Author Topic: Controversy on Transistor Testing: Can't be both!  (Read 2324 times)

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Offline rhb

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Re: Controversy on Transistor Testing: Can't be both!
« Reply #25 on: December 09, 2018, 09:49:43 am »
I think that you're correct.  If you design ICs you have to know the semiconductor physics.  I'm sure that the prof I got the method I use from knew because those details were what he specialized in.

And then, of course, there was Bob Widlar, who not only knew the physics, but could come up with ways to calculate things no one else could such as the resistance of an irregular patch of silicon using a sheet of resistive paper.

It bothers me that every intro book on transistors tells you how to identify the base, but not C & E.

 

Offline Jwillis

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Re: Controversy on Transistor Testing: Can't be both!
« Reply #26 on: December 10, 2018, 11:24:41 am »
Correct me if I'm wrong but I believe that is a common mode choke in the first stage filtering.
 

Offline nsrmagazin

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Re: Controversy on Transistor Testing: Can't be both!
« Reply #27 on: December 23, 2018, 09:14:14 am »
Age doesn't need to be bad, but it can be.
Hi all!
If you like the post, please press "thanks".
 

Offline spec

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Re: Controversy on Transistor Testing: Can't be both!
« Reply #28 on: December 26, 2018, 10:02:08 pm »
Hi billbyrd1945,

Just read your thread. :)

As a general point. The classic transistor test only applies to a single bipolar junction transitor (BJT), not a transistor with another speed up transistor built in, a Darlington transistor, logic transistor or an IGBJT. So before you do a test it is best to check the device datasheet to establish what the device is. And, as has been stated, the pin-out can vary even for devices with the same part number.

But there is another gotcha that you may like to know about: leakage current. Some BJTs have high leakage current which can make them appear to be faulty on an an elementary DMM test. Devices with high leakage are germanium transistors, schottky transistors, and high power transistors.

There is also a little trick. If you have a small signal NPN transistor, say a BC546, or BC337, and you want to check that it is operating as a transistor, you can set your multimeter to the high ohms range, then connect the positive lead to the collector and the negative lead to the emitter and you should get a high resistance. But if you then touch the positive lead (collector) with with your index finger on one hand and the transistor base lead with your other hand the DMM will show a lower resistance reading, and the harder you press with your index fingers,  the lower the reading will be. This is quite a good test because it demonstrated that the transistor actually has current gain (it works because dry index finger to dry index finger is around a 100k Ohms for most males).

This test is best done with analoue multimeters, but remember that with analogue multimeters on the ohms ranges, the positive lead has a negative voltage while the negative lead has a positive voltage.

Of course, to test a PNP BJT in the same way, you you just reverse the multimeter leads.
« Last Edit: December 26, 2018, 10:29:21 pm by spec »
 

Offline glarsson

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Re: Controversy on Transistor Testing: Can't be both!
« Reply #29 on: December 26, 2018, 10:38:30 pm »
This test is best done with analoue multimeters, but remember that with analogue multimeters on the ohms ranges, the positive lead has a negative voltage while the negative lead has a positive voltage.
This "reverse polarity" was/is very common, but not all analogue meters are like this.
 

Offline spec

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Re: Controversy on Transistor Testing: Can't be both!
« Reply #30 on: December 26, 2018, 11:16:53 pm »
This test is best done with analoue multimeters, but remember that with analogue multimeters on the ohms ranges, the positive lead has a negative voltage while the negative lead has a positive voltage.
This "reverse polarity" was/is very common, but not all analogue meters are like this.
I didn't know that- thanks for the information. :-+
 

Offline 6PTsocket

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Re: Controversy on Transistor Testing: Can't be both!
« Reply #31 on: January 02, 2019, 02:09:33 am »
This test is best done with analoue multimeters, but remember that with analogue multimeters on the ohms ranges, the positive lead has a negative voltage while the negative lead has a positive voltage.
This "reverse polarity" was/is very common, but not all analogue meters are like this.
I didn't know that- thanks for the information. :-+
Now I have to go check out my old Simpson 260. I always liked it for checking transistors. The forward conduction was always around 10-15 ohms.

Sent from my SM-G900V using Tapatalk

 


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