Troubleshooting Mosfet Lock on DC Motor Remotely

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Discussion Overview

The discussion revolves around troubleshooting a circuit involving a DC motor controlled remotely using PWM and RF signals. Participants explore issues related to the operation of a MOSFET used as a switch in this setup, particularly focusing on the inability to turn off the motor after it has been activated.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes their project setup, noting that the RF signal can turn the motor on but not off, raising questions about the MOSFET's behavior.
  • Another participant suggests that the lack of a resistor from the MOSFET gate to ground might be causing the MOSFET to remain on due to high gate capacitance.
  • A participant provides links to external resources for the RF and PWM circuits, along with specific resistor values used in the MOSFET circuit.
  • One participant reports success after using a voltage regulator to supply the RF receiver with the appropriate voltage, which resolved their issue with the circuit functioning correctly.

Areas of Agreement / Disagreement

Participants express various viewpoints on the potential causes of the issue, with no consensus reached on the exact problem. Some suggest specific circuit adjustments while others share their experiences, indicating a mix of agreement on troubleshooting steps but disagreement on the root cause.

Contextual Notes

There are unresolved questions regarding the impact of the resistor configuration on the MOSFET's operation, and the discussion includes assumptions about circuit behavior that may not be fully validated.

dcd528
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I have a hobby project that involves controlling a dc motor remotely. I used a pwm circuit to control motor speed. I also have a rf circuit that sends a signal that can be used to turn the motor on/off. Both circuits work when used independently, that is the rf signal will turn a motor on/off (no speed controller involved), the pwm circuit will modulate the speed of the motor (no rf component involved). the problem is I tried to use a mosfet as the switch, controlled by the rf signal. The rf signal turns the motor on but it doesn't turn it off.
The entire circuit is being powered by a 9 volt battery (small one with the snap on clips).
Any suggestions as to what I am doing wrong would be welcome. I am a tinkerer not an EE.
thanks much
Dave
 

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I'm having an issue with seeing the text on all of the items. Can you possibly switch to the PNG format? It is a lot better for text/line items when compared to JPEG. Also can you give the resistor values?
 
it's hard to see some of it, but one of the mosfets doesn't have a resistor from gate to ground. perhaps the gate capacitance is high enough to keep the mosfet on for some time?
 
for the rf circuit see http://www.rentron.com/Files/TWSAKIT.pdf
for the pwm circuit see http://www.electronickits.com/kit/complete/motor/ck1400.pdf
the resistor values for the mosfet that is receiving the signal from the rf are 1K between rf and gate and 100 K between gate and ground.
The mosfet that doesn't have a ground resistor is within the pwm circuit, that circuit works on its own. Could the lack of the ground resistor cause an issue when coupled with the rf and switching mosfet?
thanks
 
Last edited by a moderator:
I had the receiver wired to 9 volts, it has a range of 4.5 to 5.5, when I added a 7805 voltage regulator so the receiver was supplied at 5 volts and delivered a 5 volt trigger to the mosfet the circuit worked fine.
One more hurdle and the project will be complete!
Thanks to all for help and suggestions
 

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