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0-30 Vdc Stabilized Power Supply


Sallala
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Another thought (dangerous, I know :-))

Could I use a capacitance multiplier right after the rectifiers somewhat like this:

http://sound.westhost.com/project15.htm

and adjust the 12k resistor to lower the voltage into the psu circuit?


Regards
soje

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Your positive unregulated supply voltage is too high when there is no load but is fine at max load.

The capacitance multiplier circuit has a high output voltage when there is no load and its output voltage drops at max load. The opposite to what you want.

Use a 43V zener diode to regulate the voltage of a TIP120 darlington power transistor. The darlington has a max loss of 2.5V at 3A. If a resistor biases the zener diode that feeds the darlington then the max DC voltage will be 41.8V. The darlington will be saturated at max load. The voltage will be 5% higher or lower due to the tolerance of a 5% zener diode.

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I've attached a schematic, is this what you were thinking?

I'm really not on top of the theory behind this circuit, but I've tried to sim it, and play around with the values. The cap after the darlington I believe is there to stabilize. Any idea how low you can go on this value?

Thanks!
soje

cap_mult.pdf

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I've attached a schematic, is this what you were thinking?

Yes, but:
1) The datasheet for the TIP120 spec's its max base-emitter voltage loss only when its base current is 12mA. Your 1k resistor limits the base current to almost nothing when the darlington needs to be turned on very hard because its current and the current in the load are max at 3A.
The value of the 1k resistor must be reduced but then the zener diode will get hot when there is no load.

So you must compromise with the current in the zener diode and its power rating.

The cap after the darlington I believe is there to stabilize. Any idea how low you can go on this value?

An amplifier with voltage gain needs to be stabilized. This circuit has no voltage gain so the output capacitor does not do much. Try 0.33uF.
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I've been reading thru all the posts (phew) to try and learn a few things. I've also tried to calculate the values of the resistor and zener using the info here:

http://www.satcure-focus.com/tutor/page5.htm

I measured the rectified (4 x MBR1060) output filtered with 4700uF loaded with R1 (2k2), giving 54.7VDC. Loading it with a 200R resistor gives 44.5VDC.

Choosing a zener arbitrarily (BZX79C43, Iz=2mA) I end up choosing a 100R resistor, giving 5W for the zener so I need a higher rated zener. However this raises the needed current for the zener, and thereby again the current in the zener. Is this right?

Since high wattage zeners are not readily available, could I do something like this?

http://sound.westhost.com/appnotes/an007.htm


Another approach:

The OPA445 is within it's voltage raiting, but how about the rest of the circuit? On that note, I've seen the OPA551 mentioned. Could this be used (it's cheaper than OPA445)?

And another question:
What's the purpose of R1?


Thanks for helping out :)

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I measured the rectified (4 x MBR1060) output filtered with 4700uF loaded with R1 (2k2), giving 54.7VDC. Loading it with a 200R resistor gives 44.5VDC.

The 200 ohm load resistor has only 223mA to only 274mA in it but the voltage dropped over 10V so your transformer must be cheap and very small. It cannot be used for this 3A project.

Choosing a zener arbitrarily (BZX79C43, Iz=2mA) I end up choosing a 100R resistor, giving 5W for the zener

The supply is about 49.7V. Without a transistor and load, the zener diode and the 100 ohm resistor have a current of 67mA. The current is much too high for the little zener diode. If you used a transistor emitter-follower like in the article then the zener diode will have 2mA and the transistor can have a regulated output with a current of 200mA.

Since high wattage zeners are not readily available, could I do something like this?

Yes.
But why?

The OPA445 is within it's voltage raiting, but how about the rest of the circuit? On that note, I've seen the OPA551 mentioned. Could this be used (it's cheaper than OPA445)?

The OPA445 was used as an impreovement years ago because the original TL081 opaps were operating at a supply voltage higher than their max allowed supply voltage. The OPA445 opamps are made about every 6 months then they quickly sell and none remain for sale until the next 6 months. They are also very expensive.

Your transformer is not suitable for this project. A 28V to 30V 127VA transformer should be used with inexpensive MC34071 or TLE2141 opamps that are used in the latest version of the project.

What's the purpose of R1?

It wastes a small amount of power when the project is turned on and it slowly discharges the main filter capacitor for no reason when the power is turned off and there is no load.
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Hello everybody,
I wish I could increase the max. current from 3 AMPs to 5 AMPs  e.g. by changing the transformer, diodes and Tran sisters and PCB tracks width and heat sink etc.

Anyone with good suggestions?

Alternatively, is there a good switch mode PS circuit with variable voltage and variable current limit that has max current 5 Amps or more? if you need you can join with me. This is a good help line for all of you.

Thanks

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There was a thread about modifying this project for 5A output.

Use the latest schematic.
Use a 200VA 28V transformer, three output transistors with emitter resistors and use a 0.27 ohm 10W resistor for R7. I think 20,000uF to 24,000uF is good for the main filter capacitor.

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  • 2 months later...
  • 2 weeks later...

hello
I made the original power supply(30V 0-3A) with all the original parts.( http://electronics-lab.com/projects/power/001/index.html )
I have the maximum output of 11.5V!!! and the led is always on.
The voltage after the bridge is 33V dc.
I measured   the reference ( pin6 to pin4 at U1) and I have 10.6V (I disconnected the P2 and P1).After this i disconnected the D9 and I measured the voltage at pin 6(u2) and is 0 to 11.5V depends of P1. All this measurements was made without load.
Please help me

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I made the original power supply(30V 0-3A) with all the original parts.
I have the maximum output of 11.5V!!! and the led is always on.
The voltage after the bridge is 33V dc.

