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Circuit Issue with MOSFET

wallarug

Hey,

 

I am having issues with this circuit:

 

image

 

Background:  This circuit is to control an RGB LED strip (so three of the above would achieve this).  The reason for this particular design is: IRF540 to drive load (large current) and the two 2N7000 drive the MOSFET, reducing current drain from Controller (possibly a 3v3 Arduino).  The double 2N7000 allows the IRF540 to be active when logic high.

 

Can someone please tell me if there is a problem with the above schematic, otherwise I think it would be my breadboard layout is not correct. (too hard to post that right now).

 

Any comments would be much appreciated.

 

Many thanks,

 

wallarug

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    To test the circuit as drawn connect the free end of R1 alternately to Vcontrol and ground - the output should switch.

    If it doesn't, check your wiring or swap components if the wiring is correct.

    If it works, you could replace the first FET with a lower threshold FET such as a TN0702.

    If you really need to use the FETs you have, you could put a pull-up resistor on the first gate and a pull down resistor on the arduino output such that the voltage at the arduino output is 3.3 V (without the arduino connected. This would allow the gate to be at a higher voltage, hopefully turning the FET on. When the arduino is connected it can pull the gate down, hopefully enough to turn the FET off. It may take some calculation or trial and error to get it set properly, but just be sure the control input voltage is 3.3 V before connecting the arduino.

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  • 0

    To test the circuit as drawn connect the free end of R1 alternately to Vcontrol and ground - the output should switch.

    If it doesn't, check your wiring or swap components if the wiring is correct.

    If it works, you could replace the first FET with a lower threshold FET such as a TN0702.

    If you really need to use the FETs you have, you could put a pull-up resistor on the first gate and a pull down resistor on the arduino output such that the voltage at the arduino output is 3.3 V (without the arduino connected. This would allow the gate to be at a higher voltage, hopefully turning the FET on. When the arduino is connected it can pull the gate down, hopefully enough to turn the FET off. It may take some calculation or trial and error to get it set properly, but just be sure the control input voltage is 3.3 V before connecting the arduino.

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