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MOSFET N ch & P ch motor switch Schematic


MOSFET N ch & P ch motor switch Schematic

A dual supply is used to drive a motor. Only one supply is used at a time to actually drive the motor. This simplifies the circuit design down to two MOSFETs. The disadvantage is that each supply much provide adequate voltage and current to drive the motor.

Using the two MOSFETs allows the motor to be controlled in the stop, forward and reverse modes. An optional fuse test mode is provided also.

A P ch MOSFET is used to switch the positive supply to the motor for forward direction and an N channel MOSFET is used to switch the negative supply to the motor for reverse direction.

Figure 1- (8k) is a schematic of the dual supply N ch and P ch MOSFET switch discussed here. Figure 1 is a GIF file that can be opened by any Web Browser.


IMPORTANT:

MOSFETs are extremely static sensitive but more important is that if the Gate is left open (no connection), the MOSFET can self- destruct. The Gate is a very high impedance device (10+ megohms) and noise can trigger the MOSFET. Resistors R2 & R7 of Figure 1 have been added specifically to stop the MOSFETs from self destructing. It is very important to install these resistors FIRST before installing the MOSFETs. You will find that after these resistors are installed that the MOSFETs are quite stable devices. The resistors pull-down (or up) the Gates and turn off the MOSFETs, not to mention add some static protection.


Back EMF protection

D1 & D2 route back EMF from the motor back to the power supply. Some MOSFETs (actually most) have these diodes built-in, so they may not be necessary.


Control Transistors

Q1 & Q3 are NPN transistors that invert the control signals to Q2 and Q4 respectively. Q4 is a PNP transistor that controls Q5.

Looking at Q1 and Q2 operation only, we can see that a High at A will turn on Q1. This lowers the Gate voltage of Q2 to 0V. Q2 turns on, +12V is connected to the motor's positive lead.

When point A goes low, Q1 turns off, which turns off Q2 .

When point B goes High, Q3 turns on and current is allowed to flow from Q4's base. Q4 turns on and raises the Gate voltage of Q5 from -12V to ground potential (0V) by the voltage divider consisting of R6 and R7. Q5 turns on, -12V is connected to the motor's positive lead.


STOP Mode

When A=0 and B=0, the motor is stopped. R2 pulls up Q2's gate and R7 pulls down the Gates of Q5 and turn off the MOSFETs.


REVERSE Mode

When A=0 and B=1 (+5V), the motor is in reverse. Q1 is turned off, Q2 is turned off due to R2.

Q3 is turned on by the voltage at B. Q3's collector pulls Q4s Base to ground. This turns on Q4 which raises Q5's gate to +0V (due to the voltage divider of R6/R7). This turns on Q5. The +ve side of the motor is connected to -12V. The current path for the motor is from ground to thee motor's -ve contact to motor's +ve contact to Q5 to -12V.


FORWARD Mode

When A=1 and B=0, the motor is in forward. Q3 is turned off, Q4 is turned off and Q5 is turned off due to R7.

Q1 is turned on due to the voltage at A and Q1's collector goes to ground. This turns on Q2. Q2 raises the motor's +ve side to +12V. The current path for the motor is from +12V to Q2 to the motor's +ve contact to the motor's -ve contact to ground.


NOT ALLOWED Mode (or fuse test mode)

If A=1 and B=1 then both MOSFETs turn on which shorts out the power supply and lets the smoke out of the components - Not recommended.


Miscellaneous Info

The tricolor LED allows you to test the circuit without connecting the motor. The LED will be green for one direction and red for the other. Handy test.

Motors make a lot of electrical noise from the brushes when running and huge electrical spikes when stopping, starting and especially changing direction. C1 and C2 try to suppress the noise spikes. Negative spikes are shorted to either ground or the power supply by D1 and D2. Z1 tries to clip the positive spikes.

MOSFETs turn on very fast, if you have problems with noise, you may want to put 0.1 uF capacitors in parallel with R2 and R7 to slow the turn-on time. This will reduce the EMF generated by the motors. If the rise time is too slow the MOSFETs may heat up excessively!

Try to keep the motor supply separate from the logic supply if possible or go to extreme filtering techniques using coils, diodes and capacitors to filter out the motor noise.


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Disclaimer: The circuits described here are for reference purposes only.
There is no guarantee in any way shape or form that they will work for your specific application. Use them at your own risk.

Please send any corrections, suggestions or errors that you may have caught to me.

Copyright Eugene Blanchard August 2007

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