Electronics > Beginners
10 Amps power supply with LM723
mike_mike:
Hello. I build the power supply from the attached schematic, but I found that there is a problem regarding the current limit control.
If I short between the pins 2 and 3, the output voltage varies from 1V to about 30V. But if I connect pins 2 and 3 to the circuit, the voltage varies up to about 12V. I also tried to use an external NPN small power transistor (BC548) and I found the same problem is happening again. I used only POT1, POT2 and POT3 from the original schematic. I did not used any Ammeter, I just connected a wire. The BE voltage of the small power NPN transistor is always about 0.55-0.6V.
I am uploading the original schematic: original_schematic.png
And I am uploading the schematic when I used the external small power NPN transistor: schematic_img.png.
More details about the problem:
1. Input voltage 33Vdc, from 24V/160VA transformer.
2. Voltage on pin 12: about 33Vdc.
3. Voltage on pin 11: about 32.5Vdc.
I tried the following solutions:
1. I changed the potentiometers with some new ones, but the problem is the same with the new pots.
2. I checked the PCB for bridges and it is OK.
What should I do to make this circuit work correctly ?
iMo:
In the original schematics if you short 2 and 3 the current limit is off.
If 2 and 3 is wired into the circuit and it triggers at 12V it means at 12V output the current through the shunt 0.15ohm is high enough to create 0.6V voltage drop on the shunt and open the the current limit transistor.
Probably a problem with your wiring or shunt value.
You do not use 723's pins 2 and 3 in your schematics (eagle). The external transistor T4 is doing the limiting. So let 2 and 3 shorted (and connect to COMP maybe).
Double check the voltage drop at the 0.15ohm shunt while varying the output voltage.
Doublecheck the value of the 0.15ohm shunt!
mike_mike:
I changed the 0.15R resistor with a new one and I checked the voltage on the 0.15R resistor and it is about 2.8 miliV, when the output voltage is 12V.
The BE voltage of the transistor is still 0.55-0.6V.
I connected the pin 2 and pin 3 together but the situation is the same, the output voltage is still about 12V, and sometimes it is varying around 12V.
Edit: When I powered again the power supply, the voltage was about 17V and not 12V as it was when I powered it initially.
When I set the output voltage at about 2.1V, the voltage on the 0.15R resistor is 2.1mV.
I also changed LM723 with a new one, but without succes.
iMo:
--- Quote from: mike_mike on April 10, 2019, 04:20:43 pm ---I changed the 0.15R resistor with a new one and I checked the voltage on the 0.15R resistor and it is about 2.8 miliV, when the output voltage is 12V.
The BE voltage of the transistor is still 0.55-0.6V.
--- End quote ---
How you can have BE=0.6V when the voltage at the shunt is 2.8mV??
It has to be the same when CL(base) and CS(emitter) are wired to the shunt..
mike_mike:
I found that the 0.5-0.6V voltage drop at BE junction is because there is 0.5-0.6V voltage drop on the 120R resistor ...
I changed the 120R resistor with a new one, but it did not changed the situation.
Somewhere on the PCB is a component that makes the BE jonction to be at 0.6V... i think.
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