What should be the readings on DMM when testing a transistor ?

Thread Starter

Younes Thabet

Joined Jan 9, 2019
144
Hello all,

I have read some articles on how to test a transistor with DMM diode mode and what I understand is that measuring from Base (red probe) to Collector\Emitter (black probe) should give a reading and all other probings (C-to-E, E-to-C, C-to-B, E-to-B) should give O.L.

But that isn't the case with my SOT-23 transistor.
Here is what i found (image attched)! Is this still an NPN transistor or something else?

Does the readings differ when transistor is in the circuit or out of it?

Thanks,
IMG_20230123_104821.jpg
 

WBahn

Joined Mar 31, 2012
30,300
The transistor needs to be out of the circuit, otherwise you are measuring the part you are trying to measure in parallel with whatever the hell else happens to be there.
 

SamR

Joined Mar 19, 2019
5,080
You shouldn't be measuring Voltage but rather resistance (continuity). From the base to collector or emitter. Depending on NPN or PNP, it should be open or short in one direction and opposite in the reverse. IF both open or short then the device is toast.
1674501718000.png
 

WBahn

Joined Mar 31, 2012
30,300
He's using a meter that has a diode test feature. The voltage he is reading is the voltage required to forward bias the diode to a particular current, with a fairly low max voltage that will read overload if the current isn't achieved by that point.
 

dl324

Joined Mar 30, 2015
17,137
Does the readings differ when transistor is in the circuit or out of it?
You can't take quantitative measurements for a transistor, in circuit or out, with a DMM/diode checker.

If your meter can measure beta, it's worth is only qualitative and not particularly useful. For a beta value to be useful, it needs to be at a meaningful current and collector-emitter voltage. For that kind of information, you need a curve tracer.
 

SamR

Joined Mar 19, 2019
5,080
The Ohm meter test is simply a go/nogo test... It seems he is taking in circuit measurements. I also have DMMs with the diode test feature but use the ohm meter test more often. That along with my 20USD cheap chinesium component/transistor tester or my DCA PRO or B&K curve tracers when I need more information. Never been a real fan of Diode/Continuity testers since with my bad hearing I don't hear the BEEP signal. In the interest of full disclosure...

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Thread Starter

Younes Thabet

Joined Jan 9, 2019
144
The Ohm meter test is simply a go/nogo test... It seems he is taking in circuit measurements. I also have DMMs with the diode test feature but use the ohm meter test more often. That along with my 20USD cheap chinesium component/transistor tester or my DCA PRO or B&K curve tracers when I need more information. Never been a real fan of Diode/Continuity testers since with my bad hearing I don't hear the BEEP signal. In the interest of full disclosure...

View attachment 285902
View attachment 285903
Thank you for the clarification.
Would you mind sharing the book title, it seems like a great one.
 

SamR

Joined Mar 19, 2019
5,080
ELECTRONIC DEVICES AND CIRCUIT THEORY by ROBERT L. BOYLESTAD, LOUIS NASHELSKY, PEARSON [ 11th Edition] - PDF Drive
Or
Electronic Devices and Circuit Theory by Robert L. Boylestad, Louis Autor Nashelsky | 9780131189058 | Get Textbooks | New Textbooks | Used Textbooks | College Textbooks - GetTextbooks.com

Edition is not super important as the theory is still the same. Lot of good stuff at both links. For used books, I look for the ones classified as "Very Good" as they will be unmarked and usually in like new condition with maybe a bit of cover scuffing. FWIW
 
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