Transistor - unexpected measurements - Page 1
Forum section: electronics / beginners

author: xboxgamer
author_rank: Contributor
timestamp: December 31, 2020, 09:23:52 am
content: Hello.On my Xbox motherboard I had exploded transistor after accidentally creating a short. Base was permanently shorted to collector.I got a donor board and before proceeding with soldering I wanted to measure how the transistor is acting on the donor board and the measurements surprised me. It's a PNP transistor MJD210GI Understand the basics of measuring of a transistor and I already know where is the base, emitter and collector since I have a datasheet. Per my understanding when in diode mode on a multimeter there should be a reading between base and Collector only 1 way, between base and emitter only 1 way. Is that correct? To my surprise Base to Emitter and Base to Collector gave me exactly the same reading .768What's even more odd is that when I moved red probe on collector and black probe on emitter I received a reading of .660 (no reading after switching probes)Please correct me if I am wrong but I assumed that there should be absolutely no reading between collector and emitter?Is it because I am measuring the transistor still in the circuit and the transistor should be measured after taking it out of circuit?Also why base-emitter and base-collector has the same value?

author: ledtester
author_rank: Super Contributor
timestamp: December 31, 2020, 09:51:35 am
content: QuoteIf you keep one probe on the base and move the other probe between the collector and emitter you can definitely see these readings -- see the attached pic fromhttps://instrumentationtools.com/testing-a-transistor-with-a-digital-multimeter/QuoteIf you are testing it "in circuit" it is possible other components on the circuit board are responsible for the diode reading.

author: Peabody
author_rank: Super Contributor
timestamp: December 31, 2020, 02:44:16 pm
content: Can you measure on the original board after removing the remnants of the blown transistor? That might isolate the effect of the other components.

author: bob91343
author_rank: Super Contributor
timestamp: December 31, 2020, 07:58:45 pm
content: Before drawing conclusions from your measurements, remove the transistor from the board. If you don't know what's in the circuit you don't have any idea how to interpret the results.

author: Zero999
author_rank: Super Contributor
timestamp: January 01, 2021, 11:00:01 am
content: Yes, the transistor should be taken out of the circuit, before being measured. It's true that base-collector and base-emitter are diode junctions, but the latter should have a higher voltage drop, than the former. This is because the base-emitter junction is much smaller, than the base-collector junction. Although diodes drop roughly 0.6V to 0.8V, the exact voltage depends on the current density through the junction. The higher the current density, the higher the voltage drop.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 02, 2021, 01:22:20 pm
content: I assume you are using a multimeter with a diode checker . Do you have a transistor checker??? Sometimes when a transistor is damaged, but not totally disrupted, the transistor checker will show two diodes, not a transistor. If you check for junction volt drop, as a diode checker does, you will see two diode junctions, whether the transistor is good or somewhat damaged.Check it out of circuit. In circuit testing is fraught with errors, combining the other attached components in the measurement.But it is an easy place to start measuring.

author: xboxgamer
author_rank: Contributor
timestamp: January 02, 2021, 11:49:12 pm
content: Thank you everyone for your replies. Unfortunately I have a delay becaue I am waiting for a hot air station. Since its a dpak transistor I don't want to use standard iron.I just wanted to clarify something. I was exactly expecting to have different voltage drops between base emitter and base collector. That's what every video on YouTube suggests but they all measure it out of circuit. So my question is is it possible that I will receive different voltage drops after getting the transistor out of circuit?Also please help me determine best temperature in my hot air station to remove the transistor without damaging the board. Any suggestions for dpak removal?Of course I will keep you all updated once I receive the hot air station and do the measurements.