I got a few isolated MOSFET modules recently to test with fans and LEDs. They are basically MOSFETs, but the gate is controlled by an optocoupler to isolate the control circuit from the load circuit.
I tried to see if I can get it to work with a 5V fan on the output side as I was thinking of using my 3D printer control board (running on 24V) to drive a 5V fan. But it seems the minimum voltage for the output side needs to be 9V; it would not run at 5V. The MOSFET gate just does not get enough voltage to open.
I could not find the schematic of this, so I had to do a bit of reverse engineering to try and find out why.
The MOSFET is a IRLR7843. Optocoupler is PC817. Standard stuff, I guess. There is a red LED when the load is being driven, connected to a current limiting resistor. Standard stuff too. The zener diode seems to be there to clip the gate voltage. I had to figure out how the rest of the resistors were connected using a multimeter.
This is the schematic that I think should fit this module.
The resistors R2+R3 and R4 will actually form a voltage divider that halves whatever is the voltage on the output side (VDD in the schematic). This is bad news if VDD=5V, since the gate voltage is then 2.5V, which according to the data sheet for the IRLR7843, seems to be too low for the MOSFET to work normally.
Otherwise, this module should work quite well; just remember to have VDD=9V at least. And the connectors used, while stamped with "10A" on them, don't look like they can handle that much current. I would probably limit any use to 2A.
I tried to see if I can get it to work with a 5V fan on the output side as I was thinking of using my 3D printer control board (running on 24V) to drive a 5V fan. But it seems the minimum voltage for the output side needs to be 9V; it would not run at 5V. The MOSFET gate just does not get enough voltage to open.
I could not find the schematic of this, so I had to do a bit of reverse engineering to try and find out why.
The MOSFET is a IRLR7843. Optocoupler is PC817. Standard stuff, I guess. There is a red LED when the load is being driven, connected to a current limiting resistor. Standard stuff too. The zener diode seems to be there to clip the gate voltage. I had to figure out how the rest of the resistors were connected using a multimeter.
This is the schematic that I think should fit this module.
The resistors R2+R3 and R4 will actually form a voltage divider that halves whatever is the voltage on the output side (VDD in the schematic). This is bad news if VDD=5V, since the gate voltage is then 2.5V, which according to the data sheet for the IRLR7843, seems to be too low for the MOSFET to work normally.
Otherwise, this module should work quite well; just remember to have VDD=9V at least. And the connectors used, while stamped with "10A" on them, don't look like they can handle that much current. I would probably limit any use to 2A.
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