Transistor as a Zener Diode

Discussion in 'General Electronics Chat' started by RobeeJ, Aug 17, 2013.

  1. RobeeJ

    Thread Starter New Member

    Mar 19, 2013
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    I was looking at an early 80s vintage music centre yesterday looking for what might be popping the fuses, so I'm looking for anything unusual, and then I saw a transistor with what looked like the base leg had been cut short and soldered to either the emitter or collector.

    Thought... that can't be right, so I found the schematic in the service manual, and in that spot it showed a zener diode.

    So a quick google later and I find out that you can in fact use an NPN transistor as a zener. Interesting!

    My questions are:


    • Was this primarily done for cost reasons?
    • Is this a cheap and nasty alternative? Or is it a perfectly valid option for most uses?
    TIA!
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
    266
    It might be part of a noise generator circuit.

    Here is a "pink noise" circuit I found using a transistor in reverse bias mode.
     
    Last edited: Aug 19, 2013
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Only in this connection NPN transistor work as a Zener diode.

    [​IMG]

    But the Zener voltage is not well defined. This voltage will be in range between 6V up to 9V. Depending on the type of transistor we use in the circuit. So this BJT Zener diode is poor man's Zener diode. So we almost never use it.
    Sometime we use NPN transistor in ordinary diode connection.
    We simply short base with emitter. WE use this to achieve better Vbe matching.
     
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  4. tindel

    Active Member

    Sep 16, 2012
    568
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    If the schematic has a zener, I'd use a zener - I wouldn't trust a bjt breakdown voltage to be reliable (voltage level wise) or low noise.

    Many zeners are designed specifically to be low-noise - get one of those.
     
  5. RobeeJ

    Thread Starter New Member

    Mar 19, 2013
    24
    1
    Thanks for the replies. I thought it would be a cheap alternative, and this was supposed to be top of the range at the time too. :)

    I'm pretty sure this was factory fitted as well.
     
  6. #12

    Expert

    Nov 30, 2010
    16,278
    6,791
    Once upon a time, I worked at a factory where they used a reversed base emitter junction as a zener, but they had a deal with the manufacturer to sort the transistors for a breakdown voltage between 5 and 6 volts. You can't trust a random transistor to be the voltage you want.
     
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  7. ian field

    Distinguished Member

    Oct 27, 2012
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    Depends which way round it is - if the junction is forward biased you can replace it with a diode of the same semiconductor material, most B/E junctions avalanche when reverse biased - just like a zener. You best bet then; would be a TL431 programmable zener and a pre-set pot to set the right voltage.

    The old germanium transistors were sometimes used for their exaggerated temperature coefficient, to give temperature compensation in the circuit.
     
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  8. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Connect a trimpot ends between collector and emitter and the wiper to base.

    Voila you have a two terminal device - an adjustable 'zener' between collector and base.

    This was also known as an amplified diode connection.
     
  9. ramancini8

    Member

    Jul 18, 2012
    442
    118
    Some companies used defective transistors as diodes or zener diodes. Sounds el cheapo, but if you make enough product the savings are significant.
     
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