I have two H-Bridge drivers controlling two geared DC motors. These are the modules I am using : https://www.handsontec.com/dataspecs/module/BTS7960%20Motor%20Driver.pdf

I have two of the above modules, sharing a common 8V power supply. I get some odd behavioir when attempting to control one of the motors with PWM. At low speeds, the other motor will turn slightly. If I turn one of the motor shafts by hand, the other motor will also turn.

I'm assuming the issue is due to the sharing a common supply. Is there anything I can use to isolate the two?


  • \$\begingroup\$ When you say "turn slightly" do you mean turn momentarily or do you mean turn slowly? And under what conditions are you turning the shaft by hand? Powered? Unpowered? \$\endgroup\$
    – DKNguyen
    Commented Mar 29, 2021 at 19:57
  • \$\begingroup\$ I was turning it by hand, unpowered.. i.e. The system completely disconnected from the power supply. \$\endgroup\$
    – Dibly
    Commented Mar 29, 2021 at 20:03
  • 1
    \$\begingroup\$ That's just normal because what's happening is it is acting like a generator and the current is finding its way through the parasitic body diodes of the MOSFETs or flyback diodes in the H-bridge to the power terminals and then finding its way through the same avenue through the other H-bridge to the motor. Do you really need to handle that while unpowered? Turning slowly while the other motor is running is a lot stranger. I would think that's noise but I would have to see it. Is it erratic? \$\endgroup\$
    – DKNguyen
    Commented Mar 29, 2021 at 20:04

1 Answer 1


I see that you reckon that you've found the cause but it is possible for one motor to act as a generator and power the other.

enter image description here

Figure 1. When turned by hand the motor will act as a generator and current will flow in through the intrinsic MOSFET diodes (not shown on the schematic symbols) as shown by the red arrows. The reverse polarity MOSFET at (2) is good at preventing reverse polarity but not reverse current. Since its gate is pulled low at (1) it may turn on and pass current out onto your 8 V bus.

This in turn powers up the other board which may run depending on what happens its microcontroller when it gets some unexpected and badly regulated voltage.


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