Trouble analysing this power supply ckt..

Discussion in 'General Electronics Chat' started by pradeep_1993, Feb 27, 2016.

  1. pradeep_1993

    Thread Starter New Member

    Feb 27, 2016
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    Power Supply ckt.PNG The capacitor is a 33 micro farad/400v
    Zener diodes 2 & 3 are used in forward biased mode according to me.
    If anyone can relate this to something helpful...please revert back...
     
  2. #12

    Expert

    Nov 30, 2010
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    It's true. The zeners are forward biased.
    This is a current limited power supply with the 33 uf capacitor being the dominant limit at about 100 ohms of reactance.
    Another hundred ohms at R14 and That allows about 1/8 amp total. about a milliamp through the forward zeners and that makes the transistor conduct about 6ma.
    It doesn't make a lot of sense without knowing what it is supposed to serve for its load at PL2.
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
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    Welcome to AAC!

    Strange current source/load; what a waste of zener diodes. What is it used for?
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    It's a transformerless psu with a small transformer put on the mov, the zeners are drawn backwards and will drop 1.4v, it maybe a constant current circuit, or you have drawn it wrong.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    It's not constant current, but the 24V zener and the transistor will perform some filtering of the voltage peaks. The 24V is a sort of peak clipper to absorb any voltage >24V. The transistor seems to be set up as a varying dummy load. These two loads combined with the high input reactance give some filtering - peak dampening - without a capacitor. It's a silly design, though. R4 is redundant to R2, the zeners are backwards and don't need to be zeners.
     
  6. pradeep_1993

    Thread Starter New Member

    Feb 27, 2016
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    Any idea what a KA431AZ zener works like????
     
  7. dl324

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    Mar 30, 2015
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  8. Dodgydave

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    Jun 22, 2012
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  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The red node is clipped pulsating DC, steady at 24 V for most of the AC half-cycle, then dipping down to 0 V (WRT D1 anode) for each AC zero crossing. At anything above 1.4 V the transistor b1 kicks in as a constant current load of 7 mA, assuming Vbe = Vf.

    ak
     
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