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Depletion mode MOSFETs

A project log for Reboot-o-matic

Automatic power-cycling made safer

Nick SayerNick Sayer 10/23/2019 at 23:294 Comments

Just before someone brings it up... the two P MOSFETs could be replaced by a depletion mode P MOSFET. Depletion mode MOSFETs work just like the more ordinary enhancement mode devices, with the sense of the gate being backwards. Where increasing the amplitude of the gate-source voltage would turn an enhancement mode MOSFET on, doing so with a depletion mode device turns it off instead.

Unfortunately, depletion mode devices are out of the ordinary, so the prospects of using one - particularly one that can pass 2 apps continuous - are poor. 

Discussions

Ken Yap wrote 10/23/2019 at 23:40 point

Couldn't you give up having a common negative rail for RPi and the router, and use a N MOSFET in the negative lead? N MOSFET switch breakout boards are very cheap, a buck or two IIRC.

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Nick Sayer wrote 10/24/2019 at 00:07 point

Don't forget that the actual control of the power is being given to the local microcontroller so it can enforce added safety rules. Lifting an N MOSFET gate up from the supply rail means coming up with a boosted supply for the controller. That's certainly doable, but it's more complicated than the 3 MOSFET solution I've got here.

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Ken Yap wrote 10/24/2019 at 00:13 point

Ah,  you're leeching power off the router's wall wart then.

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Nick Sayer wrote 10/24/2019 at 00:16 point

Yup. Check out the schematic in the files section.

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