I have some TIP29 transistors that I pulled from some device years ago. I think I have a pretty good handle on what transistors theoretically do, but when I try to use these they don't seem to work right, even though they test good and I've tried 3-4 of them.
The data sheet says they're NPN, but when I connect a simple circuit (VCC -> resistor -> LED -> collector, emitter -> Ground) with no voltage at the base, the circuit is closed (albeit the light is faint)! When I then apply voltage to the base, the circuit opens. When I remove the voltage to the base, the circuit sort of fades back in.
What am I missing here? I thought an NPN would be open from e->c without a voltage at the base, and that a voltage at the base would close the circuit?
What is VCC? What is the resistor for the LED? What kind of LED? What is the value of your base resistor? What voltage are you applying to the base via the base resistor?
Are you certain that the transistors are good? How have you tested them to verify that they are?
The device is a power device, so all current ratings (including leakage current) will
be higher than standard "small signal" transistors. The leakage current is 200uA
with the base connected to emitter, 300uA if base left open, this is plenty to light
a high-efficiency LED.
For comparison a BC109C small signal transistor has a collector leakage current
rating of 15nA, over 10,000 times less (the TIP series of transistors are ancient
technology these days, modern super-beta devices are far more capable).
admoore:
What am I missing here? I thought an NPN would be open from e->c without a voltage at the base, and that a voltage at the base would close the circuit?
Nope. It's the opposite.
With BJT transistors, "voltage" isn't the really the way they work. BJT transistors conduct when current (amps) flows from base to emitter. You have to apply a voltage to get some current to flow but transistors are a lot easier to understand if you think in terms of current.
(nb. For MOSFETs it's voltage that counts...)
This page is a good intro to BJT transistors: BJT H
admoore:
I have some TIP29 transistors that I pulled from some device years ago. I think I have a pretty good handle on what transistors theoretically do, but when I try to use these they don't seem to work right, even though they test good and I've tried 3-4 of them.
The data sheet says they're NPN, but when I connect a simple circuit (VCC -> resistor -> LED -> collector, emitter -> Ground) with no voltage at the base, the circuit is closed (albeit the light is faint)! When I then apply voltage to the base, the circuit opens. When I remove the voltage to the base, the circuit sort of fades back in.
What am I missing here? I thought an NPN would be open from e->c without a voltage at the base, and that a voltage at the base would close the circuit?
Sorry if this is really newbie-ish.
If it is faint, it is not "closed", that is just leakage. As pointed out, a power transistor is going to have a lot more leakage than a smaller transistor, enough to barely light a small LED.
How are you applying voltage to the Base? It -must- be through a current limiting resistor, or you've probably burnt out the BE junction. Bipolar Junction Transistors (BJT) are current amplifiers. The BE junction is treated as if it were just a diode junction, it drops about 0.6V. To act as a switch, the Base should be supplied with about 1/20 to 1/10 of the desired CE current.
Are you grounding the Base? That might explain why even the faint glow goes away. Use a 4.7K resistor on the base, and connect the other end of the resistor to 5V. The LED should light up.
Thanks for your responses. I was working with VCC of about 3V, and I was not using a resistor when applying VCC to the base. (oops).
I'm guessing the TIP29s are just showing leakage then, the LED I'm using is quite small and I believe the resistor on it was a 220 or maybe 1K. This makes sense.
Looks like I'm going to need to dig that college electronics text out of the attic after all :).