# How does this power supply circuit work?

This is the circuit of a power supply with adjustable output voltage and current, and I wish to understand how it works:

What I did understand:

R1 T1 VD5 - they form a voltage divider which provides a voltage referrence that is applied through a voltage divider formed by R3 to the input of a common collector amplifier formed by T2, T3 and T4. This allows us to set a desired output voltage by varying the base voltage on T2.

R6 is used to create a voltage drop, to measure the output current, which is set by R8. To limit the current, T5 loweres the referrence voltage.

What I don't understand:

Is C3 used to filter fluctuations which occur too fast for the zener to react?

What is the purpose of R4 and R5? And what is the purpose of R7, C4 and C5?

How exactly does T1 work? And why does the LED only come on when the current limit is reached? It looks to me that it should be on all the time.

Sorry for my english. And I hope my questions aren't too dumb.

PS: Is it possible to replace the output transistors T2, T3 and T4 with a MOSFET?

• Adjustable voltage or adjustable current? There's only one potentiometer to adjust things. Commented Oct 9, 2015 at 15:13
• @pjc50: R3 controls voltage, R8 controls current. Commented Oct 9, 2015 at 15:17
• If you ignore the additional circuitry, the basic idea is the one shown in the last few figures (5,7) of bristolwatch.com/ele/zener_power_supply.htm Basically a triple-transistor (souped-up) version of the Darlington is used as the pass element. The basic regulator idea is en.wikipedia.org/wiki/Linear_regulator#Simple_series_regulator Commented Oct 9, 2015 at 15:20
• Also, I suspect that using a TL431 instead of that ad-hoc variable Zener (R1, T1, VD5) will probably be superior in practice... and won't cost much more. Commented Oct 9, 2015 at 15:35
• @pjc: there are two pots actually (R3 and R8). Commented Oct 9, 2015 at 16:12

What I did understand:

R1 T1 VD5 - they form a voltage divider which provides a voltage referrence that is applied through a voltage divider formed by R3 to the input of a common collector amplifier formed by T2, T3 and T4. This allows us to set a desired output voltage by varying the base voltage on T2.

Not quite. R1, T1 and VD6 form a constant-current source (about 10 mA) that feeds the zener (VD5). This improves line regulation by making the zener current independent of the input voltage.

R6 is used to create a voltage drop, to measure the output current, which is set by R8. To limit the current, T5 loweres the referrence voltage.

Only indirectly. T5 steals current from T1, but the voltage won't actually be reduced until the zener current drops to zero.

What I don't understand:

Is C3 used to filter fluctuations which occur too fast for the zener to react?

Sort of. The current through the zener can be somewhat "noisy", and C3 serves to bypass this noise to ground so that it doesn't appear at the output.

What is the purpose of R4 and R5?

Mainly to drain away leakage current through T2 and T3, respectively, so that the output voltage can be reduced to zero. However, they also waste some of the output current from each transistor, reducing the overall gain of the T2-T3-T4 combo.

And what is the purpose of R7, C4 and C5?

These components are connected across the output terminals. The capacitors provide additional bypass (decoupling), while the resistor drains any leakage current.

How exactly does T1 work?

See above. The base voltage is a fixed value relative to the positive rail, controlled by the forward drop of the LED. This means that the voltage across R1 is also constant, and this sets the value of the collector current.

And why does the LED only come on when the current limit is reached? It looks to me that it should be on all the time.

Does it? The operation of the current-limit circuit should have no effect on the current through the LED (VD6).

• Thanks. The original description of the circuit said that the LED only comes on when the current limit is reached, so i thought it's VD6, since there are no other LEDs Commented Oct 9, 2015 at 15:14

It's s triple emitter follower. Whatever voltage is on the base of T2 determines what is on the emitter of T4 (minus three BE volt drops totalling somewhere around 2.5 volts). So if you have 10V on the wiper of the pot you'll have about 7.5 volts on the output.

Assuming the current limit circuit isn't activated there will be 24V on the top connection of the pot due to VD5. When current limiting this 24V gets reduced and hence the output voltage drops to maintain the same current limit into the load. If the load tries to draw more current, the 24V is reduced even more.