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PIC and Mosfet

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Barry

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Hello all,
When controlling a DC motor's speed with PWM and a MOSFET with a PIC ( FET I believe stands for Fire Emitting Transistor!!!! lol ) - the DC motor draws approx 10 Amps - what would happen if the Mosfet goes open circuit? ( and I beleive this happens with FET's )
As I understand the working of the FET, the motor would go max rpm. ( or would it keep it's current setting or would it shut down? )

If this control with the PIC and MOSFET is very critical ie pumping fuel into a turbine engine, if the motor ( pump ) goes max rpm, the turbine rpm would go out of control. The rpm of the turbine is monitored by the ECU electronics and when rpm gets too high, the ECU would decrease pump rpm to get the rpm within set limits. When MOSFET goes open circuit, will I still have the normal PWM control over the pump? This is quite a serious safety issue!

My questions are the following:
1. Is there an alternative way to control the DC motor without using MOSFETs ( but still applying PWM )
2. Is there a possible way that the PIC can monitor the MOSFET and cause the system to shut down in the event of a MOSFET going open circuit? ie cutting supply voltage to the pump.
3. Can a PIC monitor itself or another PIC and in the event of a PIC dying or resetting, shut the whole system down?
All these issues are relevant to my gas turbine engine control project. With a turbine out of control in a radio controlled model aircraft it could be potentially VERY dangerous.

Thank you in advance.
Barry.
 
FETs do tend to fail as a short. A normally open mechanical relay would let you cut power to the motor. A current sense resistor would let you check if the motor is running away.

If you use a watch dog timer to keep an eye on the PIC you can use the watch dog reset to cut power to the motor.

Ive heard of safety critical systems that use 2 Micorcontrollers both running on the same clock running the same software. The outputs of the two systems are compared - if they are different something bad has happened and things should be shut down. However you might be better off just spending time testing and making your software more reliable.
 
MOSFETs are quite reliable as long as you've designed the circuit properly and used a quality component & vendor. Radio Shack isn't a great idea in this context.

You might explore the possibility to redundant transistors as well. Simply putting two MOSFETs in series and connecting their gates together will operate normally if one experiences a low impedance short. If it fails with a higher impedance, the worst that happens is the device gets less or no voltage. And the transistor may catch fire.

This will signficantly slow down the switching time, probably double, since the transistor tied to the rail must turn on first so the second even has a Vgs. The rdsOn of this setup is also double.
 
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