i checked today my 0-30 0-3 psu
we got the following:
psu works fine with no load , minimum volts can be set to 0.02 VDC and max volts with my 28VAC trafo can go up to 31 (didnt check max volts). i used the trimmers to set the range from 0.02-26volt to be on the safe side.
first problem thats gonna BLOW YOUR MIND:
if you increase the voltage slowly , at around 7 volts you got a JUMP to 8volts on the analog voltmeter. at oscilloscope you see a strange wavy pattern . after around 9 volts the wave are more like a flat line.
second problem
lower than 0,5VDC the led of the current regulation stays on
third problem
if you bridge the output you can get max amps 0,3ADC and the psu makes a buzzing noise , sometimes the analog amp meter needlle jumps to 3A and then returns to 0,3A and that happens again and again. R7 goes smoking hot.
if you use a 10ohm load , and with a max current of 2,5amps you wont notice any problem at all, the psu looks like its fine , apart from the tiny jump from 7 to 8 volts (hard to notice on a 0-30vdc analog meter)
keep in mind i made the version R7 of the mod of the original greek psu from 1997.
the mods i did were the latest from this site:
https://www.electronics-lab.com/community/index.php?/topic/40835-0-30v-0-3a-latest-data/any ideas where to look what to check?
i can upload video too.
Here is some info for the circuit:
There are three important voltages in this circuit that can be used when troubleshooting. The first is +38 volts shown at the top. This is determined by multiplying the ac input voltage to the bridge rectifier by 1.414: 28 x 1.414 = 39.6. Subtracting 1.4 volts for two diode drops in the bridge gives about 38 volts which is expected with light loads.
Another important voltage is +11.2 volts shown at the output of OA2. The Zener is a 5.6 volt device so it is reasonable to assume that the voltage across the amplifier output and its inverting input is 5.6 volts. Knowing that both operational amplifier inputs are normally at the same voltage leads to the conclusion that the voltage across the 10 k ohm resistor to ground is also 5.6 volts. Thus, the voltage at the output of OA2 is expected to be 2 x 5.6 volts = 11.2 volts. Last, the second Zener diode at the bottom of the schematic regulates the negative supply terminals for OA1 and OA3 to -5.6 volts. These three voltages are important for understanding how this supply works and they should be verified early in the troubleshooting process.
How does the current limiting work? Let’s make another calculation by assuming the current potentiometer is set to its midpoint. The voltage from the wiper to ground will be:
Vwiper = 11.2 V x (5 kΩ)/(110 kΩ) = 0.509 V
The 33Ω resistor has been ignored. Notice that this voltage is applied to the non-inverting input of OA1 along with the drop across the 0.47 Ω resistor. If the load current happens to be 1.083 amps, then the drops are equal and OA1 is on the verge of crossing into negative saturation. OA1 serves as a comparator in this circuit. When the power supply is not in current limiting, the output of OA1 is at positive saturation and it has no effect on OA3 which serves as the error amplifier for the power supply. When the drop across the 0.47 Ω resistor is more than the voltage across the wiper and the bottom of the current set potentiometer, OA1 switches to negative saturation and OA3 turns off and so does the pass transistor. Also, the overcurrent LED comes on at this time.
Last, looking at OA3 shows it to be a non-inverting amplifier with a gain of 3. This gain is set by the 56 kΩ and 27 kΩ resistors. With a gain of 3, the range of output voltage for the supply is 0 to 3 x 11.2 volts, or 0 to 33.6 volts, ignoring the drop across the output transistors. This supply also has a 10-turn 1 kΩ variable resistor that serves for fine adjustment of the output voltage. Generally speaking, this is a 0 to 30 volt dc supply with a current capability of about 2 amps. If you analyze the comparator trip point when maximum current is flowing through the 0.47 Ω resistor, it is about 2.17 amps. This same circuit can be used for higher current by decreasing the value of the 100 kΩ resistor in series with the current limit potentiometer. Other components will have to be modified as well.