0-30 Vdc Stabilized Power Supply

[audioguru2]

Hey,

couldn't you replace the 2n3055 with an MJ2955? This would reduce the dropout voltage ... like this:

http://www.elektronik-kompendium.de/public/schaerer/kdarl2.htm

What do you think?

The 2N3055 is an emitter-follower without voltage gain. Its dropout voltage might be only 2V. Because it has no voltage gain then the stability (possible oscillations) of opamp U2 is not affected.
Using a PNP common-emitter transistor adds voltage gain which might cause U2 to oscillate.

Our project uses a fast opamp and a fast driver transistor for good transient response. Your circuit uses an extremely slow LM358 opamp and the same driver as our circuit so it might have a slow transient response.

You do not gain much but try it and let us know if it works.

 

[adyhansolo]

Hi,
Can I  use for the 3 single opamp only one quad opamp MC34074 ?

 

[audioguru2]

Hi,
Can I  use for the 3 single opamp only one quad opamp MC34074 ?

Maybe.
But it might get too hot.
U2 works hard but U1 and U3 do not. You need to calculate how much heat they produce to see if it is OK or if a single MC34074 quad opamp will fry.

 

[soje]

Hi.

I'm gathering parts for this project, and I have a 2x15VAC 4A trafo at hand. Unloaded it delivers 34,4 VAC with the two sides in series. Will this be too much for the circuit?

I've seen 30vac mentioned for the trafo, so I hope this isn't a stupid question  :-[


Regards
soje

 

[audioguru2]

The latest version uses a 28V or 30V transformer.
The voltage regulation of your transformer is poor. A 30V Hammond transformer is 31.5V without a load, not 34.4V.

Simply calculate if it will cause damage:
1) 34.4VAC has a peak voltage of 48.6V and the full-wave bridge rectifier drops 1.4V to 47.2VDC with no load. But the opamps have a max allowed voltage of 44V so they might be destroyed.

 

[soje]

That's what I was afraid of. Darn

Thanks for your reply.

Regards
soje

 

[soje]

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

 

[audioguru2]

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.

 

[soje]

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

 

[audioguru2]

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.

 

[soje]

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

 

[audioguru2]

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.

 

[hossain4]

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

 

[audioguru2]

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.

 

[audioguru2]

China-man,
Please do not post SPAM in this thread about the technical details for a lab power supply project.

 

[tzbuu]

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

 

[audioguru2]

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. 

 

[tzbuu]

Thank you very much audioguru
The problem was Q1 and Q3. I switch the Q3 with Q1. In my circuit Q3 was a npn and Q1 a pnp...
I will change the tl081 with TLE2141, but for now it's ok with tl.

 

[audioguru2]

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.

 

[tzbuu]

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

Thank you