Mosfet on Breadboard seems to not be working

[hendrsop]

In my experiment I am using a breadboard that connects to a conductivity sensor that is placed in a neutralized solution of baking soda and vinegar, which is then reconnected back to the breadboard, then runs through a mosfet which is suppose to close its gate to complete the circuit to run a pump that is also connected to the breadboard. The mosfet's threshold is only 1-2 volts but it won't send the electricity through to the pump. I used a multimeter to test the source leg and drain leg and each of those legs were receiving 11 volts which I thought would complete the circuit since it exceeds its threshold but no electricity was hitting the gate leg. Is there a specific way to get a mosfet to close its gate? Why isn't the mosfet closing its gate if the voltage it's receiving is exceeding its threshold?

[rmarz]

hendrsop - Not knowing what your circuit diagram is, there are several questions I would ask.

1) Is your MOSFET a p-channel or n-channel device?
2) Is your MOSFET an 'enhancement' or 'depletion' mode device?
3) Are you biasing the gate with the proper polarity voltage?
4) Have you been very careful handling the device so that it hasn't been damaged by stray static electricity damage?

If you are sure your selection of MOSFET is correct for your circuit, and it hasn't been damaged, this should be a fairly straightforward application for your experiment. If you can tell us more about your device part number and perhaps share your circuit diagram or reference to the experiment you are working on we can probably be of more assistance.

Rick Marz

[bfinio]

Rick - just for your reference, based on the students' description it sounds like they are doing this project, which has the circuit diagrams in the procedure: https://www.sciencebuddies.org/science- ... l-pancreas

[rmarz]

hendrsop - Ben Finio and I have been communicating about this issue. I have seen the schematic diagram in the 'Help' section of the experiment - (https://www.sciencebuddies.org/science- ... l-pancreas) - and it is correctly described in the 'breadboard' wiring diagrams. We're confused with your result of measuring 11 volts on both the Drain and Source terminals of the MOSFET. In essence, measuring voltage across the Drain and Source. Assuming you are measuring from the negative polarity of the battery, what I would call 'ground', you should only get a voltage measurement on the Drain terminal. Where are you making this measurement from? All the comments so far seems to suggest these two possibilities:

1) The breadboard is not wired correctly, check against the breadboard photos in the experiment
2) While unlikely, the MOSFET itself could be damaged and not operating.

As a quick test, after checking the breadboard wiring, place a jumper wire into the breadboard replacing the Conductivity sensor. That should guarantee that the MOSFET gate is biased to turn on and the motor would start. Let us know if you have success.

Rick Marz