0-30 Vdc Stabilized Power Supply

[audioguru2]

Hi Stanley,
Excellent! ;D
You have a good idea to use trimpots for fine tuning.

 

[audioguru2]

Hi Helder,
Yeah, the problems with this project are frustrating. I would have updated it but I don't need it. A few people have built the improved one with success so maybe I'll update the project.

It is a kit. Maybe errors were made on purpose in its schematic and parts list so that copies won't work. Maybe it would be replaced by a better or improved one if you built the kit and complained about it.

 

[kankundachi]

Hi HelderS :

    About your opinion,I can realize your feeling.But I think everyone to bring up his experience in this project is very good.Because it will be make this project to tend to perfect.Besides we can study this project by everyone experience.Isn't it?


Best Regards,

              Stanley

 

[HelderS]

Ok, my BC557 have already arrive. So, i've put him at his place, and turned on my PS, but I'm having a doubt wich is, how can I measure the output current ??

I've built my PS entirely original, I've done none of the changes proposed in this forum... (I thought the project on the website was CORRECT)

 

[audioguru2]

Hi Helder,
A current meter is made by using a low-value resistor in series with the load, and measuring the voltage created across it. R7 in the circuit is already in series with the circuit, so the voltage drop across it is caused by load current through it, but its voltage drop doesn't reduce the output voltage like a current meter in series with the load would.

Connect you voltmeter across R7 to measure load current. Ohm's Law says that 2A through the 0.47 ohms of R7 creates a voltage drop across it of 0.94V, 1A creates 0.47V, 100mA creates 0.047V etc.

Many projects on the web have obsolete parts, parts backwards or schematics with shorts on them. Unfortunately, this project is one of them but we recently designed fixes for it. ;D

 

[HelderS]

Hi, Audioguru, sorry about the stupid question, but to measure the voltage in R7, I have to short circuit the output, right? That's What I've done, and I measure at R7, a voltage of 0.022 V. wich gives 46.8 mA. What's wrong? Why can't I have more than that measured current (46.8 mA) ?

 

[audioguru2]

The short-circuit current is determined by the setting of the current-regulation pot, P2. In your original project's circuit with the pot at max I think Q2 and Q4 will melt with about 2.5A.

The voltage across R7 is the load's current times 0.47.

 

[HelderS]

Ok, audioguru, I know how to make the claculations, I would like to know why can't I have more than 50 mA???

  Regards, Helder Silva

 

[audioguru2]

Hi Helder,
It's troubleshooting time!

1) Turn the current regulation setting pot P2 to max and connect the neg lead of your voltmeter to the output 0V terminal.
2) Without a load on the project measure the voltage at the slider of P2. It should be about +1.7V. If it isn't, check the vaues of R7, R17, R18 and P2.
3) If the values are correct but the measurement is wrong, measure the voltage at pin 6 of U1. It should be about +11.2V.
4) If the +11.2V is wrong, check the part number and polarity of D8 and the values of R5 and R6.
5) Turn up the voltage setting pot P1 to max. Short the project's output and the current should be about 2.5A and the current-regulation warning LED probably won't light. Turn down the current-regulation pot P2 to about 70% and the current-regulation warning LED should light and stay on as the pot is turned to zero. 

 

[HelderS]

Hi, Audioguru, here it goes my results:


2) Without a load on the project measure the voltage at the slider of P2. It should be about +1.7V. If it isn't, check the vaues of R7, R17, R18 and P2.

  Here, I measured +1.8V

3) If the values are correct but the measurement is wrong, measure the voltage at pin 6 of U1. It should be about +11.2V.

    Here, I measured +10.94V , IS IT TOO LOW???


wHAT DO YOU THINK ABOUT THE VALUES i MEASURED, I STILL CAN'T HAVE MORE THAN 50 mA AT THE OUTPUT...
  R2 BECOMES REALLY HOT AT YOUR POINT 5) CONDITIONS? aNY CLUE?

THANKS

 

[audioguru2]

Hi Helder,
2) Your measurement is high because the value for pots could range over plus and minus 20% which is fine.

3) Your measurement is low because you probably used a zener diode designed to be operated at 45mA, but R4 in the circuit gives it only 1mA. That's why I recommend using a BZX79C5V6 zener diode that is designed for 5mA and changing R4 to 1k to provide 5.6mA.

Of course R2 gets hot and will melt and burn the pcb. It dissipates a lot of power. I recommend using a 2W resistor.

