Connecting diodes to a motor in H-bridge

Question

I'm attempting to design a three wheeled toy that is controlled by a remote connected with it via a cable. I first designed it with relays, however I had to change to transistors due to contrast between low power required by the toy itself and a high power demand of relays. I know that if there are vulnerable components in the circuit with a coil such as a motor, you should connect a diode in parallel with the coil in the reverse bias direction.

Here's a problem, in my H-bridge, current will flow both ways through the motors and I'm confused as to how to connect diodes to my motors to protect the transistors.

^{simulate this circuit – Schematic created using CircuitLab}

Also, if you have any idea how the design could be improved further, I'm all open for criticism. Note that on the schematic there are no specifications of the devices as these are liable to change with the choice of components available.

Answer 1

The diodes aren't protecting the motor; they are protecting the transistors. So put them across the transistors. The current-both-ways problem isn't present at the transistors.

Answer 2

Firstly I must point out that your circuit will NOT work as you have wired the transistors but assuming this is just a picture rather than an actual schematic read on...

You connect them across all four transistors in the H bridge. For instance Q1 emitter has a diode anode and Q1's collector has the diode's cathode. For Q2 the emitter has the anode and the collector the cathode. Here's a typical circuit: -

Note that this uses PNP transistors for the upper devices but the diodes still remain connected the same way as if you were using NPN devices in the upper positions. Note also that this circuit is likely to work (for a low power motor). For higher powers I'd use this FET version: -

What you will also find is that if there is a lot of energy stored in the motor, when the transistors are deactivated, that energy is dumped back to the supply voltage so you may want to consider a large capacitor to soak up that energy.

Answer 3

One option would be to not use any protective diodes, but to parallel the winding for each motor with an MOV and a smallish capacitor, while making sure that your transtors' VCE is higher than the MOVs' clamping voltage. The capacitor will help level the inductive flyback, and the MOV will prevent that spike from getting very tall.