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Convert 0-30V 3A PSU to 5A or more


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Let's give a try to this:

How can we convert the following adjustable power supply which is rated at 3A max to 5A or more. It has a current limiter circuit and 0-30V output.

Curent Specifications:
- Input Voltage: ................ 24 VAC
- Input Current: ................ 3 A (max) ------> make it 5A or more
- Output Voltage: ............. 0-30 V adjustable
- Output Current: ............. 2 mA-3 A adjustable
- Output Voltage Ripple: . 0.01 % maximum
- Reduced dimensions, easy construction, simple operation.
- Output voltage easily adjustable.
- Output current limiting with visual indication.
- Complete protection of the supplied device against over loads and malfunction.

If we succed, it will be the best bench power supply available on the internet. Let's discuss the ways we can do that.

You can find it here: http://www.electronics-lab.com/projects/power/003/index.html

The schematic is the following:

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Okay, i belive this is a good PSU, but it have a few drawbacks! Lets discuss it and how to change it to be one of the best PSU:s on the internet. Has any actually buildt this PSU and tested it? Really tested it! What was the ripple at 30V / 3Amp output? And what happens when you try to get 3 Amp at very low voltage out? Okay, i know this is extremes, but especially at low output voltages and high currents you must be able to rely on your PSU. Any inputs on this subject would be appreciated. To change this PSU for higher currents is not very hard.

//Staigen

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I build it and give me around 4A output max, but i didn't let it for long time. I have not made any measurements on ripple because i haven't an oscilloscope at home, but i will do the measurements as soon as possible and i will post the results here.

What you suggest to use as a low voltage - high current load?

Also can you drive us to the right direction to convert it to a 5A or more PSU?

Q4 = 2N3055 it's rated at 15A max current as datasheet mentions. So it's ok to use it for higher currents. The changes must be done on other parts of the circuit. Am i right?

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Where shall we start? First i will say, i have buildt several hundred PSU:s, so i have some experience about PSU:s. I belive we start with the current PSU, the 3 Amp, and its drawbacks. First the 2N3055, yes, it is rated at 15 Amps, so thats not the problem, its the power disipated in this transistor. The power dissipated in this transistor at low output voltage( in the range 0-5 Volt) at high current(3 Amp) is in the range of 70-100 Watts. OK you say, its rated at 117 watts, so thats not a problem. But it can only stand 117 Watts if you can hold its case temperature at 25 degree celsius or lower, in practice it is not able to stand even 70 Watts.

For testing low voltage - high current i suggest using three 1 Ohms resistors, rated at 2 Watts each, in paralell( about 0.33 Ohms total) at 1 Volt - 3 Amp output.

For testing ripple at 30 Volt - 3 Amp i suggest using a 10 Ohms resistor( it must stand for at least 90 Watts, and such resistors is hard to get, but you can use ten 100 ohms/10 Watts resistors in parlell).

I stop here this time, but we keep on this discussion later!

//Staigen

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As per the text on this circuit, I believe P2, Q3 and U3 are used for current limiting. P2 sets 1 volt at the input of the op amp to achieve this. The same circuitry that is turning on the current limit LED is limiting the current.
Also, T1 will have to be rated at more than 3A. This might be overlooked when changing a design.

MP

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Hi there MP, have you also built this PSU? I see you are a moderator, so i belive you are a friend to Mixos. P2 is set between 0.0094 and 1.41 Volt to achieve a current limit between 0.02 and 3 Amps. What is T1? I dont see it anywhere in the schematic. Maybee you mean the power transformer, of course you have to increase the rating of it, if you going to increase the current capabillitys of this PSU. BTW, the rating of it is a bit low at the moment, it says 24Volt/3Amp and thats 72 VA, which is to small. At 3 Amp output the rectifier is delivering about 100 Watts, so the transformer has to be at least rated at 100 VA!

Mixos, have you tested it for low Voltage/High current output yet?

Is there anyone else who have built this PSU? What are your opinions in that case? I belive it is a good PSU, but it can be better.

//Staigen

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Yes, I was refering to the Transformer as T1. It is possible I have seen this schematic somewhere else with the listing as T1 or I possibly gave it an automatic designation as it would have to be T1. I have not built this power supply but from what I read in the text of how it works, I think it is possible to make the changes in the parts I have indicated in the last post. Awaiting more comments to see if someone has any other ideas as well.
Mixos: Since you were able to get 4 Amp out of this project, I am curious if you used a larger Transformer or adjusted the current limiting set point to achieve this. Or other method?

