I'm trying to wrap my head around some basic operational concepts of a transistor. I'm more interested in practical applications, but most tutorials I find go way too deep into physics and theory.
So, what I did was just take a transistor and breadboard it with a relay, and would like to see if I can understand what is happening. I'm just trying to get the transistor to work as an on-off switch to turn on 12v. This is sort of what I am doing:
It's a 2n3906. The base is connected to a switch on 3.3v. The collector is connected to +12v, thru a 5k pot. The emitter is connected to one side of a relay coil, the other side of the coil is on ground. The only reason I'm using the pot is to make sure I don't overload my power supply. When I had +12v connected directly to the transistor, I could hear the fan on the power supply slow down, I guess drawing a large amount of current. I didn't destroy the power supply, nor the transistor, tho.
When the pot is turned to max resistance, the relay is off, which is what I expect. I actually see about 1.5v across the relay. When the pot is turned to a low resistance, the relay switches on, but I measure 6.4v across the relay.
(and fwiw, I measure 300ma current draw when the relay is on)
Since I have +12v on the collector, I expected to find 12-0.7 across the relay. At least, from what I read, it sounds like the transistor should drop 0.7v.
I've reversed the circuit, trying to have the relay across the collector and the +12v thru the emitter, and I'm pretty sure I measured the same behavior.
So what am I seeing? I did look at the transistor datasheet, but I'm not sure what parameters I need to keep in mind
So, what I did was just take a transistor and breadboard it with a relay, and would like to see if I can understand what is happening. I'm just trying to get the transistor to work as an on-off switch to turn on 12v. This is sort of what I am doing:
It's a 2n3906. The base is connected to a switch on 3.3v. The collector is connected to +12v, thru a 5k pot. The emitter is connected to one side of a relay coil, the other side of the coil is on ground. The only reason I'm using the pot is to make sure I don't overload my power supply. When I had +12v connected directly to the transistor, I could hear the fan on the power supply slow down, I guess drawing a large amount of current. I didn't destroy the power supply, nor the transistor, tho.
When the pot is turned to max resistance, the relay is off, which is what I expect. I actually see about 1.5v across the relay. When the pot is turned to a low resistance, the relay switches on, but I measure 6.4v across the relay.
(and fwiw, I measure 300ma current draw when the relay is on)
Since I have +12v on the collector, I expected to find 12-0.7 across the relay. At least, from what I read, it sounds like the transistor should drop 0.7v.
I've reversed the circuit, trying to have the relay across the collector and the +12v thru the emitter, and I'm pretty sure I measured the same behavior.
So what am I seeing? I did look at the transistor datasheet, but I'm not sure what parameters I need to keep in mind