I'm encounter following schematic in a product. enter image description here It gets 110VAC and provide 5V DC to the board. After many inspection I still can't understand what Q2 and R4 do in this design.

First of all after T1, Voltage down convert to 27.5 volt. If we assume D1, R3 and D2 reduce at most, Vin at Voltage Regulator will at least be 25V in no load which is very higher than LM123 Vin Absolute Maximum (Specified 20V in Datasheet)

And another mystery is, if we assume 5V at the LM123 output, Base-Emitter junction of Q2 will have 20V on it :|. So IMO either of Q2 or LM123 will explode.

So the question is where am I going wrong?

  • 1
    \$\begingroup\$ The base emitter junction only has 20 V across it if there is no drop across R3 or D2. \$\endgroup\$
    – The Photon
    May 4, 2017 at 0:29
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    \$\begingroup\$ This is one of those crappy designs floating around on the internet. \$\endgroup\$
    – Matt Young
    May 4, 2017 at 0:43
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    \$\begingroup\$ Here is a useful link about it. Scroll down to the title "Pass transistor". It basically says, the purpose is to provide the extra current which regulator cannot. talkingelectronics.com/projects/TheTransistorAmplifier/… \$\endgroup\$
    – packt
    May 4, 2017 at 0:54
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    \$\begingroup\$ @packt except that's a pnp circuit, which makes more sense... sort of. \$\endgroup\$
    – Trevor_G
    May 4, 2017 at 1:01
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    \$\begingroup\$ Should I be disturbed that the ground pin isn't directly connected to ground? \$\endgroup\$
    – Bradman175
    May 4, 2017 at 3:16


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