You know what? The power in R2 should always be the same. Did it heat more when the output was shorted? Check the values of R13 and R14. Check the pins positions on Q1.

Are the opamps hot? They are operating with a total supply voltage much higher than their absolute max. Measure the positive supply voltage at pin 7 of U2 and the negative supply voltage at its pin 4.

Did the LED ever light?

 

[audioguru2]

Yup. You would light-up too, like U2 and U3 if your supply voltage is way above your absolute max. It looks like you need some OPA445AP high voltage opamps.

I can't remember what you are using for Q2. If is the original little thing and it fails, it might wipe-out your new opamp. Get a real power transistor to replace it like a TIP31A and a real little finned heatsink to be bolted to it.

You didn't post the voltage measurements I requested.

 

[HelderS]

  Hi, Audioguru, so I've to change my Q2 to a TIP31A, and can I use a LM741 ampop instead the OPA445AP, once it may be more difficult to get where I live. If not, can you give other alternatives.
 
  And I have to say that ALL my project has been made with the original parts proposed in the website, except my transformer wich have 26.7 VAC @ 2.5 A, for maximum ratings.

  I'm not at home, so I can not measure now the opamps supply.

  Regards, Helder Silva

 

[audioguru2]

Your fully loaded transformer produces 26.7VAC which could be 28VAC without much load. Its peak voltage will be 39.6V then is reduced by the rectifier bridge to 38.2VDC without much load and is the positive supply for 2 of the opamps. These opamps also have a negative supply of about -5.6V so the total supply voltage for the opamps is 43.8V.

The original project's TL081 and an ordinary 741 opamp have an absolute max total supply voltage rating of only 36V and they aren't guaranteed to work so high.

The OPA445AP (Texas Instruments, Burr-Brown division) has a 90V rating, a 741A opamp, an MC34071CP (used to be Motorola, then ON Semi and now maybe another name), a TLE2141CP (Texas Instruments) and a National Semi opamp have a 44V rating, but they aren't guaranteed to work so high, just withstand it for a moment without damage. "Continuous exposure to this stress level may affect product reliability".

I hope you find some 90V opamps.
I also hope you have a huge heatsink for your single 2N3055 transistor with thermal grease and aren't using an insulator (electrically insulate the heatsink from the chassis). It will dissipate 61.5W at 1.77A with the project set to a low ouput voltage. It and your transformer will probably smoke if the current setting is max and the output is shorted.
I also hope you have a 6A to 10A rectifier bridge module bolted to the chassis with thermal grease, instead of the wimpy little diodes in the original project.

BTW, your transformer is rated at 2.5A AC. It really is a power rating so since the supply's C1 charges to the peak of the AC voltage which is higher, then its DC rating in this project is only 1.77A DC. ;D

I could go on and on so here are my recommendations again:

View attachment 37912

 

[audioguru2]

Hi Helder,
2 of the opamps have a positive supply voltage of your 39.7V plus the negative supply of -5.6V = 45.3V.

The NE5534D has a tiny surface-mount package (the "D") and has an absolute max supply voltage of only 40V. It has diodes across its inputs and its offset adjustment is completely different.

What's wrong with Portugal? Doesn't Farnell serve parts there? www.farnell.com
Walk or drive to France or Spain or c'mon over here. ;D

 

[HelderS]

    Hi, Audioguru, why do you added de negative supply voltage??? I measured the voltage betweeen pin 4 and 7and it were +/- 40 V, doesn't this volage includes already the negative voltage supply??


  Regards, Helder Silva

 

[audioguru2]

The project has always had a -5.6V negative supply. U2 and U3 won't work properly without it. If you measure only 40V from pin 4 to pin 7 and your positive supply is 39.7V then your negative supply isn't working. Check the polarity of its diodes.

Actually, since the opamps get hot then they are probably blown and are overloading the negative supply, causing its voltage to be only -0.3V.

 

[audioguru2]

Hi again Helder,
You didn't say so I looked: Farnell in Portugal have lots of OPA445AP opamps. ;D

 

[HelderS]

Hi, Audioguru,

If you measure only 40V from pin 4 to pin 7 and your positive supply is 39.7V then your negative supply isn't working. Check the polarity of its diodes.

 

[audioguru2]

I think you misunderstood what I said, what I said was that the voltage between the pin 4 and pin 7 is 39.7V, I didn't measured the positive supply voltage, neither the negative supply voltage, BUT BOTH.