MP

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The transformer i bought was rated at 3A, but seems to be able to give more that this :D ! Also i didn't touch anything on the current limiter circuitry, I just set the current limit to full, that the strange!

Staigen: I haven't done the testing yet. Sorry for that, you will have to wait some days.. ::) By the way, I am happy you are an expert in PSUs that's give me some hopes to make this power supply even better :D, we hope to continue sharing your knowlebge with us! But can you be a little more detailed...

Why the case must has a 25 C, and not let's say 120 C (200C is the max temp that can affort this part) with a radiator of course.

"At 3 Amp output the rectifier is delivering about 100 Watts" Why so much watts?

Please help us understanding how you think in every result you post. So less experienced peoples will learn something!

Hope i am not getting bored.

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Ok, i will try to explain things, but its not easy for me, i'm not so good in the english language.

You have the data for the 2N3055 on your site, named MJ2955.pdf . If you look at figure 1 in that document you see that the maximum allowed dissipation power is only about 50 Watts at 120 degree celsius. This is so because there is a thermal resistance between the silicon chip and the case. To make things worse, there is also a thermal resistace between the case and the heatsink and also the heatsink have a thermal resistance. This is usally measured in C/W and often called K. I belive you can do a search on googgle for this and find a real description of it there, or maybee you have somthing about it in your articles.


And so the 100 watts. When you rectify 24 Volts ac you get about 35-36 Volts DC when the rectifyer is not very loaded, and when it is loaded its a little bit lower, thats why you can get 30 Volts out of the PSU with a 24 Voltage transformer.
I am not sure whath exactlly the rectifier gives out at 3 Amps load, but lets assume its about 33 Volts. And the wattage is current times voltage, in this case 33 x 3 = 99 Watts. Thats why i said that the transformer is rated a little bit low. Watts and VA is about the same, VA is ac and Watts is dc.

I hope i explained things a little, but it is not easy in english.

//Staigen

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Staigen, thanks alot for the info.

For 3A and 30V output, you believe the 24V/3A transformer is a little light. What transformer would you recommend to get the 99 VA you are suggesting? Maybe around 25V and 4A?

What tranformer would you suggest if this power supply is converted to 30V and 5A?

Thanks again.

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I belive we have to wait a little for Mixos to test his PSU. Btw Mixos, have you tested your PSU yet? For 30Volts/3Amps PSU i belive a 24 or 25 Volts/100 or 105VA transformer should be okay, but there is an another problem if you rise the voltage of the transformer too much, to get more headroom for the pass tansistor. Look at U2 and U3, they get to much supply voltage. I dont know what they can stand, at the moment they get around 38-42 Volts or so, but acording to the datasheet they are only guaranteed to stand 36 Volts .

For 5 Amps output i belive a transformer with a rating of 150 - 200 VA will do it.

Mixos aso wanted an explanation why we get so high voltages out of the rectifier. First, a transformer rated at 24Volts/3Amps delivers 24 Volts only at the rated load, and thats when it is loaded with a resistor. When its not loaded or loaded with a smaller load, the output voltage will be higher.
The voltage is the RMS Voltage, and when we rectify this voltage an smooth it with a condenser the voltage on the condenser will reach about the peak voltage instead, and the peak voltage is about 1.41 times higher than the RMS Voltage.

//Staigen

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No, I haven't built it yet. I haven't had enough free time lately.

I also took a detour to try the laser toner/inkjet photo paper transfer method for making PCB's. IMHO, don't waste your time.

Now I'm waiting to see what comes out of this discussion about the proper transformer to use as well as bumping it up to 5A. I'm relying on those who know something about this stuff. I'm not including me in that group yet! :)

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I am also going to build a PSU, maybee its going to be this one, at the moment i dont have a PSU worth the name, it can only deliver 1 Amp at 12 Volt maximum.

I have purchased a toroid transformer rated at 24V/10Amp, 240 VA,an expensive one. I also have a suitable box for it. But at the moment i dont have the money to buy the heatsink, its gonna be a real fat one, and it is very expensiive.

//Staigen

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So, I am also building this PSU and have box, panel meters, transformer, etc. I have also finished the circuit board.

I am thinking to use a thermistor circuit and 12 volt fan to keep the heatsink cool. Have not decoded to mount the heatsink inside or outside the box yet.

I think of two ways to power the fan circuit;

1. Use the rectified 30 volts and a voltage regulator to get 12 volts, or,
2. Use a dedicated 12v 1a transformer with rectifier and filter cap to provide unregulated 12 volt for the fan circuit.

What do you think?

DerFly.

