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

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nope, you did not understand me right. :P
i was suggestiong to use the heatsink and the transistor that are already there and another resistor in series with the transistor, wich will be short-circuited(the resistor) when the voltage required at the output of the power suply is highter. what i was trying to say is that for the resistor, a heatsink is not required.

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I did understand you from the beginning, but i dont think the idea is so good, it complicate things, and it can make the PSU less reliable. The idea is not so dumb, it is much power dissipated at low output voltages and high currents, and that is transferred to heat, so if you can dissipate that in a resistor there will be much to gain, but it have moore disadvantages to, for example, where to put the trip point etc. And lamps dont have constant resistance, they have heavy positive temperature constant.

//Staigen

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yes, lamps are not more reliable.
i think that the best option here is to either put tu 2n3055 transistors, but it think that tha will require having a 30V a.c. transformer to obtain 30V at the output, or to use a highter power transistor.
or, maybe what about using 2 2n3055 transistors in paralel?
i have seen this in many schematics. maybe it will work better than having 2 transistors in series?

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Of course the transistors have to be connected in parallell. And i will suggest moore than 2 transistors. I dont think we can rise the transformer output to 30 Volts, the op-amps would not stand that voltage.

//Staigen

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Which is the relation between the number of the output transistors and the output voltage and current?

If we put for example 2 output transistors in parallel, the output current we can draw is doubled (theoritically if the rest of the circuit can affort it)? shall we change the rectifier diodes or something else to something more powerfull?

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There is a relation, but it is not that simple, you must also doubble the heatsink! If you put the 2 transistors on the same heatsink you cant double the current. Also, you cant just parallellconnect the 2 transistors, they are not going to share the current equally, you must also put a small resistor at the emitters, one at each emitter.

Then we have the transformer and the rectifier and its asociated parts. All you who is going to build this PSU, either the original PSU or the upgraded version we are discussing here, read this! First, decide the current output you want, than multiply this with 1.5! Thats the right rating of the transformer. Mixos have tested his PSU and he is using a transformer rated at 3 Amps, as stated in the projects text, and his transformer/rectifier brooke down at about 2 Amps. The rectifier must at least stand that current too, but here it is better if you can use a rectifier of a higher rating, the reason for this is that a bigger rectifier have a lower forward drop. Than theres the reservoir cap, usally you use 1000 uF/Amp, but in this PSU the cap must be much bigger, 10 to 20 times bigger, and the reason for this is that the ripple is raising when you take out large currents if the cap is too small. Usally when you design a PSU of this type the transformer is designed to deliver a voltage equal to or slight above the maximum output voltage, but in this PSU the maximum output voltage is higer than the transformer voltage, hence the heavy rectifier. Here i will suggest to move the whole rectifier and the reservoir condenser off the pcb and use a rectifier of the type that is bolted to the box of the PSU, also use a cap that is also bolted. The cap shall also be a "high grade" or "computer grade" type with very low ESR. Also the transformer must be of a very good type, with a very low ouput resistance, therefore i suggest a toroidal type. They cost a little bit moore, but nowadays not much moore, and the rise in performance is considerably higher!

//Staigen

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yes, i think that a 10 or 15A rectifer could be used....
why not use a bridge from schotty diodes? expensive, but more performant.
a good transformer would be a toroidal one, probably a 30V, 200W would be suitable.
i think that having 30V a.c. output rather than 24V. if we use 2 transistors in paralel, than a resistor must be bonnected to the emitter of the transistors. cannot use them directly.
also, 2 heatsinks must be user, or maybe a bigger one for the 2 transistors. like mentioned before, the putting the 2 transistors on a heatsing will not increase tha current. since the power lost as heat on the transistor is dependent on the current, having half of the current troug one transistors means that only half of the power is disipated on that one. and there are 2 transistors, you still have the same total power didpated.
anyway, i would be curious of something. say that i will cool a 2n3055 transistor with water and maintain the temperature at a low value, less than 50 degrees, do you think that i could make a current more than 15A pass trough it? i think that it is possible...but not tried it yet.

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Have you thougt of the IC:s? They cant stand to much voltage!

//Staigen

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Oops! I think i missed some of your reply. First the rectifier bridge, yes, and even higher rating, the difference here in sweden is minor between at 10 amp bridge and a 35 amp bridge ( 18:- Sek and 22:- Sek ) ! Schottky rectifier bridges i dont have any experience of, so there i can not give you an answer, but i think that i read somewhere that they dont perform so well in a regular rectifier at 50 or 60 hertz, or maybe it was that you did not get much advatage by using them so. Regarding the transformer, look above. And then the pass transistors and output current, of course you can buld it for moore and moore current, but i belive that is beyond the discussion here. You only have to get higher ratings of evrerything thats related to to it( transformer, rectifier, resevoir condenser, the number of pass transistors, the heatsink etc etc). Than the watercooled heatsink, of course that can be implemented, it is your money, you must in the end get rid of the heat anywhere, so you must cool the water somwhere else. Then the number of pass transistors, i will suggest using moore than 2 transistors, not moore than 1 to 2 amps per transistor, the higher maximum output current the lower current per transistor. This also depends on the heatsink. And here i think you also should think of safety and the SOA of the pass transistor(s)!

//Staigen

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what do you mean my SOA???
well....i forgot about the ic's. maybe they can be replaced by something else?

soooooo....
in conclusion the idea is to put 2 transistors in paralel....right?
and change the bridge....
yes, you are right, there is a small difference in price between a 10A and a 35A bridge, but still, i think that a low drop one should be used....

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SOA = Safe Operating Area, its in the datasheet of the 2N3055. Its often overlooked in constructions of this type, the constructor only see the 117 Watts of dissipation possibilltys.

