Posts posted by Jeff A.
Sorry, I was referring to Q2. I'm using TIP31C for Q2.
Some time ago I asked you to help me solve a voltage drop, but now I come again to ask for your help to solve a completely opposite problem: voltages are rising when I connect small loads at different voltages. Below is a short report:
voltage without load voltage whith load current
3V 3.11V 372mA
5V 5.24V 462mA
10V 10.67V 641mA
5V 5.06V 127mA
10V 10.19V 188mA
20V 20.5V 279mA
24V 24.8V 311mA
For transistor Q1 I am using the TIP31C.
The R 15 resistor is 100 ohms.
The resistor R4 is with 1K.
The zener D8 is a common type of 5.6V, as I can not find the one recommended by you.
All other components are in accordance with your recommendations.
Your answers will be very well received,
6 hours ago, audioguru said:
The original old Greek kit used TL081 opamps that have a problem called "Opamp Phase Inversion" where the output goes positively as high as it can if an input voltage gets within a few volts from its negative supply voltage. The improved project used newer opamps that do not have this problem.
When the power supply is turned off then the huge positive filter capacitor for the rectifiers takes time to discharge but the low current negative supply capacitor discharges quickly. Then the output of this power supply project will have opamp U2 to force the output of the project to go positively as high as it can which will probably destroy whatever you were powering with it.
The resistors on the base of Q1 detect that the negative supply voltage is dropping and causes it to conduct and short the output of opamp U2 to 0V so that it cannot cause the output of the power supply to go positively as high as it can.
The circuit does not have a huge high current diode at its output to block a negative high current at its output from damaging it. A huge high current diode might be too slow to protect against damage.
Now, finally, I have been able to understand the function of Q1 and resistors R13 and R14.
I'm doing tests on a Prontoboard, and during the power-off, the output voltage goes to the maximum when the transistor Q1 is removed from the circuit.
Thank you very much for your new explanations, I had not been able to understand them in the past.
I know I have a specific topic for the improved project, but I'm still trying to understand some things in the original project.
I can not understand the operation of transistor Q1 in the circuit. It has been said here in the forum that this transistor works as a protection system, could someone explain to me how this works?
Does the power supply need no protection fuse in the output? How does it protect itself in the case of a simple polarity change, for example: a battery being connected with the polarities exchanged in the output with the whole circuit without power in its entrance? Or even if it is powered, but with the polarities of a load on its output inverted? That is, is she going to protect herself against dodgems like me?
Thank you again,
On 8/1/2018 at 1:46 PM, audioguru said:
Simply test voltages in the circuit with a voltmeter. The opamp U2 controls output voltage regulation and it has an extremely high internal voltage gain (220 thousand times for a TLE2141).
The negative feedback of the circuit detects any output voltage loss and directs U2 to correct it.
I think your poor voltage regulation is caused by the wiring from the output of the circuit to your load is too thin and causes the voltage loss.
I understood your explanations and I already know what I did wrong.
Thank you very much.
I followed the discussions on the topic of the power supply project and during the last few years I set up different boards, all with the suggestions and improvements that you have suggested. In all the tests and prototypes that I made happens a small drop in voltage every time a load is turned on. This voltage drop goes from 0.02V up to 0.08V at the most different voltage and current. That is, at lighter loads it is at 0.02V, but at heavier loads it can reach at 0.08V. Is it possible to correct this little problem? What do you think about it?
Thank you for your attention and I sincerely apologize for my English because I am from Brazil and I am using Google translator.
0-30 Vdc Stabilized Power Supply
in Projects Q/A
I discovered ...
The problem has nothing to do with the power supply.
I was using a voltmeter based on the ICL 7107 to make the measurements, and when I put another meter I did not find any variation in the output voltages.
The readings of my small home-mounted voltmeter go up only when the source has loads on the output, otherwise these metaphysical follies do not happen.
When using a common multimeter with a scale of 6,000 counts I did not find any variation in the different loads and voltages that I put in the output of the power supply.
Even so, I switched TIP31 to BD139.
I am just a hobbyist in electronics and here in Brazil people like me can not afford professional or sophisticated equipment to study and experiment, we need to build our bench equipment by dismantling old appliances and taking advantage of the pieces taken from these scraps. That's why all the information about this project is very important to people like me.
Thanks again for the help and information.