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# 0-30V Stabilized Power Supply

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Let me ask maybe an stupid question. If NPN transistores used are 2N3055, that support 15A. Why i need to use 3 in parallel to get 5A? (as i read on the thread).

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Well, one will probably work, but gets VERY hot and needs a huge heatsink (with fans).

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A 2N3055 transistor has a maximum allowed collector current of 15A but works poorly at 10A and more because at 10A its minimum current gain is only 5 and it has a maximum saturation voltage loss of 3V when its base current is a whopping 3.3A.

If you can keep its case at 25 degrees C somehow, its maximum dissipation is 115W.

In this 5A project, the power supply to the 2N3055 output transistors is 40V and there is a loss of 0.55V in their emitter resistors and a loss of 1.35V in the current-sensing resistor. Then when the output current is set to 5A and shorted, the output transistors must dissipate a total of 5A x 38.1V= 190.5W. One output transistor will be much too hot and two will try to dissipate 95.3W each which would be impossible to cool. So three output transistors are used and each dissipates up to 63.5W which is a lot of heat to remove but is possible.

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oh, thanks very much for the answer! are incredible easy to understand. Btw, so if i just want to allow 10A current, can use 6 x 2N3055 in parallel . Right?

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If you want an output of 10A then of course you need a transformer with more current, a bridge rectifier with a higher current rating and a bigger main filter capacitor.

What will drive 6 output transistors? The single little BD139 driver transistor cannot. Replacing the driver transistor with a more powerful transistor will not work because it must be fast and powerful transistors are too slow. So use two paralleled driver transistors.
But then the opamp will not be able to drive them. So you need to boost the current from the opamp.

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Well, thanks again, really dont know what to drive, i just want to make an bigger Power supply for my bench.

Of course that bigger transform, rectifier and bigger caps will be used. Its the easy part (just more expensive). The problem will be parallel BDs and to boost the opamp current.

Thanks again.

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• 2 weeks later...

I almost finished my PS. The two boards are completed, tested and they work wonderfully! I have ordered and I am expecting power transformers and digital panel meters (voltmeter and ammeter).
Here some pictures of my work: http://diyfan.blogspot.com/

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a Hi everyone, I´m a electronic engineer student and I need to develop a project for March, it´s a PSU with one criteria, the “way out” has to have more than 1A. I chose it for understanding that this might be a great supply for my next projects. I tried to make the original first but I had some issues with the Proteus software (a friend told me that it does not work properly with the negative power supply) and I´ve been reading some old posts and I noticed that this is exactly what you have eliminated. Which one should I use, Redwire or Audio? I downloaded  the Tompa archives and I liked the PSU from Redwire, because it seems to be easier to deal with, but Audio is pretty good too, so now I don’t know which I should try to use. Do you know if there´s a link to Multimeter, that Redwire created. I liked very much, it has everything I need, even temperature, if not, i´d like to make a multimeter with PIC, so I could learn now what I will have to study in college in the future. Both archives from Tompa are in AtMega, is it really good?

Thank´s all !

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Redwire's and my versions are almost the same. We both added the calibration trimpots at the same time without seeing the other one so the trimpots are in different locations.
I think there is a pcb designed by Redwire but I didn't design one. There are also a few other pcb designs.

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What about the multimeter? is there a link with a PIC or maybe a link from Redwire? Redwire is pretty good, it gives us tons of information.

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I was wondering because I saw one of the multimeter in the documents that caught the tompa, but already got some information ...

could you tell me please how the VA is essential for the PSU work well? According to his part list, a transformer of 28V / 4.3A, I provide 120.6 VA correct? I read about 45 pages of the topic and saw that will warm the 120VA transformer .. I'm from Brazil and here there is no standard transformer of 28V, if I know the VA will be able to make a relationship volt / amp and find the ideal transformer am I correct?

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I was wondering because I saw one of the multimeter in the documents that caught the tompa, but already got some information ...

Please translate "caught the tompa" into English.
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First, Sorry for not introducing mysef, I registrated on forum just to say that this is probably one of the best circuits that I have built (Picmaters pcb, 5A version). Everything just works.
I almoast started to build 1st version...but fortunatly I didnt.
This 3rd rev. is great one. Big TNX to all here!!!

