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From: TTman on 15 May 2010 07:21
Posting again as it appears not to have got through with a jpg attachment...
I need some help with this . I understand how some of it works, but not all.
Q1e is clamped by the 2v5 Vref.This provides a constant current for the 12V
zener of ~ 2.5/780 = 3mA. The zener clamps Q2b @ 12v and thus Q2e @ 11.4
nom.This supplies some CMOS circuitry. So far, so good , I think. I know it
looks like an overkill, but it's from a circuit that requires high
reliability.....
My questions are:-
What is the function of the 3 x 51R + diode? Is it some form of current
limit?
And the purpose of the other 2 diodes? Temp compensation?
I have one circuit where the Q2c is at 15V. What is failing ?Both
transistors seem to measure passive diode drops ok...
I'm guessing here, but I think if the load across the 11.4V is excessive,
then that voltage drops in an attempt to limit the current into the load....
Any help here would be appreciated as my analogue understanding is really
poor :(
Circuit can be found here http://fleetcomputersystems.co.uk/psu/.


From: John Larkin on 15 May 2010 12:55
On Sat, 15 May 2010 12:21:20 +0100, "TTman" <someone.pc(a)ntlworld.com>
wrote:

>Posting again as it appears not to have got through with a jpg attachment...
>I need some help with this . I understand how some of it works, but not all.
>Q1e is clamped by the 2v5 Vref.This provides a constant current for the 12V
>zener of ~ 2.5/780 = 3mA. The zener clamps Q2b @ 12v and thus Q2e @ 11.4
>nom.This supplies some CMOS circuitry. So far, so good , I think. I know it
>looks like an overkill, but it's from a circuit that requires high
>reliability.....
>My questions are:-
>What is the function of the 3 x 51R + diode? Is it some form of current
>limit?

They're probably to reduce the power dissipation of the PNP, and/or to
be a current limit.

>And the purpose of the other 2 diodes? Temp compensation?

Protection of the transistors on powerdown or unusual conditions,
probably.


>I have one circuit where the Q2c is at 15V. What is failing ?Both
>transistors seem to measure passive diode drops ok...

Too much current load on the 11 volt stuff? 14v/153r = 91 mA. What
load current do you expect?

One LM7912 would replace this whole mess.

John


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