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Reverse Transistor/Diode measurements??

Discussion in 'General Electronics Discussion' started by mooginnyc, Feb 17, 2019.

  1. mooginnyc

    mooginnyc

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    Feb 3, 2010
    Hello,

    I was testing a 2N3904 transistor at my work with a newer Radio Shack analog multimeter, how I usually do, by measuring resistance between base and collector, and base and emitter. It seemed to measure the exact opposite, meaning no current would flow when forward biased, but current passed reversed. So I thought the transistor might be bad.

    So I brought a new 3904 from home, and tested it with my Simpson 260. It measured as I would expect. The + lead on the base passes some current when the - lead is on the emitter or collector, and passes no current when reversed. I also brought a 1N4148 diode that measured the same way with the Simpson.

    But at work, the Radio Shack meter measures both of those exact components opposite. There is no polarity switch. We also have an even cheaper, hardware store analog multimeter, and I tested the transistor and diode with the exact results as the Radio Shack.

    All three meters are set to Ohm. The Simpson and the Radio Shack were set to R x 1, the third meter is set to its lowest, R X1k

    Can anyone explain why I’m getting opposite readings on these meters?

    Thanks!
     
  2. Bluejets

    Bluejets

    3,489
    706
    Oct 5, 2014
    Some meters are arranged that way.
    It is best with analog meters to test with a known diode beforehand.

    I usually do a test, and if reverse readings, I flip the leads in the meter.
     
    Cannonball likes this.
  3. mooginnyc

    mooginnyc

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    Feb 3, 2010
    Thanks for the reply!
    Is there any reason analog meters would be arranged one way or the other? Seems confusing! I guess I just never came across it before. I'll know now!
     
  4. BobK

    BobK

    7,631
    1,657
    Jan 5, 2010
    The Ohms setting is not intended for testing diodes. Does the meter have a diode test function? If so, use that.

    Bob
     
  5. Bluejets

    Bluejets

    3,489
    706
    Oct 5, 2014
    The ohm setting on these meters is usually used as there IS no diode check facility and the voltage is high enough to bias the diode junction on.
     
  6. mooginnyc

    mooginnyc

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    Feb 3, 2010
    Correct. There are no diode checks on these analog meters. But there are on the digital meters. I can use a DMM to check, but I was just curious why the method I've always used with the Simpson (and other analog meters) was suddenly the opposite! I guess since measuring resistance isn't necessarily polarized, that's not always a given. I had no idea! Thanks for the replies!
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    9,301
    1,887
    Nov 17, 2011
    Look at the way an analog instrument is constructed:
    upload_2019-2-21_7-48-28.png
    To the left is the circuit for measuring voltages. A series resistor "converts" the voltae to a small current (usually a few µA) which is indictaed by the instrument. The instrument's scale shows the measured voltage.
    (For current measurements the resistor is in parallel to the instrument).
    The direction of current is such that a positive voltage at the input deflects the indicator. A current in the reverse direction will not defflect the indicator, at least not in a meaningful way. This is assuming a typical instrument with "0" at the left or right end of the scale, not centered.

    To the right is how such a setup can be altered to measure resistance. In its simplest form an internal battery is inserted into the current path (this is why an analog Ohmmeter usually needs to be zeroed to compensate ageing of the battery before use). The battery is inserted such that the direction of current through the instrument is the same as for e.g. voltage measurement (left). Consequently the "+" input for voltage measurements becomes a "-" output when measuring resistances. In the same way the "-" input becomes a "+" output. This is irrelevant for measuring resistances and may therefore not be stated explicitly in the meter's manual.

    Of course the manufacturer of the instrument may use more refined setup of selector switches to compensate for this effect, e.g. by swapping the internal connections to the terminals when going into resistance measurement mode.

    Doing a test with a reference diode (post #2) is really the safe way to get you going with your measrements.
     
  8. mooginnyc

    mooginnyc

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    0
    Feb 3, 2010
    Thanks for the thorough explanation!
     
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