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Power supply | need help deciphering the schematic

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Pixelmarth:
I need some help deciphering the circuit :(
I know that P1 controls the voltage and P2 controls the current, but I can't figure out how the opamps work.
My instructor wants me to build it, but I want to understand it first.

A universal power supply unit

So that we don't always have to rely on batteries for experiments and testing our circuits, we should get a universal power supply unit for our workshop. As true hobbyists, we will of course build it ourselves. As a blueprint for this, I offer my laboratory power supply, which I built more than 30 years ago and which still works today.

The terms voltage source and current source are often used in the same context. Strictly speaking, however, they are two different things. A voltage source provides a stable output voltage that does not change when the load at the output changes. A current source supplies a constant output current and regulates the voltage required for this. For our laboratory power supply, it is desirable if it combines both properties. It supplies a constant voltage to operate our circuits. However, if a certain maximum current is reached, it no longer allows the current to increase further. In this way, we can prevent the components from being destroyed by excessive current in the event of errors in the test circuit. In addition, our power supply unit is not affected by a short circuit.

At the time, I opted for a power supply unit that delivers two independent voltages of 1-30V at a maximum current of 1A. The circuit described is therefore completely duplicated.

A few comments on the circuit. The power transformer with an M102a core is self-wound. The choice of active components is actually quite uncritical as long as the limit values for voltage, current and power dissipation are adhered to. A Graetz bridge 3PM1 was used as the rectifier. Of course, you can also use 4 individual rectifier diodes. This is followed by a large charging capacitor. When selecting it, remember that it charges with the peak voltage of the AC voltage before rectification. At 40V effective AC voltage, this is about 56.6V! The Z-diode SZX21/1 at the input of the first operational amplifier supplies a reference voltage of approx. 0.75V. As Z diodes cannot be manufactured for such low voltages, this is actually a normal forward biased silicon diode. I have simply used the base-emitter diode of a miniplast transistor for this.

A rotary switch can be used to preset the desired maximum current in 5 stages (10mA, 30mA, 100mA, 300mA, 1A). The fine adjustment is made with the potentiometer P2. The output voltage can be set with P1. At the time, I did not use precisely calibrated scales and always set the desired values beforehand using a measuring device. Of course, you could also integrate suitable measuring instruments in the housing. The two power transistors (SU 169 and SD 337) are screwed onto aluminum heat sinks for heat dissipation. Of course, sufficient ventilation must also be ensured in the housing. The resistors on the output switch must be designed for the power losses that occur there, especially in the high current ranges.

Here is the Link to the German Website: https://technik.reicke.de/bauanl1.php

Also, if anyone could explain to me how you go about deciphering schematics that you might not quite understand at first, I'd be grateful!

The first opamp sets the voltage at the emitter of the first transistor at twice the first zener voltage.  The second opamp controls the output transistors (a compound emitter follower type thing) to make the output voltage equal twice the voltage at the slider of P1. The transistor most further to the right senses a controllable (by P2) fraction of the voltage dropped by the output current through the switchable resistors.  When the voltage seen by the base-emitter junction of that transistor gets to about 0.6 Volts the transistor begins to conduct and its collector current steals the drive to the output trio.

Kim Christensen:

--- Quote from: Pixelmarth on April 11, 2024, 02:50:20 pm ---I need some help deciphering the circuit :(
I know that P1 controls the voltage and P2 controls the current, but I can't figure out how the opamps work.
--- End quote ---

First off, those Op-Amps (TAA761?) are a bit unusual in that they have an open collector output instead of a push pull type.

What puzzles me is why they use a 1V zener. (SZX21/1) Especially since this means that the reference voltage out of the 1st OpAmp will be around 2V. The 3.3K resistor in series with the POT P1 means 1.5V is the max possible voltage into the non-inverting (+) input of the 2nd OpAmp.
But then, they use two equal value (12K) feedback resistors for the 2nd OpAmp which means that the output voltage will not go higher than 4V.

Pixelmarth:
well I've also tried simulating the circuit in falstad if you'd like to have a look:
https://tinyurl.com/2behxkwa

maybe you can help me figure things out that way!

--- Quote from: Kim Christensen on April 11, 2024, 04:34:51 pm ---What puzzles me is why they use a 1V zener. (SZX21/1)

--- End quote ---
I think that is an error. Meant to be a 15 Volt one same as the other two but the trailing '5' has been destroyed by the circuit line?

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