Electronic – MOSFET always slightly on and other strange behaviour

ledled stripmosfetrgbtransistors

I'm trying to control my RGB LED strip with my arduino, using IRLZ34N MOSFETs according to this schematic

schematic

simulate this circuit – Schematic created using CircuitLab

I tried it out, everything worked. I soldered it to a circuit board just like that, except for that I used 4 copper wires on the Arduino connections. Everything worked, I unplugged the copper wire from the Arduino and checked back ~3 Days later, then this happened:

The strip was slightly glowing, even when the gates of the MOSFETs are directly connected to ground. Connecting a Gate to +5v made that color glow fully though. That's pretty much my problem, some deeper examining got me those Results:

I desoldered the mosfets and my multimeter reads a resistance of ~290;310;350Ω between Drain and Source. Another strange behaviour is that the conduction seems to go from 0-100% really fast. My LED strip goes from the constant "slightly on"-State to 100% brightness in a matter of some mV. Also strange is that this happens at ~4.7V on the Gate, while my fully functional IRLZ34N gives the strip 0-100% brightness at around 1.5-2V.

Best Answer

You are missing pull-down resistors in the gate input of mosfets. They are possibly floating. Picking up noise, and getting ON randomly. Use 10k or so.

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