I'm trying to use a MOSFET, an N-channel (specifically IRLB8721PbF), with a gate controlled by an AVR to power on/off a Raspberry Pi (powered through GPIO), I'm using it on the low side. (Both share the same +5 V rail).
When I measure the Raspberry Pi's current draw, it seems limited, and it doesn't operate properly.
On that MOSFET:
- Gate: To AVR digital pin (and pulled low with a 10k resistor)
- Drain: To Raspberry Pi GPIO ground
- Source: To GND
Naturally, my 5 V from my power supply is directly to the Raspberry Pi GPIO 5 V.
As a kind of experiment control: I just power the Raspberry Pi directly from my power supply 5 V/ground. I measure the current the Raspberry Pi draws through 5 V with a multimeter, and during power up it fluctuates from 0.15 A (up to about 0.45 A) and settles at about 0.35 A.
However, when I measure how much current the Raspberry Pi draws when connected to the MOSFET and when I turn the gate on -- it seems the current is limited and it gets up to about 0.30 A (approx) and never gets higher, and the Raspberry Pi seems to never fully power on.
I'm curious if I'm missing something here with trying to power this on. It is indeed a logic-level MOSFET, but, as an experiment I've tried powering the MOSFET at varying voltages with a bench power supply -- but no noticeable change by using more voltage (up to 10 V I tried.)
Granted, I am more of a programmer and electronics technician -- so I appreciate any electronics engineering input and advice, but, pragmatic solutions are most appreciated. I'm just so puzzled as the MOSFET should be rated up to 60 A, and I would've thought that just applying a logic-level voltage I would've been able to draw more current through the MOSFET.
Here's a diagram of the circuit (please note: I can re-create this situation without the microcontroller at all, and just with the Raspberry Pi + bench power + MOSFET, as diagramed). Note that in this representation the Raspberry Pi GPIO connection is simplified and in this diagram pin 1 on the jumper = 5 V, and pin 2 = GND.
Update: It turns out the voltage drop on the MOSFET is enough to give the Raspberry Pi fits. A friend of mine pointed it out in the data sheet, and here's a screen capture of the exact graph that shows where approximately 0.15 V are dropped, which can be measured on source and drain (as pointed out early on! great eyes guys, I really appreciate). I have decided to instead use a relay.