Greetengs from Croatia

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Hi everybody. I'm a newbie. I've spent some time reading this topic, and for me was very helpful to understand the functioning of this power supply. It’s true that I didn't finished yet the entire topic, but I will do this in the next period. With your permission I will ask you a few questions about this project.

First of all, obvious I have started this project from the original faulty diagram, buying a kit of this PS, but without transformer. Now, I'm trying to finalize a good upgraded diagram for the 3A version. The questions are:

1. What do you think about using a MJE3055 instead of 2N3055. I intend to use two of them mounted on a PC processor heatsink with fan?

2. I know that in the latest version of this PS, U3 is powered through a 10V zener. Why we don't use the same method for powering the U2? Assuming a 30V transformer @ no load, we will have around 41V between pin 4 and 7, that is close to the 44V limit of opamp.

3. It is correct if I calculate the voltage drop, from transformer to the output, like this: 2V on the transformer, 2V on the rectifier bridge, 1.4V on R7, 2V on Q4/Q5 transistors? All losses are assumed for a 3A load.

4. Do you think it is possible to have a voltage ripple below 10mV using a 10000microF and a 28V @ 5,35A transformer.

Thank you

MJE3055T_ST.pdf

2N3055_ST.pdf

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1. What do you think about using a MJE3055 instead of 2N3055. I intend to use two of them mounted on a PC processor heatsink with fan?

Two little MJE3055's are not good enough for this project. Maybe if you use four of them.

2. I know that in the latest version of this PS, U3 is powered through a 10V zener. Why we don't use the same method for powering the U2? Assuming a 30V transformer @ no load, we will have around 41V between pin 4 and 7, that is close to the 44V limit of opamp.

The output voltage of U3 goes only as high as about +14V so its collector voltage can be reduced by the zener diode.
But the output of U2 must go much higher than the maximum output voltage of +30V.

3. It is correct if I calculate the voltage drop, from transformer to the output, like this: 2V on the transformer, 2V on the rectifier bridge, 1.4V on R7, 2V on Q4/Q5 transistors? All losses are assumed for a 3A load.

U2 and the driver transistor also cause a voltage drop. The main filter capacitor also causes a voltage drop.

4. Do you think it is possible to have a voltage ripple below 10mV using a 10000microF and a 28V @ 5,35A transformer.

The ripple voltage that is about  3V p-p.
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What I meant was that I saw a multimeter in Tompa´s archives which was able for download, but It was in a folder called Redwire, I´ve already sent a MP to it and I got some information.
Once I´m posting again, one more question came up to me about my last request.
I read about those VA and found out that the energy wire is different for South America, USA and Europe, knowing this, how could I choose my transformer ?Could it be by the VA output? if so how many VA should i choose for a good performance? ( PS: in Brazil it is 110/220v 60Hz)

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I am in Canada where nearly all electric things use 120V/60Hz but high power electric things like the electric stove, air conditioner and electric clothes dryer use 240V.
So I use 120V transformers.
In Brasil you probably use 110V transformers.
VA is how much power passes through the transformer.

If the power supply has a maximum output current of 3A then a 30V transformer must have a VA rating of 127VA.
If the power supply has a maximum output current of 5A then a 30V transformer must have a VA rating of 212VA.

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so, back to VA, i´d like to know how many VA is the best for the transformer, or maybe you´d tell me how many you used. Thank´s again

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Just a quick question:

Is it possible to use larger transformer like: 2x38 VAC 272,5 VA.

Probably by lowering voltage to 30 V after diodes? Just asking becouse i might get one cheap.

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A 38VAC transformer without a load might be 39.5VAC. When rectified and filtered the raw DC is 54.4VDC.
But the U2 opamp has an absolute max allowed supply of 44V so you have a lot of power to throw away.

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I can use a laptop charger instead of a transformer in this design, or any other design? Only for space in the housing that will use.

Thank you.

PS: excuse my bad English, I'm spanish.

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A laptop charger has only 19V.
This project uses 40V.

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I know. I do not really need more than 20 volts, so I think a 24 volt charger fits my needs, could use this design with the charger I mentioned earlier?

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I know. I do not really need more than 20 volts, so I think a 24 volt charger fits my needs, could use this design with the charger I mentioned earlier?

The resistors and voltage drops all must be re-calculated to use a much lower supply voltage.
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