Identifying PNP and NPN Transistors

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Identifying PNP and NPN transistors can be challenging, especially when they are unmarked. Common methods include using a diode tester to check the emitter-base junction, which can differentiate between the two types but may not identify the collector and emitter leads. The discussion highlights the limitations of using standard multimeters and emphasizes the importance of recognizing the pinout variations across different transistor models. Suggestions include borrowing specialized transistor testers that can determine gain and lead configuration. Overall, the need for clearer labeling on transistors is acknowledged to aid in identification.
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Hello,

I just bought lot of pnp and npn transistors. They are not labeled. How do i tell them apart?!
 
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Are they distinct in any way at all? A lot of times, they make one lead longer than the others, or one side of the body flat.
 
well the leads are weird at one set. they are spread. Does that mean anything?
 
not usually - assuming a standard TO-92 package, you can get most transistors with different lead forming (p or n channel).

Is there no marking (screening or laser etching) on the body of the components?
 
http://media.digikey.com/photos/Toshiba Photos/2SC1815GRTE2FT.JPG

one set, and that set of transistors also has a little dot-like hole in the non-flat region.

other set is http://cdn.sigma.octopart.com/8893201/image/ON-Semiconductor-MPS751G.jpg

item bought from here

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=160557979973&ssPageName=STRK:MEWNX:IT
 
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figured it out. But really, 2 types of bjt and they both look same to the eye? This is flaw of EE community i think. They should have come up with some specific label or something .
 
You might find this gadget interesting.
 

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  • #10
Sure you can use an ohmeter to check the base emitter junction. Now tell me how you will tell it from the base collector junction. This method will tell you the difference between NPN and PNP but it won't likely tell you which is the collector and which is the emitter.
 
  • #11
Averagesupernova said:
Sure you can use an ohmeter to check the base emitter junction. Now tell me how you will tell it from the base collector junction. This method will tell you the difference between NPN and PNP but it won't likely tell you which is the collector and which is the emitter.

Yeah, you really need something that can check the AC gain, to distinguish between the collector and emitter leads. I like the find by Studiot:

Studiot said:
Can you borrow one of these?

http://www.peakelec.co.uk/acatalog/jz_dca55.html
 
  • #12
Sencore (others possibly as well) made a transistor checker that just hooked 3 leads to the device under test. You rotated a permutator switch until you got a reading on a meter. This narrowed it down to 2 possible positions. Then hit the gain switch to determine which was which. Read out the position on the switch which lead was connected to emitter, base, and collector. This was way before the digital revolution. Berkeman, the link you provided looks kinda cool at a glance anyway.
 
  • #13
Bassalisk -- do you have an idea for a simple circuit you could make with a signal generator and DVM that would let you figure out the forward and reverse gains of your transistors? Might be a good learning project for you.
 
  • #14
I like the find by Studiot:

Do you really not have these in the States?
Peak do a range of little automatic component testers in the same format.
 
  • #15
I used DIODE tester. It let's current in one direction and measures how much current is getting back. If no current is getting back at the meter then I reverse biased the junction.

I have diode tester in my multimeter.
 
  • #16
Are you aware how and why the diode test function in your multimeter works?
 
  • #17
Well I tested it with an actual diode. I hook red wire(positive) on the anode, and the black wire(ground/negative) to the cathode. It gave out a reading. Its says here its measuring continuity voltage, I believe that is voltage in conducting state?

But when i hook it the other way around, i get a zero. Pretty straightforward.

I used it on the transistor, emitter-base junction because I know that works in forward bias.
 
  • #18
http://www.produktinfo.conrad.com/datenblaetter/100000-124999/124401-an-01-ml-VOLTCRAFT_VC_130_DMM_de_en_fr_nl.pdf

here is a link to my product manual, page 25 is English, page 35 is diode test operation manual
 
  • #19
Pretty straightforward.

But it is not always that straightforward and an electonics/electrical major should known the pitfalls.

Yes, I seem to remember you have recently bought or borrowed a digital multimeter.

Older type Analog meters, with pointers respond to current not voltage.
We make a voltmeter out of them by putting a high value resistor in series with them.