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Just use the 30-35 Volts from the rectifier, a fan dont load much. Why not use 2 fans in series to cool both sides of the heatsink. But then you must do somthing to hold the middle point between the fans at a proper voltage. Can be done with a op-amp, i belive. But costs goes up!

//Staigen

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Hey Mixos, have you tested your PSU for low voltage/ high current yet? When you are testing, also check the voltage over the reservoir cap (C1). Also check the voltage over the resistor R7, and also the voltage over D7 and the output voltage from U1 (at point 5 ). Also the temperature on the case of Q4, you dont have to measure the exact temperature, just put your finger on it, it must not get to hot!

//Staigen

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Hi all, i belive we should take the most of the discussions about the PSU and its coversion to higher currents in this forum. Mixos and me has have a small discussion about how to do the tests in ICQ and via email, and he have performed some tests now. The tests did not fall out as good as the promises in the projects text. Trying to get more then about 25 V at 2.1 Amps show increasing ripple in the output, and trying to get higher voltages than that was not possible. At lower output currents it was of course possible to get higher voltage out. This is so because the reservoir condenser C1 is to small, maybe his transformer is not enough good to, i am not shure about this, he have to test that to. Can you test the output AC voltage from the transformer when the PSU is loaded at 1 V / 3 Amps? Also the pass transistor Q4 get very hot at low output voltage / high current (1V/3A)!

//Staigen

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I measured the AC voltage across the transformer at

1V/3A the output was a distorted sine with 32V amplitude

and it goes till:

1,4V/4,4A where the output was also a distorted sine with 31V amplitude

Is the transformer goes to current saturation?

At these outputs except the Q4 also the rectifier diodes becomes hot.

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Yes, the transformer is rated at about 72 VA, and you try to take out about 90-100 VA! Is there any place on the internet that describe rectifiers, somewhere? I have difficulties explaining exactly how a rectifier works, i know how it work, but explaining it in a few words is harder. And i mean rectifiers with its asociated transformer and reservoir condenser, also under load.

Shortly, the transformer have to be rated about 1,5 times the actual load, in this case 24 Volts times 3 Amps times 1.5 = 108 VA! Thats a 4.5 Amps transformer.

Then we have the ripple. This is because the current you draw from the reservoir cap when the voltage out from the transformer is lower than the actual voltage over the cap. The current is unloading the cap during that time and the voltage is dropping. The remedy for this is to heavily upgrade the cap to about 10-20 times the normal value. Also the rectifier diodes have to be at somewhat higher rating.

Now to something else. How much shall we upgrade the PSU to? I have a little suggestion here, how about 0 to 30 Volts and up to 5 Amps with good regulating and ripple and 7 Amps at somewhat lower voltage, also with good regulation ripple and 10 Amps intermittent but not moore than about 25 volts output with good regulation and ripple. I belive this is possible. At least i am going to build the PSU that way, if we success!

//Staigen

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ok. i have a suggestion. what if the 2N3055 transistor could be replaced by 2? this will lower the dissipation ok each transformer.
but the drop on the transistors means that the output will not be as high as 30V....
or, another suggestion. put a resistor in series with the transistor. and a relay with a control circuit. when the output voltage is more than 15V(let's say) then the resistor is shortcircuited by the relay and so you can get the maximum voltage. when at the output is lover than 15V then the relay is OFF and some voltage gets dropped across the resistor. this way you minimise the power dissipation ok the transistor.
and for the heatsink...i think that mounting a small fan, maybe one similar to a computer cpu fan coud be used. having an artificial air flow cools the transistor better than a large heatsink. and for getting 12V for the fan, i would go for a 7812 regulator. it is cheap, and for the current that a fan will draw, less than 200mA it is very good.
well, hope that my suggestions will do some good.

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Hi there Bogdan, shure they do! :) But we come to that later. Of course the pass transistor must have headroom, and i am also going to suggest multiple pass tansistors. Also fan cooling for the heatsink, in order to use a cheaper one. But the relay and its control circuit can easily be moore costly than a few transistors.

//Staigen

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well, yes, the realy and resistor and control circuit(a low power transistor, a zenner and some resistors) can be more expensive than the transistor itrself. but what about adding the heatsik? you will not need a heatsink for the resistors. yet, i think that the power resistors can be more expensive.
what about using something else in place of the resistors ? like car lamps or other types of lamps....?

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Adding a heatsink? The heatsink is already there. Do you mean that we shall use the pass transistors without heatsink? I dont think that will work. Of course a bigger heatsink costs moore, but you only buy it once, but the PSU you use year out and year in, so if it cost i little moore in the beginning, but lasts longer i belive its worth it.

//Staigen

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