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How to tell Collector from Emitter BJT

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fvnktion

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Hello,

Trying to design a circuit using a biased bjt. I am in the middle of prototyping a standard bjt npn, and pnp discrete to92 transistor. I am quesioning the results i am gettting and am trying to find out a way of being able to tell the collector from emitter. I can test to the 2 pn junctions to tell pnp from npn, but am not sure how to tell between the C and E. The datasheet shows the t092 package with the flat front and 3 dots, and says bottom view, i think that the bottom view means that i am looking at the pin from the bottom as they are entering the body, but am not sure.

The reason i question which is which, is because i am getting identical results when switching 5v to -5v VCE. I dont believe it is a good idea to do this, but am testing the parameters of the transistor. Also, can you tell me what would cause a high current output from base to emitter?? In a certain configuration the collector is being bypassed, and the base is running 500ma of current, way too much for standard operation.

What about good books, any good recommendations for different byassing techniques of BJT's?
 

Top view is when you are looking at transistor with pins turned down. Direction from body to pins. Bottom view direction is from pins to body.
If I understand you applied 5V between base and emitter. In that case base current is very high and may damage transistor. Base to emitter voltage is not greater than 1V normally (usually it is .65V).
 

fvnktion,
Most BJTs have a base-emitter reverse voltage breakdown voltage of about 7V. Connect a DC supply from base to emitter through a current limiting resistor and measure the breakdown voltage. The base-emitter diode will behave like a 7V Zener when reverse biased. The Collector to base break down voltage will be much higher.
~
Most planar BJTs have a Beta of about 2 when connected with emitter and collector interchanged. This can fool you, because the transistor will operate, although with very low Beta (about 2) and low Collector (which is actually the physical emitter) to base breakdown voltage.
Regards,
Kral
 

Thanks Borber, that makes sense as far as the orientation. I am just using the BJT as a switch, not caring if it is in saturation or active modes. The 5 volts is excessive on the base, but the current seems to be limited by the regulator before any apparent damage is done.

Essentially what i am trying to do is to open and close a path for current between 2 motors that are sharing a node. When one motor is running it puts the shared node volatage in between the 2nd H bridge. I am trying to use a BJT npn pnp pair to isolate the running motor from the non-running motor. I am using npn and pnp in order to allow for bi directional current flow. I do not know if i will be able to run the transitors in this configuration due to voltages changing from 0v to 12v from collector to emitter. I am not very experienced in biasing and am wondering if it is possible to run the isolator BJT's in this configuration?? Any thoughts or comments would be great.

I have attached a very rough schematic of the basic idea of what i am trying to do
 

Hello fvnktion,
In the circuit, I am guessing that the 2 motors you have you want ot have control so as to rotate them in both directions. For that you can look up H Bridge ICs which have a similar idea like you are trying to do. In terms of discrete components you can try connecting a motor between the 4 transistors you have as Q1 and Q5. Just place the Q1's (npn's with their emitters connected to the gnd line and collectors connected to the motor terminals and to the collectors of Q5s) Q5s (pnp)should be connected to the positive line with their emitters connected to the positive line and collectors connected to the motors.
You will also need to add diodes to short out the back emf that develops across the motor coil when it is switched off. Same thing you can do with the other motor. So you don't need the 4 transistors at the bottom of your diagram.
For finding the emitter, if you can find a multimeter that measures the transistor Beta then you can try both configurations and find the one with the higher Beta and thus identify the emitter. Otherwise make a simple CE amplifier setting the current through the collector to whatever the Datasheet specifies the Beta for and then measure Beta manually. Do this for both configs and you will know the config with the higher Beta is the righ pin identification. But instead of this just ask someone how to interpret the datasheet pin diagram :D
 

You can locate the base terminal easily .Check the resistance of base with other two terminals.The combination which is giving you a lower resistance is base-emitter juction b/c emitter is heavily doped which means lower resistance.

Hope that it will help you.
 

The IR drops are only visible when you go to high currents in a PN junction so I don't know if the method of measuring the resistance will work or not.
 

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