Ok, so I have no idea what is going on. My mind is blown right now. I've triple and quadruple checked the wiring, but I have no idea wtf is going on. I've been just trouble shooting the last 20 minutes or so, and I couldn't figure anything out, so I thought I'd ask here.
The problem: I have an LED hooked up to a TIP120 NPN transistor. Datasheet clearly states pins are as follows: 1) Base, 2) Collector, 3) Emitter
Currently, the LED is on, with the following pins:
1
+5V ---- <|----///----2
3-----GND
Meaning, pin 1(base) is connected to nothing, the 5V supply is connected to the LED and a resistor and is connected to the collector pin, and the emitter is connected to ground.
Theoretically, the LED should be off without any current at the base, but its still on! I've even tried connecting the base pin to ground, and it still emits! Am I crazy or do I have a fundamental misunderstanding of transistors? Thanks!
It seems that would be the case, but current doesn't increase (the LED doesn't get brighter) when I connect the 5V line to the base. I've tried this with a 1MOhm resistor, and I still get the same results.
Looking at the leads end and heat tab down the numbering is 1 Base, 2 Collector and tab and 3 Emitter. If in fact your connections agree with my description above then short base to emitter If the led turns off then the transistor is leaky and should be discarded (unlikely) if the led does nothing double check your wiring and if correct (unlikely) then discard the transistor as it has a collector to emitter short. My 'asides' about unlikely are for a new unused part, not something that was pulled or otherwise mishandled. If this is the case then I'd bet on the second or shorted transistor.
With any bi polar junction transistor the base muse be ~.6 - .7 V higher than the emitter to conduct current. With a PNP the base is less negative than the emitter to conduct - a positive shift of the base from the emitter potential. This is obvious.. with a small explanation.. and identical to the more common NPN varieties operation.
Sometimes explaining the less obvious helps to understand the process better.
Is the transistor bolted to anything? The tab is connected to the collector, so if it is bolted to a grounded heatsink, that would connect the collector to ground as well.
First things first. Check the transistor. It will not be the first time that a transistor has been blown.
Remove it from the circuit altogether and check for continuity between collector and emitter (e.g. with a multimeter). If it conducts (either way) when the base is unconnected it is, by definition, blown. Get yourself a new one. This goes for both NPN and PNP.
My apologies. I have just looked at the data sheet. The TIP120 is not an ordinary transistor. It is a Darlington pair, so what I've said does not apply. Look at the data sheet.
They will conduct with the minutest of base currents. Even just touching the base with your finger can cause it to conduct.
But it shouldn't conduct if you ground the base. The easiest is to get another one and see if that works.
Why use a Darlington pair to switch on an LED?? Get yourself a more suitable transistor.
simondumbo:
My apologies. I have just looked at the data sheet. The TIP120 is not an ordinary transistor. It is a Darlington pair, so what I've said does not apply. Look at the data sheet.
They will conduct with the minutest of base currents. Even just touching the base with your finger can cause it to conduct.
Not quite, the device has internal base-emitter resistors, so it needs around 90uA base current to conduct
Thanks for all the replies, sorry I haven't been able to post for so long.
I eventually found the problem. My power supply was hooked up incorrectly. Not sure what exactly was going on previously, but now that the power supply is hooked up properly, the transistors work perfectly. No noticeable leakby current or anything.
Thanks again for your replies though. If it helps, I now know a lot more about transistors that I previously did.