What is wrong with this circuit?


I've performed the following tests in my circuit:

  • When I set the controller output pin PD7 to HIGH, the source/drain voltage is 0V (measured with oscilloscope). Signal is clean.

  • When I set the controller output pin PD7 to LOW, the source/drain voltage I get is a aproximate 0.5V and it's a sinewave. That means the MOSFET is not switching off (completely).

The signal from Arduino (digital OUTPUT) tested with oscilloscope, it is clean. Goes HIGH and LOW 5 V. And when I remove output wire from Arduino, MOSFET switches off and then I get clean 12V Vds.

Sensor that's wired to MOSFET is a ultrasonic sensor (VALEO Automotive). All I know about it is that the on signal wire (bidirectional, half-duplex) is 12V and 1mA.

Thank you guys! It was very helpful for me. I know that there is better solution for MOSFET then the one that I used.

I have rebuilt my assembly...I made all the wires short as possibly and voila it works!

I am not sure but I think there was a problem in Ground connection!

Also i had some confusing expirence month ago, with unloaded MOSFET and Osciloscope. That was a PWM Modulator (from PWM OCB - RMCyber..... i did some measure with osciloscope! Signal that i had with unloaded and loaded device was completly different. Signal with loaded device was OK, nice und clean PWM Signal. But PWM signal from PWM Modulator without any Load was confusing.

I thank your...i learned something new! Not everything, but step by step.

  • 1
    \$\begingroup\$ Did you try changing or removing the resistance? \$\endgroup\$ – nidhin May 20 '14 at 13:18
  • \$\begingroup\$ no, i did not! Should i remove resistance on Gate? \$\endgroup\$ – user43105 May 20 '14 at 13:21
  • \$\begingroup\$ Is there a safety issue in removing it? If not try it. \$\endgroup\$ – nidhin May 20 '14 at 13:27
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    \$\begingroup\$ Sounds like the MOSFET is switching off, and you're picking up 60Hz hum on the drain from the AC mains. Is the sine wave 60 (or 50) Hz? Is your scope probe well grounded with a short lead? If you use a voltmeter to measure the drain voltage what DC value do you get? \$\endgroup\$ – John D May 20 '14 at 14:33
  • \$\begingroup\$ The first thing that comes to mind is it looks like you're not powering the MCU at all. No wonder it doesn't work! \$\endgroup\$ – Connor Wolf Aug 27 '14 at 0:55

Your selection of transistor IRF520 if ok. The required gate threshold is 4 volts or less, as seen on data sheet seen here : Data sheet Your Arduino is putting out 5 volts.

The drain load of your device seems ok with being a 12K equivalent and drawing only 1mA at 12 volts is also ok. The IRF510 will turn off even with a high resistance device in the drain circuit.

Your gate resistor is also ok.

Replace your load device with a 12K resistor and test again. You have shown that your transistor will be off when you disconnect the Arduino pin from the gate (resistor pulls gate low).

Since this is a DC circuit, use voltmeter rather than oscilloscope. The problem is in the measuring and possible ground conflicts. Make sure that your 12 volt supply ground has a good connection to the Arduino ground.

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