How do i see the polaity on a npn transistor

An NPN transistor acts as a switch, when its collector is more positive than its emitter and current passes between base and emitter, with the base more positive than the emitter. See:

If you have an ohmmeter, measure between the terminals of the transistor. There will be one combination of leads and terminals when you have the positive meter lead (usually the red one) on the base of the transistor, and you can read a low resistance to either of the other terminals. The one with the slightly higher resistance reading from the base is the emitter, and the other one the collector. Another way is, if you know the manufacturer and the exact part number, google it to get a data sheet, which will show the pin connections. If it's a plastic device with a flat on one side, and you hold it facing the flat with the legs pointing down, the terminals will usually be in the order E-B-C.

-- John

You look up the data sheet.

thanks guys, it is working now

cool regards Ragnar J

most components are looked at from the bottom side, so lets look at the bottom, i don't know if you have the plastic type package or the metal type ? lets assume the metal package here, when looking at the bottom, turn the package so that the center leg is pointing to your left. in this order, you will have COLLECTOR on top, BASE on the middle pointing to the left and EMITTER on the bottom. look below at that spec sheet.

--- now lets assume you have a plastic case package. look below and you can find plastic packages with their pin outs.

--- now to determine functionality of the transistor, using a DMM or ohm meter in Diode test mode. place the Red Lead (+) on the the base and use the black Lead (-) to test from Collector to Emitter. these two points should test like a diode. now reverse the leads and perform the same test and you should not get any reading.. P.S. since your using a relay, make sure you have a Diode of 1A or more across the relay coil to absorb returned Energy when the relay is turned off, other wise you will short your 2N2222 and maybe even the computers port. when placing the diode, make sure the Cathode side is on the side where the + voltage is being fed too. this is the side with the LINE on it.

and you want to make sure you get full voltage to the coil and not allow the transistor to float between low and high, this will cause heating on the transistor more so then should be. using the emitter to common and collector to the side of the coil that as the anode of the diode connected to it, while the other side of the coil which has the Cathode of the diode connected to it along with the main source of supply voltage is the base way.. this away you can vary the relay's needs for its coil voltage requirements with out depending on the computers signal line voltage levels. doing this, you should use between a 470 to 680 ohm resistor from the signal line of the port to the BASE of the transistor.

these are my views... i am sure others will comment greatly.

-- Real Programmers Do things like this.

I'm pleased to see that there are still guys of your age interested in this stuff. Good luck with wherever it takes you, and never be afraid to post here. Apart from a few, most of us will be more than happy to help when we can. P.S. It was a good idea to post your age because you managed to avoid some of the characters that might have replied rudely. ... Johnny