U3 compares the voltage across R7 with the voltage at current adjust pot P2. Since there is no load then there is no voltage across R7 so the inverting input of U3 is 0.0V. The non-inverting input of U3 has 10.6V across R18 plus P2 so it has a voltage of +5mV. The TL081 opamp has a max input offset voltage of 15mV which is more than 5mV so it might turn on the warning LED and turn down the output voltage all the time. It is a poor choice for an opamp, especially since its max allowed supply voltage is lower than your circuit is giving it.
Use an opamp with a lower input offset voltage like a TLE2141 which has a max input offset voltage of only 1.4mV and a max total supply of 44V.

But you have the LED lighting and the output is not clamped to 0V! Therefore D9 might be connected backwards. 

I measured   the reference ( pin6 to pin4 at U1) and I have 10.6V.

That is 5.4% lower than the expected 11.2V but it is fine.

After this i disconnected the D9 and I measured the voltage at pin 6(u2) and is 0 to 11.5V depends of P1.

Then maybe D9 was correctly shorting the input of U2 to 0V and then the output voltage could not be changed with P1 until D9 was removed?

R12 and R11 set the gain of U2 and the output transistors at (R12/R11 + 1)= 3.074 so the max output should try to be (10.6 x 3.074)= 32.6V but it can't because your unregulated positive supply is too low. The max output should be about 28V without a load or about 25V with a load.
Check the soldering and values of R11 and R12.

Maybe you have the collector and emitter of Q1 connected backwards which causes it to be an 8V zener diode. 
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You had the NPN transistor Q1 where the PNP transistor Q3 should be?
Then you had the PNP transistor Q3 where the NPN transistor Q3 should be?
No wonder it didn't work.

Your TL081 opamps are operating with a total supply voltage that is too high for them so they might fail soon.
Your tiny old 2N2219 transistor for Q2 and single 2N3055 transistor for Q3 are getting hotter than their max allowed temperature when the voltage is low and the current is high so they might fail soon.
Your transformer also might be overloaded.
Some of the resistors are too small and will overheat and might fail soon.

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  • 3 weeks later...

I ordered from tI TLE2141 and now i replace the integrates. I also replace the 2n2219 with Tip31. now what resistor i must replace?

It sounds like you made the original circuit that has many problems. A few years ago we discussed why a BD139 transistor works better than a TIP31.

Why don't you make the latest corrected version that is talked about in this forum and its shematic and parts list have been posted many times?????
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  • 2 weeks later...

Hello,
First, sorry for my english... :-[
I am new to electronics-lab forum and a beginner. I've built this power supply and now I have a problem. When I connect Q4, the output is 17V . When Q4 isn't connected the output depends on p1 and it varies from 0V to 34.6V. With Q4 conected the voltage at pin 6 of U2 is around 0V. The voltage at pin 6 of U3 is always the same as voltage across R1 and with voltage between point 7 and point 4. I've short-circuited the output without q4 and the led not lit. Help me please!

P.S. I used this circuit diagram> http://www.electronics-lab.com/projects/power/001/index.html because I 'have built it some time ago.

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My first 2n3055 has burned out.

The original project has no calibration so its max output current is too high at about 4.1A. Then if the output is shorted the 2N3055 transistor must dissipate 131W which will cause it to overheat even if its heatsink is huge.
Maybe the current regulator circuit in your project does not work so the current and power dissipation are higher.

I checked the voltage at pin 6 of U1 ad it was 11.27V (no Q4). The voltage across R5 and R6 is 5.6V.

Perfect.

Audioguru, would you like to tell me what checks should I do to see if something else has broken?

With the voltage setting pot turned up to maximum, U2 and the output transistors will try to amplify the 11.2V reference to 34.4V at the output of the project. But 34.4V is not possible if there is any output current since the transformer is too small. The max output is about 25V at 3A with lots of ripple.
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Thankyou very much audioguru for helping me. I shorted the output , the new and good 2n3055 burned instantly but it was cold! I don't know what to measure so that I could see if the current regulator works.

If I will not be able to fix my actually power supply, I think I will build the power supply from bellow. 28 V, 4 A transformer is good?
http://www.electronics-lab.com/forum/index.php?topic=7317.1512

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The current regulation works by U3 comparing the voltage across R7 caused by the load's current with the voltage across the current-setting pot and its series resistor.
When the current in the load exceeds the set amount then the output of U3 goes low which turns on the LED and reduces the output voltage until the current in the load equals the set amount.
R7 is listed as 0.47 ohms in the original parts list but 3A needs about 0.35 ohms which is not a standard resistor value. A calibration trimpot is used in the latest version.

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Hello! I have just hooked up a working prototype on a breadboard. Everything seems to be in order. Current limiting works and all.

I have some problems though. There seems to be some 50hz junk on top of the output. Is that supposed to be there? Also some high frequency stuff. It's about 10mV pk-pk. Does the breadboard bring in some stray capacitance perhaps?

Also when I attach a computer speaker that draws about 250mA peak the output voltage varies like half a volt!  :o That's unacceptable :p  Anybody know what that might be about? :-\

post-54619-14279144211383_thumb.jpg

post-54619-14279144211667_thumb.jpg

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It must be made on a pcb.
The long connection wires and tracks on a breadboard are antennas that pickup mains hum (your 50Hz junk). The high capacitance between the wire ands tracks on a breadboard cause capacitive coupling which causes ringing and oscillation (your high frequency stuff).

The voltage at the output normally changes about 3mV from no load to a 3A load.

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