Someone please explain - Page 1
Forum section: electronics / beginners

author: brezdomko
author_rank: Newbie
timestamp: February 12, 2012, 08:42:19 pm
content: So there is this power supply project...http://www.pocketmagic.net/wp-content/uploads/2010/02/LM317-30V-variable-high-power-supply-6-20A-max.jpgIt´s transformer has 240va + 6va secondaries BUT the power supply has a 600w output.... i mean waaaaaaa. I read the comments a bit but i just dont seem to be able to wrap my head around it. Someone please enlighten my stupidity.LukaEDIT: project pagehttp://www.pocketmagic.net/?p=1162

author: IanB
author_rank: Super Contributor
timestamp: February 12, 2012, 08:57:08 pm
content: I haven't read the article, but he is basically saying the output transistors could handle that much powerifyou provided a big enough transformer (and big enough bridge rectifier, and big enough filter capacitor).Another point that puzzles me is how these diagrams often suggest you can get (say) 30 V DC from a 24 V RMS transformer. At low output currents that may be the case, but if your transformer provides 24 V AC at 10 A, then themostyou could possibly get at the output is 24 V DC at 10 A. In reality various power losses and inefficiencies are going to get in the way, so you might expect 24 V DC at less than 10 A, or less than 24 V DC at 10 A. But not both at the same time.

author: 8086
author_rank: Super Contributor
timestamp: February 12, 2012, 09:00:48 pm
content: After giving the TIP3055 datasheet a quick skim it seems 600W is pushing it, though it could perhaps provide that for a short time. The datasheet reckons they max out at 90W each, so it would really be 540W max, not 600W.They seem to say that their supply doesn't provide that power though, in fact it says the transformer is rated for 240VA, so it's not really an issue. The power actually available isn't near the maximum rating.

author: Bored@Work
author_rank: Super Contributor
timestamp: February 12, 2012, 09:06:48 pm
content: You find many of those jokes on the internet. If you need a working power supply then you could do worse than design your own from the datasheet of a regulator.For this one, not only the 20A output from 10A input is a joke, but also the regulation. D4 is a lame attempt to work around the LM317's 1.25V minimum voltage limitation by removing a diode drop voltage from the output voltage. Since it is a normal diode the voltage drop over it isn't very stable. The whole regulation loop is closed via the power transistor's B-E diode. And, if it works at all, depends on the reverse transconductance (I need to look up if reverse transconductance is the right translation) of the power transistors. VBE of the transistors is also not 100% constant so you end up with a rather broken load regulation.

author: Unknown
author_rank: Unknown
timestamp: February 12, 2012, 09:13:14 pm
content: Quote from: IanB on February 12, 2012, 08:57:08 pmI don't see how you're going to get 10 A DC from a 10 A AC transformer with a linear power supply, so ~30 V DC at 6A or so would be the only option. The extra current capacity in the output transistors doesn't hurt.Quote from: 8086 on February 12, 2012, 09:00:48 pmGood luck keeping TCat 25°C when it's dissipating 90 W. You would need some sort of insane cooling configuration at normal ambient temperatures. Just the case to heatsink thermal resistance will probably cause at least 50 K temperature rise at 90 W.

author: IanB
author_rank: Super Contributor
timestamp: February 12, 2012, 09:37:14 pm
content: Quote from: alm on February 12, 2012, 09:13:14 pmOK, yes. The transformer would have to be de-rated due to the non-ideal load. I missed that.However, I just did a few calculations and allowing for rectifier voltage drop and regulator drop out voltage, I don't see away to get much more than about 24 V DC from a 24 V RMS AC supply if any reasonable current is required. It looks like a regulated 30 V could just be achieved with a minuscule load current.