Hi There!
I am using a Standalone Atmega328P with some BS107 Mosfets. Somehow the mosfets wont switch. I supply the arduino with 3.3V and the Gate-Source threshhold voltage of the BS107 is at 3 Volts does someone have any idea why the mosfets wont switch? i tested the gate inputs the Mcu does output 3.3v
Hi, @Jan_Agner
The threshold figure is the voltage that the MOSFET begins to switch on, as the BS107 is not classed as a logic MOSFET, the response you are getting is understandable.
Also the fact that its Rds ON is typically 6R4 would not make it a very efficient switch.
What is the load that the MOSFET is supposed to be supplying current to?
Thanks.. Tom.. 
PS. you did measure the output voltage at the drains with loads in circuit?
Are you sure it is a BS107 and not a BS107A?
Otherwise, I see no reason why it should not work.
Thanks for your fast reply
I didn't notice the RDS-on 
The mosfet would be Switching some OPTOCouplers that consume roundabout 100mA (according to the manufacturer)
I did test it with an led and a 1kOhm resistor in series.
Its a BS107P but it shouldn't matter because the datasheet seems similar.
Was the LED connected to 5V or 3.3V?
I see no ground connection on your screw terminal.
Was the LED power supply ground connected to the 328 ground?
You are right on the edge of it being usable at 100mA
the led is connected between the +24V and the Drain Pin of the mosfet (Through the screw terminal).
The source pin of the mosfet is connected to the ground.
the problem is that the led is constantly on which means somehow the mosfet doesnt block the current, and i never get more than around 600mV from Drain to Ground.
the led is connected between the +24V and the Drain Pin of the mosfet
With a 1K resistor inbetween I hope.
If the gate is at zero volts, then I would say that the BS107 is damaged or you have a short circuit somewhere.
Hi,
The 24V supply negative is connected to the Arduino gnd?
Can you post some images of your project?
So we can see your component layout.
You have got the MOSFET oriented the correct way?
Have you measured the gate to drain voltage when the MOSFET is supposed to be switched OFF?
Tom...
Do tell.
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Schematic.pdf (42.6 KB)
I can at the moment only show the schematic.
It is wired exactly like in the schematic (ordered a custom pcb) so ground is connected everywhere. The mosfets should be oriented the right way ( i checked like 3 times).
I didn't meassure the Gate-Drain voltage directly but i meassured it over the pulldown resistors(the arduino does infact output the 3.3 V and pulls the pin low to 0V)
It is for a 750W industrial servo with integrated controler-
Measure the gate voltage with the ground lead of the meter connected to the MOSFET Source and let us know what you get. That part is border line with 3.5Vs on the gate, it does not get out of the ohmic reagon, I suggest you pick a logic level one with a lower Vgs.
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Seems unlikely that a digital input would need 100mA on any kind of controller. Same for an opto-coupler. Before you waste any more time with these MOSFETs, share some details of the controller so we can be sure you really need them at all.
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But you are also using a 16MHz crystal. Atmega328 isn't guaranteed to work at 3.3V and 16MHz. I think you need something more like 4.5V for 16MHz.
Why do you need 16MHz?
See post #8
If it was flipped it would always be On.
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Hi @Jan_Agner
Do you have a component tester?
Have you tested your MOSFETs with a DMM, check on you have used and an unused device.
Can you post an image of your PCB please, the CAD should be able to export a jpg image.
Tom..
OK I found the issue i've rotatet top mosfet (Q6 in Picture) and this one works fine now.
somehow the drain and the source are rotatet in mine
Thanks for your help everybody.
Did you find it on the 4th check?