1K is fine if 1V is the minimum output. There is an explanatory note at the bottom of page 1 showing 909 Ohms for 0.9V.
The instability could be due to several things, as it has a floating reference it will be very prone to noise and voltage drops in the wiring but I would first check what voltage is going into the second stage. The first stage is a variable voltage source with its output decided by the drop across the 0.33 Ohm output resistor so it works somewhat like a constant current supply. The second stage is a voltage regulator which gives the selected output provided that the first stage gives it enough current. You have to determine whether the pulsating is caused by the output stage oscillating or it's input voltage varying and going below the level needed to sustain the output.
Brian.
Hi, I hope you are all well.
Here is the current schematic of the PSU. The 100R and the 5v incadescent bulb at the output (used as indicator) helps to set the min current limit for the regulators.
However I cannot make the PSU produce not even close to 10v. The max voltage at the output is 8.8v. I measure the voltage at the tip36 collector and it is about 9.5v whereas the voltage at the bridge output (loaded) is 15v. I measure 10.5v at the collector of the bd139. Why such a drop in the transistor pair? Maybe tre pair is starving current at the base?
This might explain the inability of the PSU to provide more than 0.5A of current?
What should I do, maybe change the base resistors but to what values?
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I had left the emitter of the powr transistor unconnected by mistake! So I guess we could ignore the max current limit problem now that the problem is corrected. However, I can still get a max voltage output of 8.83v.
I measure the voltage at the tip36 collector and it is a bit less than 11v whereas the voltage at the bridge output (loaded) is 15v. I measure a bit less than 11v at the collector of the bd139.
I measure the in-circuit max value of the voltage set pot (200k) and it is 180k. Maybe this is the problem?
Any clues?
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Yes this voltage set pot was the problem. Instead of 200k it was 180k (Chinese crap). I added a trimmer in series (100k) and I am able to precisely set the voltage now to 1.2-10v which was the requirement. I can actually set this to 12v as well when I increase the value of the series trimmer, at the expense of higher lower voltage. It is difficult to access the voltage pot now that the circuit has assembled, but in the future I may replace this 180k pot with a higher value one and remove the trimmer.