//Staigen

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• 2 weeks later...

so................... ??? ???
isn't anyone still inerested in modifying the suply? ???

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Im probably going to build this one :), but i dont have all the comonents yet :(. The reason for the delay is that the PSU Mixos have and do some testing on, brooke down. He has now repaired it, and hopefully he can do further tests this week.

//Staigen

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I have been following this project for some time now and I think all the problems with heat, SOA, big expensive heatsinks, double fans and so on can disappear by going switchmode. This is more economic, much higher efficiency, lover component count and a lot less heat. And it will fit in a smaller case too. ::)

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• 2 weeks later...

Thank you very much for your previous posts on the component sizes and issues. I just want to make the original 30V/3A supply reliably live up to its specs! Just so I understand would you please answer the following. For the original 30V/3A supply:

Transformer should be about 4A to 5A, right?

Do you feel that "C1 = 3300 uF/50V electrolytic" is suitable? If not, what would you recommend?

Are the "1N5402,3,4" diodes adequate for the bridge?

Any other substitutions you would recommend?

Thanks alot.

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Yes right FireFly :D, that's what we are trying to do with this power supply right now, to make it reliably live up to its specs !!

The transformer is good to be 4A or even 5A

Staigen recommended a high value capacitor, something like 10000uF/50V if you can find one. Otherwise you can use some connected in parallel. The sure is that a higher value than 3300uF is needed.

Rectifier bridge diodes 1N5402,3,4 are easilly overheated and for that reason a ready made bridge rectifier (at higher power rating, even 8A-10A) is proposed to be used off the board, attached to the box of the power supply for better cooling.

We waiting Staigen to make his proposals, because he is a real PSU expert!! Where are you staigen :D ?

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OK, I got the hint ;D
We go linear on this one. :'(
Where are you Staigen!

Ante ::)

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• 2 weeks later...

I've almost collected all of the parts I need for the original 30V/3A project. A new, old-stock, Hammond 25V/5A transformer will be arriving any day now. I've got the presinsitized board, chemicals, etc. and have added extra points to drill to allow meter attachment (someday!). But I don't want to burn it until all of the components have been suitable "upgraded".

Could someone suggest what part to use for the diode bridge upgrade? The PCB appears to have enough room on that corner to easily allow changes to the copper trails for this if needed.

Can the 1N5402,3,4 diodes just be upgraded or is there a performance advantage to a one piece unit? (In this case, I don't care about a few extra holes or dollars -- just performance.)

Do these one piece bridges normally need to be heat-sinked? Would the smaller "L" shaped sinks attached to the board be needed/good enough?

Thanks yet again, everyone!

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well, a bridge is better than 4 diodes, because it cools easier.
this is because the surface of the birdge is highter than the one of the 4 diodes(maybe .....?!)
and buying a 10A bridge is cheaper and uses less space than 4 diodes.

now a switching mode suply could be done with about the same costs, but i am going off topic, since we want to modiy the original suply.

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This construction it is very flexible i have see the same circuit to have an output 0-30v and 10A with limit curent the only ghange it was to same parts and of course the promlem with the temperature of the two tranzistor and the registor. i have this PSU but the 0-30v-3A for 5 years and i don't have any problem i'm thinking to do some changes to make it more useful like to increase the curent and to make it with a digital control of A and volt. I recommend this circuit to every one that they use electronics. The only way to change this PSU it will be a regulator that it will control the A and the V. I have found same of them like the L200 or the L123 but i have'd try this part yet.

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Here did you see it at 0-30v and 10A setup? Can you give more info?

Does it gives you 3A at 30V?

In tests i made i could't take more than 2,3A

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Lisen the curent set from a trimer check that and the 10 A it is from the corect transform all the project it work with two comparative the one is for the Volt and the second it is for the Amber, you may use anything you want with this theory but you mast consider the transformer it has a registor that you must make her in the watt of the difference of curent limit with the max limit for examble the PSU has 10A max and 0-30v and you set the curent limit to have 1A out in a load of 5A you have 4A difference. Now with the Volt that you have for examble 20V the watt in the registor it is 80W this is the value that have the tranzistor for exable 2n3055 but be carefull the 2n3055 has some type with max A diferent from other. so you can use same similar to 2n3055 i don't remember the number but i will find them. but it is litle immoderete to have 10A to your place i have this PSU with 0-30V 3A and a 13,8V 10A and the only that i need is a PSU with -30V +30V 3A for some amplifier.

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Hi,

[move]I am Gaark. [/move] [glow=red,2,300]Thank you [/glow] for your response about the variable psu.I read your discussion on the PSU only in the transistor/heating phenomena.If yo need to arrest more heating, one of the easiest way ( other than using any mechanical ) to simply control the AC input voltage to the transformer using a Traic by closed loop.If you adopt this the collector voltage cant reach more and the transistor is safe at any loading condition ( With in the limit of the source you have )

But i need formal calculation for a particular voltage / current output form the PSU.Before switching ON the PSU i need the predetermined setting. Please suggest me.Hope I get all sugestion from your side. Because i am a biginer in electronics.

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Hi gaark.

I have seen a PSU that was equipped with a triac regulation on transformer primary. The way they done it was using a dual potentiometer for the voltage control, not a closed loop. The set-up was about 8 volts difference with no load. I think if you want a closed loop control you have to use a digital potentiometer with

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I am wishing to build a power supply..somewhat like the one being discussed here...but my needs are for it to deliver no more than 13 or 14 volts DC...with no more than 8 amps max.....usally it shall never reach more than 4 amps.....my problem has been is that I dont know what transformer to use....and what to change in the original plans to make this supply....please help with what transformer to use...and any other changes that would need to be made to the original plans to acheave this power supply...

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