# Low-side MOSFET: Current seems limited

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.

• What do VGS and VDS measure? Sep 2, 2013 at 18:03
• Could you put together a schematic or if not, a hand-drawn connection sketch of what is hooked up to what and at what voltages, please? Sep 2, 2013 at 18:07
• What gate voltage does the MOSFET get to power the raspberry? Sep 2, 2013 at 18:11
• @AnindoGhosh -- diagram added, good suggestion. Sep 2, 2013 at 18:59
• @IgnacioVazquez-Abrams -- Vgs (is that the voltage at the gate? I'm assuming) is 5.1v in the posted diagram (with just bench power, no micro), from the micro it's more realistically 4.3v. How do I measure Vds? It's on the low side, should it not be zero? I'd love to learn. Sep 2, 2013 at 19:00