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I have created the following circuit where I use a VNH7070BASTR full bridge motor driver to control a DC-motor. The goal is to open and close a drawer using a rack and pinion system. A new requirement that I have is that the drawer can ALWAYS be opened in case of failure somewhere in the circuitry controlling the driver. I want to use an external switch to move the motor in one direction. But I am not sure how to design this correctly.

I thought of shorting directly using a relay or a FET. However I am not sure how to deal with an open state of the H-Bridge. How could one implement such override?

circuit

wardn
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  • I am using a rack and pinion mechanism in the center of the drawer. Thx for noting the error. – wardn Sep 20 '20 at 17:35
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    `failure somewhere in the circuit` includes the motor ... install a cutoff switch at the motor wires ... pull out drawer manually – jsotola Sep 20 '20 at 17:36
  • The goal is to not pull out the drawer manually – wardn Sep 20 '20 at 17:39
  • There's no way to do that which is immune to all failed states of the H bridge other thn a switch which disconnects the H bridge; if your backup solution has to use the same power supply, you'll need to disconnect both sides of the motor from the H bridge. Really a manual pull out is the only sensible choice here. – Chris Stratton Sep 20 '20 at 17:42
  • No crush protection? seriously? As long as some person can reach into the drawer while it operates you should design with compliance to functional safety – schnedan Sep 20 '20 at 18:05
  • if you want powered opening during failure, then disengage the main motor and deploy a secondary motor ... you could also deploy something like an automotive airbag system behind the drawer to push it open – jsotola Sep 20 '20 at 18:05

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