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The voltage on the + and - inputs of the opamp stay at the same voltage.
The ouput voltage gets divided by the 5K and 1K resistors.
IF the divided down voltage from the psu out is lower than the reference voltage the opamp output will swing negative causing the transistors to let more current through so the output voltage goes up.
The 2n3904 and the 0.3ohm resistor form a primitive foldback current limiter that will act when the output current is at about 2A or more.
The lm1588 is not a rail to rail op amp and the data sheet glance at the lm188 data sheet suggests it is not intended to be used with the inputs within 3v of the supply rails so this may be a poor regulator at lower voltages.
A quick calculation suggests that vin=5V should give 30V out. (5mA flowing through the voltage divider, 5V across the 1K resistor, 25V across the 5K resistor)
Increasing the value of the 5K resistor a bit will give more volts out for the same vin, providing the output of the transformer and bridge rectifer is high enough.
You need the unregulated DC to be 3V higher than required output all the time (there will be some 100/120HZ ripple on it).
I think you could get 30V at low current out of this. If you want 2A at 30V you might need to use a transformer with a higher secondary voltage.
The voltage on the + and - inputs of the opamp stay at the same voltage.
The ouput voltage gets divided by the 5K and 1K resistors.
IF the divided down voltage from the psu out is lower than the reference voltage the opamp output will swing negative causing the transistors to let more current through so the output voltage goes up.
The 2n3904 and the 0.3ohm resistor form a primitive foldback current limiter that will act when the output current is at about 2A or more.
The lm1588 is not a rail to rail op amp and the data sheet glance at the lm188 data sheet suggests it is not intended to be used with the inputs within 3v of the supply rails so this may be a poor regulator at lower voltages.
A quick calculation suggests that vin=5V should give 30V out. (5mA flowing through the voltage divider, 5V across the 1K resistor, 25V across the 5K resistor)
Increasing the value of the 5K resistor a bit will give more volts out for the same vin, providing the output of the transformer and bridge rectifer is high enough.
You need the unregulated DC to be 3V higher than required output all the time (there will be some 100/120HZ ripple on it).
I think you could get 30V at low current out of this. If you want 2A at 30V you might need to use a transformer with a higher secondary voltage.
In your last schematic (without current limit) you can replace this diode with a small (100Ω) resistor and completely remove the 3kΩ resistor, as in this case transistors can be driven directly from the opamp's output.
There is no problem with 30V output voltage.
The problem is at 0V end.
To achieve output voltage of 0V you will need extra small negative voltage of, say, -5V. This negative voltage (has to be regulated) will be used as supply voltage to opamp (opamp will be then supplied of +35 and -5) and as bottom reference for potentiometer-resitor, so they will be connected between power supply output and -5V.
The other opamp input (+) will be connected to 0V.
Opam type can be LM725, LF411, or any opamp with supply voltage of >40V.
Bear in mind that for 30V output voltage opamps output has to be close to 33V !!!
You already have input and output capacitors, but to make this circuit working you will need to limit the opamps banwidth by adding a small capacitor of, say, 10nF between its (-) input and ouput pins ..
In your last schematic (without current limit) you can replace this diode with a small (100Ω) resistor and completely remove the 3kΩ resistor, as in this case transistors can be driven directly from the opamp's output.
There is no problem with 30V output voltage.
The problem is at 0V end.
To achieve output voltage of 0V you will need extra small negative voltage of, say, -5V. This negative voltage (has to be regulated) will be used as supply voltage to opamp (opamp will be then supplied of +35 and -5) and as bottom reference for potentiometer-resitor, so they will be connected between power supply output and -5V.
The other opamp input (+) will be connected to 0V.
Opam type can be LM725, LF411, or any opamp with supply voltage of >40V.
Bear in mind that for 30V output voltage opamps output has to be close to 33V !!!
You already have input and output capacitors, but to make this circuit working you will need to limit the opamps banwidth by adding a small capacitor of, say, 10nF between its (-) input and ouput pins ..
The 10nF cap has to be connected between (-) input and output and not between output and 0V ..
If you use different type of opamp you will have to check pin allocation in its data sheet. For example, LM725 has (-) input on pin 2, (+) input on pin 3 and output on pin 6, and on top of that you may need to add compensation circuit ..
If you, however, decide to stay with 3-24V range then you can use readily available common opamps such as LM741, LM358 oe CA3140 ..
Regards,
IanP
Below you will find the way of compensating LM725 opamp.
In your case you will need only R1 and C1 (top option).
For more info on this IC go to:
**broken link removed**
If, however, you will ensure that the unregulated voltage will not exceed 36V then you can use CA3140 or similar, which is internally compensated opamp. To esure that this voltage will not be above 36V you can add a 10Ω resistor and a 36V (33V +2.7V) zener diode to the supply pin of this opamp ..
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