Sounds like the gate circuit is working correctly. You are measuring the pwm signal after is is averaged by the meter. That seems to be working OK and that is the hard part IMO.
You should have 13.8 volts positive tied to the panel V+ and the and the drain pin (also the metal tab) of the mosfet tied to the V- input. The source pin should go to your power supply V-.
It sounds like you may have the tab grounded. If that is the case you would get full brightness.
Think of the FET as a switch - when a pulse is applied to the gate - When the pulse is removed - the switch turns on and shorts the drain to the source - when the pulse is over, the switch turns off. The PWM is a switch turning on and off very fast. It varied thee duty cycle
to change the brightness. The panel is on full each time it gets a pulse - but the time it is on varies to the full time of each cycle.
It seems almost certain that the tab or the drain pin is shorted to V- . If not measure the resistance from drain to source and make sure the fet isn't shorted - not likely.
Also you can test the setup by removing the gate signal. the panel should turn off. Shorting the gate to V+ will turn it on - removing it off. For testing you can use a cheap LED and a 1k resistor in series.
Thantkeeps you from worrying about messing up the panel.
Edited by pbunn, 20 August 2016 - 09:28 AM.