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
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.
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
(or up) the Gates and turn off the MOSFETs, not to mention add some static
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
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,
is connected to the motor's positive lead.
When A=0 and B=0, the motor is stopped. R2 pulls up Q2's gate and R7 pulls
the Gates of Q5 and turn off the MOSFETs.
When A=0 and B=1 (+5V), the motor is in reverse. Q1 is turned off, Q2 is
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
to motor's +ve contact to Q5 to -12V.
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
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
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.
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
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
out the motor noise.