# Can I use an h-bridge as a changeover circuit?

I have solar controller which outputs battery voltage when available. I also have a switching psu as backup.

What I would like to do:

Use an h-bridge, connecting its battery negative terminal to my load and the two motor terminals one to psu negative and the other to solar controller negative.

The idea is: When solar controller is active, feed a voltage to the h-bridge direction input so that I can select my power source.

Can this work?

• Please draw a schematic or block diagram! Commented Sep 2, 2019 at 21:19
• What is B and what is M? But the answer is probably still no since your H-bridge probably does not use switches that can block current flow in both directions. I don't know why you wouldn't just use a much simpler relay. Commented Sep 2, 2019 at 21:51
• B should normally be connected to battery and M to motor. Commented Sep 2, 2019 at 21:59
• You can't just use things backwards like this. Even a relay based bridge likely wouldn't work unless you carefully thought through how the coils were powered. Commented Sep 2, 2019 at 22:16

The idea is: When solar controller is active, feed a voltage to the h-bridge direction input so that I can select my power source.

H-Bridge circuits typically use power MOSFETs, which have internal body diodes (shown in the simplified circit below as D1..D4). The diodes are intrinsic in FETs, but would still be necessary anyway to steer back-emf current through a motor or other inductive device.

Unfortunately, even if all the FETs are 'off' both supplies are connected to the load via diodes D2 and D4. So you cannot select the source using this method. It will however automatically pass through whichever source has the highest voltage.

Another problem is that the H-Bridge controller circuit is not getting power (normally supplied through B+ and B-). To power it properly you would need an isolated DC/DC converter with output voltage equal to or higher than both the solar charge controller and power supply.

simulate this circuit – Schematic created using CircuitLab

So all-in-all your idea just won't work. A practical solution for automatic switch-over would be to just use two high current Schottky diodes, possibly with individual FETs across them to get lower voltage drop. "Ideal diode" controllers such as the LTC4357 can be used with external FETs for low-loss automatic switch-over.

• Thanks Bruce, you made it clear. About the h-bridge circuit, the intention was to power it from a small psu. You said: It will however automatically pass through whichever source has the highest voltage. - - this would actually do exactly what I want even without a control circuit? Commented Sep 3, 2019 at 19:00
• Theoretically it should work if the controller is powered up properly. I wouldn't do it though - too many unknowns. I would use a circuit designed for the job, perhaps using Schottky diodes and a relay if an 'ideal diode' circuit is too complex or expensive. Commented Sep 3, 2019 at 22:00