This is an old linear power supply schematic. The 78GU1C is a four pin adjustable regulator. It adjusts its output to keep its sense pin, pin 1, at 5V. In this circuit, since there isn't a divider, in the feedback loop, it's going to adjust the base drive of the D44VM4 pass transistor to keep the output voltage at 5V. Fairly straight forward regulator driving a pass transistor. What I can't figure out is what R53, in parallel with the transistor is for.

78GUIC linear power supple

Put together a simulation. The dotted box is a rough approximation of the 78GU1C. Though rough, and simplistic, it does actually behave properly when prototyped. The prototype had a CA3140EZ op amp which has a 40ma max output -- hence the extra transistor to drive the "internal" pass transistor.

In simulation the op amp will oscillate at when the current through Rl is below about 2A, depending on the opamp model being used. I'm also using "perfect" passives and generic transistors. I'm sure that with a more complex circuit, and proper models for everything, the oscillation could be tamed. I was more interested in how the circuit behaved with and without R53 present.

enter image description here

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    \$\begingroup\$ It probably limits the maximum output resistance and thus the output pole frequency to 1/(2 pi 4Ohm 1mF)~40 Hz. This does look quite slow to me, but I have no experience with circuits that old. \$\endgroup\$
    – michi7x7
    Commented Sep 12, 2019 at 20:28
  • \$\begingroup\$ Perhaps something to do with sequencing the turn on and off of the 16, 7.5 and 5V rails. That regulator IC may misbehave/latch-up if 5V rail stays high when 16V is lost due to a mains brown out. \$\endgroup\$
    – KalleMP
    Commented Sep 12, 2019 at 21:21

1 Answer 1


R53 shares the load current with the pass transistor, probably to allow a lower-rated transistor and/or smaller heatsink than would otherwise be required.

The Zener diode D9 will prevent the output voltage from rising too high if there is no load.

I'm guessing that this is a dedicated supply within some equipment. If so, knowing the expected loads, the designer may do things that wouldn't be done in a more general-purpose supply.

  • \$\begingroup\$ You are correct that it is a dedicated supply. I don't know the DC current draw, though my semi educated guess is about 3A. The incoming 7VAC is fused at 5A. R53 won't be carrying more that about 0.625A, which is very small compared to the 8A continuous current rating of the transistor. \$\endgroup\$ Commented Sep 12, 2019 at 20:08
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    \$\begingroup\$ What about minimum bias current into the sense input? I couldn't find a datasheet, but that's my guess. \$\endgroup\$
    – Aaron
    Commented Sep 12, 2019 at 20:28
  • \$\begingroup\$ Power sharing is most likely. If we allow for worst case transformer tolerances and main voltages variability the 7.5V rail may rise to 11V before the C7,8 capacitors start to fail and may easily linger there. So with a 6V drop the resistor will dissipate 9W that the transistor and heat-sink does not have to handle. \$\endgroup\$
    – KalleMP
    Commented Sep 12, 2019 at 21:27
  • \$\begingroup\$ What's interesting is that because of that resistor, the power supply actually drops out of regulation when the load is less than 0.625A. Below that, the voltage drop across R53 is less that 2.5V and so the pass transistor is completely turned off. Below that load, the output voltage will rise until clamped by the zener. \$\endgroup\$ Commented Sep 14, 2019 at 1:31

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