I was recently looking at some pictures for a linear power supply that went into a process controller I've worked on in the past.

I was able to work out the schematic of the 8V regulator circuit, but the pictures did not include angles that would have allowed me to ascertain the values of three key components, and so I'm asking the brain trust. (Seller wants $250 for it, my curiosity isn't worth that much)

The three components in question are the series pass transistor itself (it is a TO-220 component and gets mounted to the aluminum chassis instead of a heatsink), the zener diode, and a tantalum capacitor paralleling the zener (it would probably be in the neighborhood of 25uF given its appearance).

The 'bulk' power input comes from a frame-mount transformer, fed through a bridge rectifier.
I would believe it would be in the neighborhood of 12VDC; there is also a 5A fuse on that part of the circuit.

The +8V output continues to the backplane inside the process controller, where it is regulated down to 5 volts on each of the circuit boards connected to the backplane.

 

12VDC; there is also a 5A fuse

hi meter,
Those figures suggest a 12Vdc PSU rated at 3Amp, IMO not worth $250.
What is your planned use of this type of PSU.?
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Added simulation.
Using typical components, the maximum output current at 8v is 2.4Amps.
Note the 2N3055 wattage dissipation of ~35 Watts at 2.4 A
IMO a poor design.

 

Mainly historical curiosity at this point. I realize better and more efficient designs have come up in the last 40 years.

A lot of the current pricing of these assemblies probably related to the high markup the company applied to the equipment.

Edit: Could the pass transistor be a Darlington? I seem to remember something along those lines.

 

Edit: Could the pass transistor be a Darlington? I seem to remember something along those line

hi mm,
Yes a Darlington transistor could be used.
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A quick search for TO-220 NPN Darlingtons included the 2N6043, which seems to ring a bell, so I'm going with that.

Was that an 8V Zener you had in your simulation? If so, then that would seem to suggest something like the 1N4738?

 

1N4738 is a 1.3W device... so absolute max current is 1.3W/8.2V = 0.158A

if R2 is 150 Ohm and input is 24V then R2 current is
(24V-8.2V)/150 Ohm = 0.105A and that is low enough for zener to handle. reality is zener will see only a fraction of that current since rest goes to base of transistor.

but... Vout is going to be less than 8.2V. the obvious difference is due to Vbe of the the transistor (and Vbe of Darlington is even larger). then there is additional drop across current sense resistor.

 

Would it have been a 1N4739 then? (9.1V) From some notes I have tucked away, the spec for the output was 7.8 to 10V...

 

hi mm,
The 2N6043 is oldish, using a later version a 2N6056 this is a simple example of a circuit.
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Okay, I think I have what I needed to 'fill in the blanks' on this design. Many thanks!

Again, my interest is more along the lines of documentation. It appears this system entered production in mid- to late 1983, moved to 'spares support' in the 1990s, and was fully obsolete when I sent a query to the company in the early 2000s. The company changed hands in the late 1990s as well.

 

Okay, I think I have what I needed to 'fill in the blanks' on this design. Many thanks!

hi mm.
Adding a transistor, a 10k pot and a couple of resistors, you could have a reasonable PSU.
5V through 12V at 5Amp, with current limit
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