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In my circuit there is a microcontroller, that can turn on and off the Raspberry PI power soruce. The schematic is following: enter image description here

A microcontroller GPIO is connected to RP_SW and 5v_Raspberry is the power supply of the Raspberry. The Q9 FET and DT_1 transistors with R65 and R66 are making an ideal diode, I took the idea from here (The aim of this diode is to avoid back powering).

The problem is, that the 5V_Raspberry will be only ~2.3V, the rest of the voltage is falling on the drain-source of the Q8 FET. I tried to pull up the gate of the Q8 FET to 5V, but there is no change. I tried an other FET instead of Q8, but the same effect is shown. Why is this circiut not working, as excepted?

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  • \$\begingroup\$ Because that's how NFETs work. \$\endgroup\$ Oct 7, 2015 at 7:24

1 Answer 1

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Here.

Throws the manual across the room

Edit: You cannot control an NFET on the high side of a load with 3.3V. The ON-ness is controlled by Vgs, which would need to be 5V+Vgs-sat, which is like 8-10V if this is a logic level FET.

I can't see a reason you need the blocking action of the FET here, unless you have a circumstance where the Raspberry Pi is getting power from somewhere else. You can slap that circuit in series with the PFET switch if it works, or get creative and combine them which I think is possible.

PFET high-side switch

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  • \$\begingroup\$ You can downvote me, but there is so much wrong with the circuit in question, there's not much I can say to fix it other than to say "use the literature and do it the way they say" \$\endgroup\$
    – Daniel
    Oct 7, 2015 at 7:43
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    \$\begingroup\$ I liked your original answer btw +1 \$\endgroup\$
    – Andy aka
    Oct 7, 2015 at 9:26
  • \$\begingroup\$ @Daniel: I don't see, why is this offensive "Throws the manual across the room" answer is necessary. I used FET-s before always in such cases, when the source was on ground. In this case I simply overlooked it, that in this case the source is not on the ground, and tried to find solution like it would be on the ground. \$\endgroup\$
    – Milan Tenk
    Oct 7, 2015 at 12:16
  • \$\begingroup\$ I see no one here has a sense of humor... \$\endgroup\$
    – Daniel
    Oct 7, 2015 at 14:10
  • \$\begingroup\$ At least you have Daniel. \$\endgroup\$
    – Milan Tenk
    Oct 8, 2015 at 19:34

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