I am building a boombox which runs off a 3s Li-Poly battery. These are damaged if they are discharged too low, so I have designed a circuit which has two indicators (low and critical). This uses an LM358 configured as a comparator, driving an amber and a red LED. The anode of each LED is hooked up to a 5V source and the cathode goes through a 150 ohm resistor to the output of the LM358.
Since the most power-hungry device in this project is the amplifier, I want to disconnect it when the critical LED comes on. I have some N-channel MOSFETs (datasheet attached), and have been trying to get them to work, using an LED as the load. I attach the gate to the output of the LM358, the drain to the cathode of the LED and the source to 0V. When I do this the LED turns on and the MOSFET blows (the drain and source short). I have gone through 4 of my 10 MOSFETs so far so I think there's something wrong!
The fact that the LED turns on when the gate is "low" could be because the output of the LM358 goes from 0V to about 1.5V under the load of an LED, so the gate voltage is around the threshold voltage of the MOSFET. However, the datasheet says this can vary between 1.3V (min), 1.7V (typical) and 2.15V (max). It seems unlikely that the threshold would be below 1.5V for all 4 MOSFETs.
Complicating matters is the fact that my bench is next to our house's main switchboard so there is lots of 50Hz noise. My power supply is a LASCAR PSU 130, which is a fairly cheap switch-mode supply (datasheet attached). I am aware that I need to be careful with static around FETs, so I have been regularly touching my scope's ground banana terminal. When I touch this at the same time as the cathode of the LED which the MOSFET is driving, the LED turns on! I think that this could be related to the problem but I don't know that much about ground loops or whatever... I imagine the power supply is not isolated.
Any ideas on what to do?
Thanks!