Author Topic: How it works? Current Limiter and Output protector in PD Power Supplies  (Read 375 times)

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Offline 001

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How it works?  Current Limiter and Output protector in PD Power Supplies

Looks simple but tricky - see schematic in attachment

How to understand it to change current values?

Thanx
 

Online xavier60

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Re: How it works? Current Limiter and Output protector in PD Power Supplies
« Reply #1 on: September 18, 2018, 04:38:52 pm »
That is one strange apparatus. The series pass is a six way Darlington, if that's the right thing to call it.
The final 4 stages draw current progressively from 4 unregulated rails of increasing voltage steps.
 At low output voltages, Q1/Q2 draw current from  the rail filtered by C1.
With increasing output voltage, a point will be reached where the C1 rail  does not have enough voltage so Q2 takes over, drawing current from the C2 rail.
The current from Q2 passes through the B-E junctions of Q1/Q2 to get to the output.
 And so on as the output voltage is increased.
« Last Edit: September 18, 2018, 05:35:11 pm by xavier60 »
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Online xavier60

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Re: How it works? Current Limiter and Output protector in PD Power Supplies
« Reply #2 on: September 18, 2018, 04:47:33 pm »
I don't think it looks simple at all because of the horrible way the schematic is drawn. The upside down series pass transistors confused me for a while.
The power supply is actually one of those floating style designs. It has plus and minus control rails which has their ground referenced to the regulated plus output.
« Last Edit: September 18, 2018, 04:49:44 pm by xavier60 »
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Offline T3sl4co1l

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Re: How it works? Current Limiter and Output protector in PD Power Supplies
« Reply #3 on: September 18, 2018, 04:55:29 pm »
Seems like Q16 is the current limiter.

Yeah, hard to tell with the awkward way those schematics were drawn.  You'd think they went to school specifically to obfuscate them or something.  Shows up too often through history to be mere stupidity...

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Online xavier60

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Re: How it works? Current Limiter and Output protector in PD Power Supplies
« Reply #4 on: September 18, 2018, 05:06:05 pm »
Yes, Q16 controls current regulation by sensing the voltage across R30 while in "5 amp" mode. It can turn on Q17 which drags down the Base of the first transistor in the Darlington.
Q13, Q14 and Q15 do hard limiting with different thresholds. Less maximum current is allowed for transistors drawing from the higher voltage unregulated rails.
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Offline 001

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Re: How it works? Current Limiter and Output protector in PD Power Supplies
« Reply #5 on: September 18, 2018, 06:33:28 pm »
yea

Thanx a lot!

But it is works awesome
How it do that?
 

Online xavier60

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Re: How it works? Current Limiter and Output protector in PD Power Supplies
« Reply #6 on: September 18, 2018, 08:28:06 pm »
yea

Thanx a lot!

But it is works awesome
How it do that?
Do you actually own it?
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Online Wolfgang

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Re: How it works? Current Limiter and Output protector in PD Power Supplies
« Reply #7 on: September 18, 2018, 08:28:33 pm »
Hi,

all these darlintons plus diodes are there to provide automatic range switching for the different transformer taps. This was done to reduce dissipation at high load currents.
This scheme was also used (in a slightly different style) by HP and successors. For the beginning of the seventies, this was not so dumb at all.
 
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Offline 001

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Re: How it works? Current Limiter and Output protector in PD Power Supplies
« Reply #8 on: September 18, 2018, 09:07:29 pm »
Yea

See how meter current  range switch works
It looks like simple shunt switch
No real current limit, isnt it?
i can`t understand it
« Last Edit: September 18, 2018, 09:09:48 pm by 001 »
 

Offline mikerj

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Re: How it works? Current Limiter and Output protector in PD Power Supplies
« Reply #9 on: September 18, 2018, 09:50:37 pm »
Yea

See how meter current  range switch works
It looks like simple shunt switch
No real current limit, isnt it?
i can`t understand it

The output current passes through R42 and R43, creating a voltage proportional to current.  Diode CR19 adds an extra ~0.6v to this voltage, and a proportion of the total (controlled by R32) is applied to the base emitter of Q16.  The additional diode drop voltage added by CR19 means the current limit can pretty much go down to zero, and also helps to stabilise the current limit over temperature provided CR19 and Q16 are mounted close to each other.
 

Online capt bullshot

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Re: How it works? Current Limiter and Output protector in PD Power Supplies
« Reply #10 on: September 18, 2018, 10:22:29 pm »
See how meter current  range switch works
It looks like simple shunt switch

The switch shorts out R42/R43, but there's still R30.
So in the 5A range, current limiting works by the voltage drop across R30, and in the 0.5A range by the drop across R30+R42+R43.
Safety devices hinder evolution
 
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Offline 001

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See how meter current  range switch works
It looks like simple shunt switch

The switch shorts out R42/R43, but there's still R30.
So in the 5A range, current limiting works by the voltage drop across R30, and in the 0.5A range by the drop across R30+R42+R43.

I`m not so silly  ;)
What about R73 circuit?
 

Online xavier60

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I`m not so silly  ;)
What about R73 circuit?
I have a possible reason for R73. It is rather involved starting with Q11 functioning as a constant current source for the zener reference.
 I need to know how much you already understand. how much current is Q11 set to source?
Most of this current flows through CR18. I don't know what is the primary purpose of this diode besides providing a -0.7v source wrt to the ground reference.
R73 draws some current from this -0.7v source which is able to flow through the 0.5 amp shunt resistance causing a small voltage drop with applies some reverse bias to the B-E of Q16. I think this is to ensure that the current regulation can be adjusted fully to zero.
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