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Discussion in 'General Electronics Discussion' started by ayekaybee, Nov 28, 2014.

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  1. ayekaybee

    ayekaybee

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    Nov 28, 2014
    So i'm pretty new at electronics and i was wondering if i could get an explaination of reading continuity. I was testing a transistor and got different readings. From the base to the emitter i read 5.12k ohms and from the base to the collector i read 12M ohms. Does that make it a bad resistor? What reading is close enough to be OL and what reading is high resistance? I hope my question makes sense. Thanks ahead of time!
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Hi there and welcome to Electronics Point :)

    A transistor is a semiconductor, which is a class of components that (mostly) don't have resistance as such. They will give a reading if you measure them using a resistance range, but it may or may not be meaningful or useful. Also you should find that the reading is different if you swap the probes. This is a sure sign that the component you're measuring is not simply resistive.

    You also mentioned continuity. A continuity test is just a resistance measurement with a beeper that beeps if the resistance is less than a particular threshold, usually in the tens of ohms. It is designed to test for breaks in wires and tracks on circuit boards, and for shorts between wires and tracks, and damaged components. (Many semiconductors go short circuit when they fail.)

    Your question sounds simple but there is a lot to learn before you can fully understand how to measure and test components and circuits. It is all covered by electronics courses, and in a logical progression.
     
  3. ayekaybee

    ayekaybee

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    Nov 28, 2014
    I guess my question is, what is the best way to test a transistor? As in, what setting should the multimeter be on to have a more effective/accurate reading to know if a transistor is good or bad? Should I just set it to the diode symbol and test it that way? Or can I get an accurate reading from setting it at resistance?
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    The diode test range is good for:
    • Determining whether the transistor is a PNP or an NPN (and also checking that it is a bipolar junction transistor - it might be a FET or something else);
    • Determining which lead is the base;
    • Diagnosing the most common fault: a short between two or more terminals (you can also use the continuity range for this).
    The resistance range (or if your multimeter has manual range selection, the highest resistance range) is good for testing the transistor's leakage current.

    There are lots of tutorials out there that tell you how to do this - just Googling how to test a transistor gives 14 million hits! These are not all the same, and not all equally good, but Googling your question is a good place to start.
     
    (*steve*) and hevans1944 like this.
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