currentnpn
Is there any harm to having a conventional switch that can open the circuit between the collector and +V with an NPN transistor? While the base is still being excited by a pin of my controller?
I am using a simple circuit to allow a micro controller to switch a light on and off. If it matters, the transistor in question is a TIP31 I found at Radio Shack.
You MAY have collector and emitter reversed.
If you swap C and E you will usually get a functioning transistor with much lower hfe and generally poorer characteristics.
Transistor pinouts come in all possible variants.
Simple test jig:
On a breadboard where a device can be plugged in -
Connect collector via 10k to V+ (say 12V)
Connect base via 1 megohm to V+
Connect emitter to ground.
Measure drop across Collector resistor = proportional to collector current.
Now swap C & E and repeat. A large difference in current gain will be
evident.
BUT many DMMs (test meters) have transistor testers built in. Assume C&E. Test. Swap assumed C&E. Test again.
For leaded "jellybean" transistors I use BC337-40 transistors. In modest volume they can often be got for as little as most other sorts and they are excellent for most uses. 500 mA Ic, hfe of 200-600 (from memory).
IF this is a TIP 31 (and you really really should give us the whole circuit and component details) then this is the pinout:
An analogy may help to visualize this:
Think of the transistor as a valve or faucet. The base is the knob, the water tends to flow from the positive side (storage tank) to the ground (drain), if you follow the normal "current flow" directions.
The LED is like a little transparent glass section in the pipe, with a small ball loosely held in that section.
When the faucet is opened, water will be allowed to flow, and the little ball will jump around due to the water's flow.
This will happen whether the LED is above or below the faucet section.
Now for the case of electron flow, as opposed to conventional current flow direction.
Consider the same pipe and faucet, but with the ground being a source for some gas, say natural gas at high pressure underground.
The Vcc is the open air, normal barometric pressure.
Again, as the faucet is opened up, the gas will flow up the pipe, the little ball will bobble around. Again, the glass pipe section (LED) could be before or after the faucet, it won't matter.
I hope this analogy helped.
Best Answer
No problem at all. The transistor 'degrades' to being a simple diode which receives some modest (I assume!) current from your controller.