this patterns, for 4 motor of a quadcopter. Sorry but I haven't understand your question about the STM32F4, thank you \$\endgroup\$ Commented Oct 26, 2014 at 18:09
  • \$\begingroup\$ Did you test the circuit without the mcu ? Try connecting the top 220 ohm resistor to some voltage between 5 and 10 volts. Does the motor run ? Also, are you sure you have got an IRL530 and not an IRF530 mosfet ? \$\endgroup\$ Commented Oct 26, 2014 at 18:13
  • \$\begingroup\$ My question about the STM32F4 is, does it share the same ground as the FET \$\endgroup\$ Commented Oct 26, 2014 at 18:18
  • \$\begingroup\$ @JonRB The STM32F4 is linked to the usb port of my laptop, which is linked to the power supply (house electrical system), and the motor supply is linked to the same system, so I think yes \$\endgroup\$ Commented Oct 26, 2014 at 18:27

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You need to ensure that the PWM source & the FET are referenced to the same potential.

FET's turn on when the Voltage between their GATE & their SOURCE is greater than their threshold (actual characteristics are then determined by the driving characteristics.

If the 0V of the PSU powering the electrical machine is say... 5V higher than the 0V that is powering your controller ( STM32F4 ) then you will not be able to turn on the FET.

how are the the two 0V connected ? what is the potential difference between the two 0V (the laptop/USB 0V and the PSU 0V)

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  • \$\begingroup\$ Yes it was the problem...I have solved the problem connecting the GND of the STM32F4 with the power supply GND \$\endgroup\$ Commented Oct 27, 2014 at 13:53
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Use the 12V and a push/pull totem pole BJT circuit to make a 12V 'voltage' interface to the gate of the N channel MOSFET you have there. Then you can drive it hard and fast (30kHz is pretty fast, you need high current to switch the FET on efficiently) with as much of the 12V as possible. Your microcontroller will benefit from even more abstraction away from the inductive load, and you can source hundreds of milliamps rather than the limit of a little 3V low power PWM output of the STM32F4.

Page 12, in Figure 10 of this App note by Texas Instruments shows the totem pole driver circuit to drive a MOSFET.

This will fix your gate drive voltage issue, and give you awesome benefits of faster, more efficient switching, more isolation of the sensitive microcontroller from the inductive feedback of the motor, and it's a great lesson in utilising a voltage interface to drive FETs which may not have nice and low voltage gates, allowing you to use almost any standard FET, even the ones with 10V Vgs-thresholds.

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@JonRB I think is a problem about the powersupply potential as you already suggested. Trying to drive the circuit with 5V constant from the powersupply directly in the gate the motor works, providing the same 5V from the pin of the STM32F4 the motor is off. How can I solve this problem? Thanks

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    \$\begingroup\$ you need to put this as a comment to your own question, not as an answer \$\endgroup\$ Commented Oct 26, 2014 at 21:22

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