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| Power supply designing |
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| Erik:
Hello all, A couple of years ago i got a big transformer from a friend (2x18V@30A) and wanted to design a power supply for it but i failed because i didn't/don't have the knowledge for it. Recently i picked up electronics again and started from scratch learning theory. I did some self study to get atleast some basic skills but because i'm too eager to design a power supply myself i started looking at some schematics figuring out how they work and applying theory to that instead of reading 1000 pages of theory. The main problem i'm facing is the diversity of designs available on the net. I also looked at some forums about designing a power supply but that didn't helped me out much because everyone has another opinion about what's good and what's not. The first one says "use an LM317" the other one says "no!, use a LM723" The next one says "no! use a L200" and the last one says "Strap some op-amps together". Perhaps you can understand that this doesn't help out a newbie like me to determine whats the best base to start with. So my question is if someone has a good base design to start off with for a (lab) power supply which can be regulated from +/- 0V upwards and 0A upwards. I'm not asking for a complete design but just a simple but good base circuit for learning purposes which can be expanded eventually. Greets, Erik |
| septer012:
The reason everyone has different opinions on the regulators is that they all waste different amounts of power to heat. This is a trade off with current output and performance of these linear type regulators. The LM317 for instance requires something like Vin=3v+Vout to operate. In a very simple model lets say you input 9V as Vin to the regulator and you have it set up to output 3V. Whatever circuit you are running with the power supply is consuming .200 Amps of current. Power Wasted due to heat is: (Vin-Vout)Current = (9-3)*0.200 = 1.2 Watts. This is wasted power. Wasted power is converted almost entirely to heat energy and dumped through the Vout pin/heatsink on the package. Not too worrysome if you have a good heatsink in place, but it can be far worse and you can even desolder solder joints. The regulators datasheet should have a thermal joint plot which shows you watts vs temperature. An LM317 should probably not be used in a bench supply as it was really designed to be setup/designed per application and not such that its a variable supply with many varied conditions if you know what I am saying. I suggest you read this: http://www.dimensionengineering.com/switchingregulators.htm And maybe they explain things a little better than me. They are promoting switching regulators over linear regulators. There are two things that you have to worry about if switching to one of these type of regulators. 1) Noise in to your circuit on switching frequency 2) cost, they are very expensive compared to their linear counterparts 35 cents verse 15 dollars. |
| jimmc:
Just to add to what septer012 said, if you want to make full use of your transformer using a linear regulator then, under worst case conditions (max current, 0v out), you will have to dissipate about 1000w in your power supply (2 x 18V x 30A). Not impossible but not trivial either. Try looking in some of the amateur radio books, such as the ARRL handbook, hefty low voltage power supplies are popular for powering their portable gear. Jim |
| Erik:
Gonna take a look at that site for sure septer012! In the mean time i couldn't wait untill someone replied so i've built build this power supply http://www.electronics-lab.com/projects/power/003/index.html for practice. I did some quick testing and at the loos of it it works like a charm. Although there are some things i don't like for example they didn't use a bridge rectifier and didn't incorporate a fuse within, another thing i don't like is the transistor used to drive the 2n3055(2n2219) even though i'm still in the learning process it looks to me there should be a better alternative. It does have a nice led that lights up when your in current limit "mode" and seems pretty stable too. What do you think about the schematic? Like how it's designed and what could use improvements. Well, in the mean time i'll do some calculations to figure out how it's exactly working. Edit @ jimmc That would be the worst case indeed and would need some very good cooling. But in my case i think it would be better to make a dual output psu to limit the dissipation a bit but it sure is gonna need a big fat heat sink. |
| desolatordan:
--- Quote from: Erik on January 24, 2010, 09:40:02 pm ---Gonna take a look at that site for sure septer012! In the mean time i couldn't wait untill someone replied so i've built build this power supply http://www.electronics-lab.com/projects/power/003/index.html for practice. I did some quick testing and at the loos of it it works like a charm. Although there are some things i don't like for example they didn't use a bridge rectifier and didn't incorporate a fuse within, another thing i don't like is the transistor used to drive the 2n3055(2n2219) even though i'm still in the learning process it looks to me there should be a better alternative. It does have a nice led that lights up when your in current limit "mode" and seems pretty stable too. What do you think about the schematic? Like how it's designed and what could use improvements. Well, in the mean time i'll do some calculations to figure out how it's exactly working. Edit @ jimmc That would be the worst case indeed and would need some very good cooling. But in my case i think it would be better to make a dual output psu to limit the dissipation a bit but it sure is gonna need a big fat heat sink. --- End quote --- Here's a topic about how bad the design for that linear power supply is, and the necessary changes needed to make it safe and usable. http://www.electronics-lab.com/forum/index.php?topic=19066.0 |
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