Identifying transistor's PINs (Emitter and Collector)

[BlackMelon]

Hello, I was trying to identify those pins. I have already found the BASE pin by using a digital multimeter(DMM), put the + of DMM on one pin and - on the other which gave me equal ohm readings.

When I was trying to find emitter and collector, I used the technique that I put DMM's + and - on the two pins which was not the BASE (Let's call them PIN1 and PIN3) to measure resistance between them. I connected R 1k ohms from PIN1 of transistor to BASE then read the resistance. After reading ohms, I moved the 1k to between BASE and PIN3 of transistor and then read the resistance too. I almost concluded that the PIN1 is a collector cuz I've got much lower collector-emitter resistance reading than the other case

But after I swapped + and - on PIN1 and PIN3, the ohms reading is reversed with the first time (when I connected resistance from PIN1 to Base, I got a much greater collector-emitter resistance)

How can I identify which pin is Collector or Emitter

Thank you

 

[KrisBlueNZ]

Normally, you read the part number off the device, Google it, download the data sheet, and see which pin is what.

If you can't find out that way, you can probably measure for Zener breakdown of the base-emitter junction. You need a DC voltage of at least 12V (from a power supply or a battery) and a digital multimeter with a high-resistance (10 megohms) input set to measure voltage.

Assuming you have an NPN, here's how to test it.

1. Connect positive of your voltage source to the positive probe of the multimeter.
2. Connect negative of your voltage source to the base of the transistor.
3. Touch the negative probe of your multimeter to each of the other terminals in turn.

Or if you have a PNP:

1. Connect negative of your voltage source to the negative probe of the multimeter.
2. Connect positive of your voltage source to the base of the transistor.
3. Touch the positive probe of your multimeter to each of the other terminals in turn.

The pin that gives you the HIGHER voltage reading is probably the EMITTER.

This test may not work on Darlington transistors; you may need a higher voltage (e.g. 24V) for them. It should work on transistors with base-emitter resistors (although not because of Zener breakdown). It won't work if the transistor is leaky, or is a germanium transistor (the same thing, really). If the transistor has a collector-emitter diode, the voltages will be quite close together but the result should still be valid.

I haven't tried this method but it should work. Let me know how it goes.

 

[(*steve*)]

Some rules of thumb are that:

1) base to collector generally reads a lower resistance than base to emitter (and a slightly lower forward voltage)
2) if there is a metal case or tab, and it is connected to one of the pins, that will be the collector.
3) if you measure the gain it will be much lower if you reverse emitter and collector.
4) if you're brave enough to measure the reverse breakdown, the BE junction will break down at a much lower voltage than the BC junction (be careful because this can easily damage the transistor).

 

[kpatz]

4) if you're brave enough to measure the reverse breakdown, the BE junction will break down at a much lower voltage than the BE junction (be careful because this can easily damage the transistor).

Did you mean to say the BE junction will break down at a much lower voltage than the BC junction?

If you limit the current to a few microamps with a resistor, driving the B-E junction into breakdown shouldn't cause any damage for testing purposes.

 

[(*steve*)]

Yes, I did mean BC (and I have edited it).

Yes, reverse current of even a few uA can damage the BE junction. I've personally tested it and shown it to be the case. Whether it's significant or not is another question. If you have a number of transistors and one can be sacrificial, then this is obviously perfectly fine.

 

[KrisBlueNZ]

This is why I suggested using a DMM on voltage range with 10M input resistance, back in post #2.

 

[(*steve*)]

Ah, now I see it. You're effectively using the DMM in voltage mode to measure the current through the reverse biased junction. Devious!

 

[KrisBlueNZ]

Right. Or measuring current using a 10M current shunt resistor!

 

[daddles]

Kris and Steve have given some good advice. The datasheet is the best way, but not usable when you have transistors with non-standard or in-house markings (I have a box of a number of such transistors). The quickest method is probably to buy one of those $5 Harbor Freight meters that has a small-signal transistor test socket. A continuity tester or ohmmeter can tell you whether the transistor is PNP or NPN and identify the base lead. Then insert the transistor into the appropriate socket of the HF meter and note the gain; reverse the collector and emitter leads and note the gain again. The orientation with the highest gain tells you the collector and emitter leads.

You can measure the junctions' voltage drops with a DMM's diode test or measure the resistances; the emitter-base resistance or voltage drop should be a bit higher. In the transistors I've tested this way, the differences are small (one to a few parts in a thousand) and I found that the gain test is the fastest and most reliable. You may also want to look up the "wet finger test", but I've not gotten it to work well and it's clumsy for me on small transistors.

 

[KrisBlueNZ]

Data sheets are occasionally wrong, and transistors with the same part number from different manufacturers are occasionally different (or have been in the past), but I agree that data sheets are the first and most obvious source of information.

At last, a real use for the transistor tester "feature" on cheap DMMs!

 

[Reset]

How can I identify which pin is Collector or Emitter
Thank you

After determining the base with you ohmmeter place the meter leads on the other two. Take a 1k resistor and touch it to the base and one of the other leads. If the reading is less than 1k you have just forward biased the transistor by connecting the collector to the base through the resistor. If it doesn't read less than 1k connect the base to the other lead and look for less than 1k. If both of these test fail your polarity needs reversing. Switch the meter leads and redo the test.
When you connect the base to another lead through the 1k resistor and read less than 1k the transistor is forward biased and you have found the collector.

 

[(*steve*)]

A transistor will display some transistor action with collector and emitter reversed. The gain will be much lower though.

Generally you find a lower resistance between base and collector than you do between base and emitter.

The reverse breakdown voltage for the base-emitter junction is less than the base-collector junction (the exception might be for some very uncommon, very odd BJT's with very low breakdown voltages.

As noted by Kris many years ago, an actual use for the transistor test function on cheap multimeters.

 

[kellys_eye]

I always recommend the purchase of this multi-tester:

https://www.ebay.co.uk/itm/LCD-Comp...010246&hash=item3f86deb44f:g:tMMAAOSw1LRZuJDF

whilst not 100% accurate (see Dave @ EEVBlog for a review) it's more than good enough for the amateur test bench and, from personal experience, I wouldn't work without one nowadays.

It identifies practically all types of semi-conductor device and gives pin outs AND some technical spec of whatever is connected. Ingenious.

 

[(*steve*)]

I always recommend the purchase of this multi-tester:

Yeah, there are numerous versions of them and they are amazingly versatile.