I'm building an H-Bridge to drive some motors I have around. I'd like to have a bridge that can be powered by different power sources, for example batteries or atx power supplies interchangeably.

I've attached a common schematic. Let's say transistors Q4 and Q1 are ON, so that the motor is spinning. If the transistors go OFF, then the back-emf of the motor will cause a current to flow from GND to the supply through D4 and D1. Am I right?

The problem is, I don't want the power supply to sink current. Maybe a battery can handle this current without any danger, but what about a generic SMPS?

A possible solution I've come across is the use of a sort of "shutdown phase". Doing so, the current flowing in the flyback diodes is kept in the h-bridge and doesn't go into the supply. As before, let's suppose that Q4 and Q1 are conducting. Instead of switching off them together, I switch off only Q1, and let the freewheeling current flow between Q4 and D1. Only after, Q4 is shut down too. Does this make sense?

If yes, could you suggest me how much time the "shutdown phase" could take? I mean, how can I figure out how long does it take to "discharge" the back emf of the motor?

Thanks, any help would be appreciated!

H-Bridge schematic


1 Answer 1


Fundamental to this type of diode protection is a large capacitor that can soak up the energy from the motors, via the diodes when all transistors are switched off. Going deeper, you can also use a high power zener diode across the rails and, going deeper still you can switch a resistor load directly across the power supply lines to dissipate the excess energy for the duration expected.

You can also protect your supply with a diode so that excessive voltages don't force current into the power supply.

The method you suggest will still raise the supply voltage so this could cause problems on a sensitive power supply.

If you can provide a link to the motor, it may be possible to estimate the electrical energy stored but, in many cases it will depend on the mechanical load that might be free-wheeling the armature and causing it to act like a generator.

  • \$\begingroup\$ The fact is, I'd like to have an h-bridge able to drive motors with different specs. I understand the concept of a large capacitor soaking up energy: where would you put it? In parallel with the supply? \$\endgroup\$
    – Cerv
    Commented Feb 20, 2014 at 9:23
  • \$\begingroup\$ I've found electronics.stackexchange.com/questions/80137/…. I think it answers my question. However, @AndyAka thanks! \$\endgroup\$
    – Cerv
    Commented Feb 20, 2014 at 9:45
  • \$\begingroup\$ @Cerv Phew I'm glad I suggested that capacitor!!! \$\endgroup\$
    – Andy aka
    Commented Feb 20, 2014 at 9:51

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