Staigen

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

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

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

Wich make and model? //Staigen

d975 = 2SD975 d965 = 2SD965 b174 = 2SB174 They often leave out the 2S when printing on japanese transistors //Staigen

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

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

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

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

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

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

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

Have you find out who did it? If so, hang him/her out here in this forum! So someone can take care of him/her! //Staigen

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

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

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

Btw FireFly, have you buildt this PSU?

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

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

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

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

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

Jag vill önska er alla en riktigt God Jul I like to wish you all a very merry Christmas //Staigen

ExpressSCH is a part of ExpressPCB. And i dont think there is an evaluation version, only full version, but i can be wrong! ExpressPCB is freeware, i belive. You can download it here from Electronics Lab, under Downlads. //Staigen

To make scematics i use ExpressSCH, its easy to use and learn and it is fast to create a scematic. For making PCB:s i still evaluate a few programs, but i dont like non of them, and ExpressPCB i think is useless. At least if you want to make your own PCB:s. //Staigen