ScottyStills
New Member
Hi there,
I'm trying to build a circuit for switching 12V, 2A power with TTL logic (+5V, 0V) voltage levels. Right now I'm using a MJ11033 PNP power transistor (rated 120V, 50A) with the collector at +12V, and the emitter connected to my load (bilge pump). For the biasing, I'm using an inverting small gain op-amp (741) setup, with the TTL logic going into the non-inverting input, and at the inverting input I have a 1k resistor to the ground, and 3k resistor going to the emitter of the power transistor. I have a 470 ohm resistor between the output of the 741 and the base of the transistor. For some reason when I put +12V power to the circuit, my pump is running continuously, regardless of the TTL level going into the input. On another note, if I disconnect the base altogether, my load will still run, which makes no sense to me considering there's no way the transistor should be in saturation if no biasing voltage has been applied. I've considered the idea of the transistor being shorted, and have used three brand new ones with the same result. Any ideas?
Thanks,
Scott
I'm trying to build a circuit for switching 12V, 2A power with TTL logic (+5V, 0V) voltage levels. Right now I'm using a MJ11033 PNP power transistor (rated 120V, 50A) with the collector at +12V, and the emitter connected to my load (bilge pump). For the biasing, I'm using an inverting small gain op-amp (741) setup, with the TTL logic going into the non-inverting input, and at the inverting input I have a 1k resistor to the ground, and 3k resistor going to the emitter of the power transistor. I have a 470 ohm resistor between the output of the 741 and the base of the transistor. For some reason when I put +12V power to the circuit, my pump is running continuously, regardless of the TTL level going into the input. On another note, if I disconnect the base altogether, my load will still run, which makes no sense to me considering there's no way the transistor should be in saturation if no biasing voltage has been applied. I've considered the idea of the transistor being shorted, and have used three brand new ones with the same result. Any ideas?
Thanks,
Scott