Hi there.
I bought this psu kit from aliexpress. And started reading forum after forum
I made some of the modifications noted by Paul´s DIY Electronic Blog. (
http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html?m=1)
And wanted to share some of my insight.
First, modifications.
I replaced R3 (220 ohm) with an LM337. First configured to -3.3V output, then -1.8V.
This caused the maximum voltage output to fall from around 32V (with -3.3 Vref) to 29.1V (with -1.8 Vref). I´m thinking in replace it with the 220 resistor again and putting 2 1N4148 in series in place of D7 as Audioguru´s approach to leave -1.3V (needs testing)
I'm using a 24V transformer with 4A DC rated output.
I also put a 200K trimpot in series with R18 and reduced the output current (which originally was 3.81A to 2.5A) Mainly to reduce power dissipation in Q4 and the bridge rectifiers, which were pulling 4.95A from the transformer and this also reduced the input raw voltage from 35V to 26V. Also past 3A high ripple was present at the output at 100 hz rate (I believe the C1 3300 uF capacity was overloaded and the output was "sucking" the unfiltered voltage. I have plans to add a 2200uF (the one I have around) in parallel to C1).
Aside the Lm337, and the 200K trimpot I also added a 470uF cap in parallel with C7 which was 1uF, at the output (this caused that the CC mode is present in the milliamps spectrum so further testing is needed to see at which current setting the CC is disabled) the rest of the PSU is original. I'm also planning to add a cutoff switch to ground the non-inverting input of U1 in order to kill the output voltage of Q2. and some more stuff.
So it seems until now that reducing the negative voltage to -1.8V using the LM337 AND reducing the max output current to around 2.5A leaves this kit pretty much usable.
I measure the ripple with a DSO138 oscilloscope with 50mv and 5us divisions and the noise was 18mV peak to peak.
Hope this information benefits beginners (like me) who just bought this kit and are overwhelmed by its flaws and how to fix them. And also I'm not saying that all that was discussed (like changing the op-amps to 44V ones is not good)
It's just that in some countries is more difficult to come by with supplies (I live in Uruguay for example and my best chance to get good op amps are buying from Ebay. I cannot even get a D1047 or a SC5200 here)
Still my psu is under test and construction my next step is to get a K-Type thermocouple and start measuring the heat sink temperatura of the D1047.
Last but not least, I putted a 5A fast blow fuse and a NTC thermistor rated for 5A between the output of the transformer and the input of the PCB. And I can still hear a "PLOC" in my Pc speakers when I turn on and off the PSU. Even with the 3300uF cap (still not upgraded to 5500uF)