Protect circuit from current generated by backdriving DC motor

In our application we have two modes of vehicle operation: manual and autonomous. Meaning that when vehicle is operated autonomously we control a brushed DC-motor using an H-bridge. In manual mode we don't control it, DC motor will see high impedance on its leads and can be backdriven. So in manual mode we want as less resistance as possible to backdrive the motor and we want to protect our circuitry from charge generated by the motor and the motor itself. Is there a common practice for that?

Below is the relevant part of the circuitry that we are going to use to drive the motor, it consists of a gate driver and an H-bridge.

• What's backdriven? What resitance as less as possible? The electromechanical machine can be a motor or generator, meaning positeve or negative torque, with leads disconnected or high impedance, neither motor or generator mode exists. – Marko Buršič Nov 23 '15 at 23:03
• @MarkoBuršič I mean when you manually try to rotate shaft of the motor it should be easy, no mechanical resistance, no opposing torque. – DikobrAz Nov 23 '15 at 23:13

Basically you need a diode in series with the supply to the motor -- that will prevent the motor from supplying power back to the supply.

Trouble is the diode will drop 0.7 V and will dissipate power -- you can use a MOSFET in the supply or GND connection and drive it on when the motor is on, and off when the motor is off. You'll have to connect that FET with drain & source backwards from normal usage.

• Do I need to connect diodes like in this question? electronics.stackexchange.com/questions/80137/… – DikobrAz Nov 24 '15 at 4:38
• No. Basically connect the new FET between the (drain of U2, U4 & C3) and the VM supply. FET S = VM; D = U2/4/C3; Gate = charge pump high voltage when motor is on, gate = 0 V when motor is off. What supply V are you using ? – jp314 Nov 24 '15 at 5:10
• I see, I use 24V. But I don't really get it, if I turn off both U2 & U4 isn't that equivalent? – DikobrAz Nov 24 '15 at 6:49
• no, by their construction, they have diodes in parallel with them. If you are worried about the motor driving the supply, you need to isolate it. The issue will only arise if the motor was spinning at close to full speed (i.e. unloaded), and the supply dips below that value after the motor turns off -- this is the only way the motor can generate a voltage higher than the supply. – jp314 Nov 24 '15 at 15:08
• thank you! I finally got it and that explains why we see motor powering the rest of the circuitry when it's off when you backdrive it and why you experience opposing torque. – DikobrAz Nov 24 '15 at 19:38

All you need to do to transition from automatic into manual is disconnect the motor from the H-bridge using switches or relays. The switches will provide electrical isolation from the motor, preventing circuit damage. Having the motor 'open-circuit' will minimise the torque required to turn the motor shaft.

• Motor is used to drive the steering column of the vehicle and we want to be able to steer vehicle manually and programmatically. We will have a switch between manual and autonomous mode. – DikobrAz Nov 24 '15 at 2:48