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HBridge for linear actuator with PIC

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New Member
I am trying to make a HBridge using BC327 and BC337 transistors. I am planning to use it with linear actuator PQ12 (datasheet attached).

I have spent extensive amount of time getting it to work with servo circuit. I basically tried to use the Servo HS55 and couple it with the PQ12. I could get the PQ12 to move in one direction with the servo PWM input but not in both the directions.:mad:

So finally I decided that I will use a HBridge to drive the PQ12 from my PIC16F MCU. I will toggle the outputs of PIC output pins to move the linear actuator.

I had 2 worries:::confused:

1) Will the hBrighe (schematic attached herein) be adequate for the linear actuartor.

2) should I add diodes or other protection elements to my PIC processor as safety/to protect from the hbridge.

I welcome your suggestion.

Best Regards,



Well-Known Member
Most Helpful Member
Yes, you should have a diode anti-parallel to each transistor to allow the inductive flyback current of the motor to flow, or else it will produce a large voltage spike and burn out your transistors.

THat H-bridge is suitable. Most transistors should work fine since the current is only around 250mA. I would use logic-level MOSFETs (NMOS and PMOS) instead of the BJTs (NPN and PNP) in the schematic. BUt either will work. Just with the MOSFETs you probably don't need the base resistors anymore and the voltage drop is lower across the transistor.

WHy wouldn't the servo circuit work? Seems to me like it should.

Do not PWM this H-bridge or switch it at high frequency- it will have a momentary short-circuit when both transistors are partially on every time you change directions or switch transistors on and you don't want this to happen too often or the circuit will be damaged. This is because the left and right pairs of transistors are being controlled by the same signal. If you wired up the PIC to control each transistor individually (4-pins) you would not have this problem since you could make sure all the right transistors are turned off before turning transistors on to prevent this "shoot-through current".
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