# FET transistor for output channel for arduino not working

I'm trying to work with IRF640 FET transistor (picture attached).

The problem is when I put 11V on the G-S voltage. The D-S voltage is 4-6 volts or more.

Why the transistor doesn't open fully?

In the simulator, the voltage falls to 120.18mV.(as expected)

Thank you.

• Can you put an ammeter and measure the drain current too? Did you actually measure the $V_{GS }$ ? Did you check the gate-source continuity? (if it's not an open: dead MOSFET). – next-hack Oct 12 '17 at 9:04
• in real life its: 1.3A from load to drain. Vgs - 10.8V. Vds - 4.5 volts and going up as the transistor getting hotter. tested with 2 mosfet and the results are the same – Vitaly Oct 12 '17 at 9:07
• Are you following exactly the schematics (i.e. you switch the ground. The source is connected to the ground)? To be sure: aren't you trying to use it as a high-side switch? (i.e. you aren't switching the 12V, right?) – next-hack Oct 12 '17 at 9:30
• The problem might be with the diode. Have you checked if it's connected like the way shown in your circuit? – dirac16 Oct 12 '17 at 9:45
• @dirac16 I was also suspecting the diode (e.g. mounted with A and K reversed), but it would be red-hot by now (unless it is a very big one, mounted in a big heatsink). – next-hack Oct 12 '17 at 10:07