We make an ohmmeter by putting a ressitor either in series or paralle with the unknown and measuring the current through the resistor. This yields in a highly non linear scale.
In order to measure higher value resistors we need a relatively high supply voltage (my old meter had 22.5 volts.)

This high voltage was more than enough to turn on (bias on in the forward direction or sometimes breakdown in the reverse) semiconductor junctions.

Further the colours of the terminals and leads of these older type meters were reversed ie the black lead produced a positive voltage on the black lead.

So you cannot use this type of meter to measure in resistors incircuit as you will switch or destroy junctions. Electronic ohmeters were introduced that applied about 0,2 volts to overcome this.

Modern digital meters respond to voltage
So we make current meters by measuring the voltage across a known resistor with them.
We measure resistance by either passing a constant current through the resistor and measuring the voltage or using a feedback amplifier or by other means.

Whichever we do, the red lead should now be positive and around 0.2 volts only applied to the unknown resistor.

This means that we can no longer use an ohmmeter to test for forward and reverse resistance on a junction as we simply will not switch it on.

So modern meters have a special setting which applies sufficient current limited voltage to turn on the junction and presents the voltage across it as a reading in the forward direction.
 
  • #20
Studiot said:
But it is not always that straightforward and an electonics/electrical major should known the pitfalls.

Yes, I seem to remember you have recently bought or borrowed a digital multimeter.

Older type Analog meters, with pointers respond to current not voltage.
We make a voltmeter out of them by putting a high value resistor in series with them.

We make an ohmmeter by putting a ressitor either in series or paralle with the unknown and measuring the current through the resistor. This yields in a highly non linear scale.
In order to measure higher value resistors we need a relatively high supply voltage (my old meter had 22.5 volts.)

This high voltage was more than enough to turn on (bias on in the forward direction or sometimes breakdown in the reverse) semiconductor junctions.

Further the colours of the terminals and leads of these older type meters were reversed ie the black lead produced a positive voltage on the black lead.

So you cannot use this type of meter to measure in resistors incircuit as you will switch or destroy junctions. Electronic ohmeters were introduced that applied about 0,2 volts to overcome this.

Modern digital meters respond to voltage
So we make current meters by measuring the voltage across a known resistor with them.
We measure resistance by either passing a constant current through the resistor and measuring the voltage or using a feedback amplifier or by other means.

Whichever we do, the red lead should now be positive and around 0.2 volts only applied to the unknown resistor.

This means that we can no longer use an ohmmeter to test for forward and reverse resistance on a junction as we simply will not switch it on.

So modern meters have a special setting which applies sufficient current limited voltage to turn on the junction and presents the voltage across it as a reading in the forward direction.

Yes but I didn't use an Ohmmeter to test my junction. I understand now that the ohmmeter uses external voltage supply to drive current through resistor and measure the resistance.

But I have special feature on my multimeter, called diode test, designed specially for diodes. I don't think it sends large currents through test leads because its designed for diodes.
 
  • #21
But I have special feature on my multimeter, called diode test, designed specially for diodes. I don't think it sends large currents through test leads because its designed for diodes.

Isn't that what I said?

But you can still expect to encounter all types of meter in your career.
 
  • #22
Oh yea missed it. Thank you for your reply. I will keep that in mind when using test.
 
  • #23
The test polarity is important if you have an unmarked diode (or transistor) and you want to use your diode test to determine which is the anode and which is the cathode
 
  • #24
Studiot said:
The test polarity is important if you have an unmarked diode (or transistor) and you want to use your diode test to determine which is the anode and which is the cathode
They should come up with something like marking to clearly distinguish npn and pnp transistors.
 
  • #25
Even if there were markings to differentiate the PNP vs NPN, you would also need markings to indicate pinout since this changes between transitor models.

Also, the device type often changes; the TO & SO packages are used for many devices other than BJTs (voltage regulators, Diodes, FETs, etc.)

Always go for part numbers on the parts first if they exist.
 
  • #26
Look closely, you might be able to see some numbers on the part like 2222 or 3907.
 

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