We're designed a circuit based on an atmega2560 and wanted to turn on or off some parts of our pcb.

So we've installed an logic level mosfet (IRLL024N) that was supposed to act as a switch on our 5V line.

The mosfet is controlled by a digital pin of the atmega set to high or low to turn on or off the rest of the circuit.

The problem is that we are experiencing a huge voltage drop on the mosfet when it's turned on.

The source voltage is 4.93V the gate is 4.89V but the drain voltage is only 3.02V

Why is this happening?

Below is our schematic enter image description here

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    \$\begingroup\$ That is an N-CHannel device, Unless the control signal is being boosted over 5V it wont work the way you expect. You need a P-Channel. Either that or switch it on the low side. \$\endgroup\$ – Trevor_G Sep 19 '17 at 14:41

You must use a p-Channel MOSFET.

IRLL024N is an n-Channel MOSFET, therefore there will be a Vth-Vov drop, between the gate and the source (Vov = overdrive with respect the threshold voltage, to allow the load current to flow into the MOSFET).

Otherwise, you must use a VGS value much higher than the drain voltage (e.g. the gate must be 10V).

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    \$\begingroup\$ +1 or as I said in my comment, "Either that or switch the circuit on the low side." \$\endgroup\$ – Trevor_G Sep 19 '17 at 14:46
  • \$\begingroup\$ Thank you. That was what I read a little bit after I posted my question, but it's good to confirm it. Do you have any suggestion for a similar P-Channel Mosfet? \$\endgroup\$ – Lefteris Sep 19 '17 at 15:49
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    \$\begingroup\$ If you just need to switch 5V, then you can use, for instance, a FDT434P (I just went on digikey, and chose a pMOSFET with the same SOT223 package, a current larger than 3.1A, a similar or smaller RdsON, and a logic-level drive. Beware that, unlike yours, FDT434P has a maximum gate to source voltage of only 8V - yours has 16V-, and a maximum drain to source voltage of 20V -yours has 55V. It has a smaller RdsON, and a larger current). \$\endgroup\$ – next-hack Sep 19 '17 at 16:09

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