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0-30 Vdc Stabilized Power Supply


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my voltmeter has range of 0-30VDC and a full scale current of 27.5mA.
what is the value of series resistor should i put to convert it into an ammeter with 0-3A range..?

Most digital meters have a 200mV full-scale and an extremely low input current.
Most moving coil meters are also very sensitive.
If resistors can't be removed from your meter to make it more sensitive then a complicated amplifier is needed for it.
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  • 3 weeks later...

Hey Fellas.  I found the plans for this thing a while ago (O.K. a week) and it looked like fun so i built it and it smoked.  My fault though.  I did it on one of those generic solder proto-boards and i am sure i did it wrong.  Anyhow, I salvaged what  I could, bought what I couldn't, and tried my hand at laser printer toner transfer PCD etching.  SUCCESS!  After a little messing around, it works.  I am thrilled.  Yeah, I get 2A instead of 3, and a lot of ripple up there, I know, but I will do the mods you have here (already got the tip31 and some of the resistors) Now all I need is one of those fancy boxes to put it in and a digital meter.  I may incorporate the simple TTL supply as well.  Thanks Ipso-Facto for all the help in this thread, audioguru and everyone.    CmAn;D

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Hello All
I’m in school in Electronic and want to use this 30v current/voltage controlled power supply as my final second semester Fabrication project... I etched a board and soldered all the components on. When I power it up I cant get more then about 7.5 v out. Pin 6 off of U1 is fixed at 11.2v, pin 3 on U2 varies from 0 o 11.2v as I vary p1, but pin 6 never gets above about 8 volts... As well LED D12 is always lit no matter where I set P2...

Any help would be appreciated...
Thanks David

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Pin 6 off of U1 is fixed at 11.2v, pin 3 on U2 varies from 0 to 11.2v as I vary p1, but pin 6 never gets above about 8 volts... As well LED D12 is always lit no matter where I set P2.

The current regulator U3 might be bad, or a wrong value resistor was used for R7, R21, R17 or P2.
C8 might be shorted.

If Q1 has its emitter and collector pins reversed, then it would act like a 7V to 8V zener diode as it has reverse breakdown which would limit the output voltage of U2. But it would not cause the current regulation to be continuous.
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Is there any possibilities to use lower voltage transformer for lower voltage outputs?

I have a 3.2Amps rated transformer with following outputs: 13.4V, 19.2V, 26.0V and 31.9V.
So, I think if it's possible to use lower voltage outlet so PSU will produce less heat.

If that's not possible, then i propably use 24V 3A toroidal transformer and save that special transformer with some other projects.

Sorry my bad English skills.

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I put an additional 8.5VAC winding on my toroidal core transformer in order to increase the voltage so 30VDC could be delivered. Because the heat generated at worst case, which is a shorted output and full current, was far to heavy to dissipate with my little heat sink, the additional winding is switched off when the temperature gets too high. This is done by a simple PTC-based comparative circuit with a relais output. I just changed a few things to achieve that the comparator can't reset itself, but requests a push button.
I wanted to go all the way and tried to switch off the "bigger" original winding instead, but apparentely the voltage was too low so the circuit for the negative voltage stopped working. Without the negative supply, the project did act quite unstable and could not be adjusted.

Of course, your smallest voltage is higher than mine, but I just want you to know that there's a limit. Can't say for sure how low you can go, though.

There is also a moment of output instability when the switching back to full voltage occurs. I think it has something to do with C1.

Do you intend to switch the windings automatically? Your 31.9V could be just a little to less. Depending on your transformer, you could perhaps put windings in series. The current is a little low, too.

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Hi Multeri,
Some resistor values will need be be reduced so that they still supply the same current with a reduced input supply. You would need to re-design nearly every one.
Also, the main voltage reference is +11.2V, so its opamp would need a supply of at least 13.2V.

For +15V output at 3A:
1) If the 2N3055 transistor for Q4 is weak but still passes its specs, it would need +16.4V on its base.
2) If the TIP31A transistor for Q2 is also weak, it will need 17.3V on its base.
3) If R15's value is reduced to 100 ohms so it doesn't waste too much voltage and Q2+Q4 are both weak, then the output of U2 must be at least 17.6V and its input supply must be at least 19.6V.
4) R7 needs 1.4V so the input supply must be at least +21V.
5) All the parts for the negative 5.6V supply will need to be analysed and the resistor values reduced.

To get a reliable supply of +21V when the mains is 10% low and the project is supplying 3A, the transformer must have a peak voltage of 26.4V to cover the voltage lost in the full-wave bridge rectifier and ripple from the main filter cap.
Therefore the transformer must have a rating of at least 26.4VDC x 3A= 79.2VA, or 18.7VAC/4.2A.

Your transformer doesn't have enough current rating but if you used its 19.2V winding then when the project is set for 3A at a low output voltage then the project will produce about 43% less heat than if the project used a 24VAC transformer.

Using the same losses as above, with a 24VAC/4.2A transformer, the project will produce a max worst case output voltage of about +22.6VDC.

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Hi Guys,

There are a few ways to achieve the multi winding changeover trick. These techniques greatly reduce the heat dissipation in any linear PSU.  I attach two examples of how it can be done, one circuit for a split winding and another one using a four pole winding. 



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Maybe it's better to stay with original setup and mods for it.
But I have to use BD243A transistors to place of TIP31A:s.

Same TO-220 package and pins layout, same voltage rating, same current gain, better current rating and better heat dissipation. It is perfect.

Second change I have to go is 3,9k resistor, 1W version isn't available, so, they give me a 5W version.

A 5W resistor won't fit on the pcb. Use a 4.3k/1W or 4.7k/0.5W resistor instead and you won't notice that the LED is a little dimmer.
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hi men, i got one big problem, i'm gonna do matricular exams 3 days later, and i choose this circuit, but, i dont have any voltage at the emmiter of q4...at the collector of q4 its 36V, and at the output pin of U2 its 36V too...please, advice me, or give me your ICQ number...i'm hopeless...  ??? ??? ??? ???

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Hi Mat,
Welcome to our forum. ;D
Does "no voltage" mean a voltage of 0V at the output?
Then the negative feedback through R12 will normally cause the output of U2 to go high, so the voltage at both ends of R15 should also be high, then the emitter of Q2 should also be almost as high (minus about 0.7V), then the emitter of Q4 should also be almost as high (minus 0.7V to about 2V).

Did you substitute any parts?
Maybe you have the pins of Q2 or Q4 mixed-up? Look at their pics on their datasheets.

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hi audioguru :) i'm sure i dont have mixed pins of any of parts.... i did little mistake  :-[ i connected cooler of q4(collector) and output...and it little bit twinkled....and now? at the output its 0.3V...at the emitter of q4 its 0.3V.... can you give me your ICQ number? thanx 4 help anyway...

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Mistakes quickly blowup parts.

The output voltage of U2 will go as high as it can because of the negative feedback through R12. The opamp's negative feedback senses that the output voltage of the project is too low and is trying to correct it.

If the base voltage of Q2 is high but its emitter voltage is low then it is broken.
If the base voltage of Q4 is high but its emitter voltage is low then it is broken.
If the base voltage of Q2 is low then R15 is broken or its value is much too high.

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for the first tests, i powered the circuit from a lab power supply with +25 v/ -5v. so it worked well.

now that i connected the transformator and rectifier, the negative voltage dropped to about -1v.

when i remove the OP U2, i again get the -5v.

any idea what could be wrong?

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