I wanted to disconnect power to a bldc motor from a battery (84V,32AH) using mosfets with a low rdson in a highside configuration. I want to do this in a situation where I have more than one battery and if one is discharged I can use the others. I designed a circuit where I used a 555 timer to create a square wave signal then I fed it to a gate drive transformer and then rectified it. In a case where I have multiple batteries, I will need to duplicate the circuit for each battery. I did this so that I can achieve highside driving of the mosfet. I simulated the circuit in ltspice and it worked there. I have shared the circuit below.

Please review it and tell me if it can work practically. Tell me any recommendations, where I can improve and any helpful information about the design and other alternatives.

I used an application note from texas instruments to design the gate drive transformer, link(https://www.ti.com/lit/SLUA618A). enter image description here

In the circuit, I used the 2n2222 bjt to control the turning on and off of the 555 timer by disconnecting the ground connection of the 555 timer.

  • \$\begingroup\$ If it's a BLDC motor, it has a driver, and that driver has MOSFET switches. So at least internally, commanding that BLDC motor driver to turn off should shut the motor off just as effectively as external circuitry, and it should do so with less system complexity. Why aren't you doing that? (It would be helpful if you would edit your question to mention this, because I'm not the only one who will wonder). \$\endgroup\$
    – TimWescott
    Jul 18, 2022 at 15:46
  • \$\begingroup\$ @TimWescott let me edit. Actually I want to switch between to batteries by turning off one and turning on the other \$\endgroup\$
    – p_karis
    Jul 18, 2022 at 15:54
  • 1
    \$\begingroup\$ Put that in the question then -- it's relevant. \$\endgroup\$
    – TimWescott
    Jul 18, 2022 at 15:55

1 Answer 1


The body diodes of the MOSFETs are still there if you turn them off! So the current of an activated battery will flow back into all other deactivated batteries.

16 pF as timing capacitor for NE555 is not ideal, input pin capacitance will add there. 47 kohm and 150 pF are a better combination.

You don't need a full bridge rectifier, remove D3, D4 and D5 and add a jumper in position D4.

EDIT: This is a bipolar switch, just shown with one MOSFET pair. You will add more FETs parallel to meet the 50 A requirement.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ thanks a lot for your help. Do you have any recommendations of the solution I can use to drive the motor with two batteries, i.e if one is discharged I can switch to the other. My other solution was to use relays. I tried using mechanical relays rated 110vdc at 50A but it they burnt. I am not sure if it was the type of relay that was not good or is something I did not design for. \$\endgroup\$
    – p_karis
    Jul 21, 2022 at 9:39
  • \$\begingroup\$ Thanks a lot for your help \$\endgroup\$
    – p_karis
    Jul 21, 2022 at 19:15

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