Was thinking of building this...then I found the price of a mains transformer to run cost a bomb....
Did you look into toroidal transformers? They're not that expensive.
Yeah,but only on Farnell components site...pretty penny they're.
You might want to try Rapid Electronics I've got a toroidal transformer from them. They'll probably have the other components too.
Don't forget to price in the heatsinks !
Power wasted = (Voltage in - Voltage out) * current. Calculate this for worst case.
Required thermal resistance of heatsink = (Max heatsink temp - Ambient temperature) / power wasted. Say, ambient = 20degC and you're happy with a heatsink temp of 40degC then heatsink C/W = (40 - 20) / power wasted.
Check the regulator data sheet for the minimum dropout voltage, eg LM338 requires Vin to be at least Vout + 3v
Check the regulator data sheet for safe operating temperatures.
Check the regulator data sheet for info on the inbuilt current limiter.
Double check the heatsink data sheets to see if the quoted C/W is for natural convection or when used with forced air (ie a fan)
My personal thoughts on this design: attempting 21v range on a high power linear regulator is stupid.
I agree... this circuit is a good example of doing it because it can be done, rather than good or efficient practice...
Incidentally I'd always go back to calculating junction to ambeint thermal resistance and dissipation - just because your desired heatsink temp is 40degC doesnt mean the junction isnt running out of spec...
LM338 TO3 package TRjc = 1degC/W so input 20vAC * 1.4 = 28v, output 4.5v @ 15A. Pdiss = (28 - 4.5)*5 (per device) = 117.5W. Tamb=25degC, TJmax = 125degC - not possible! Assume 0.5deg/C per watt heatsink, total TRja = 1.5degC/W, max pDIss - (125-25)/1.5 = 67W, so max output @ 4.5V is 66/(28-4.5)A per device = 2.8A, 8.4A total
Best price I could find for such a heatsink was £21.50 (and you need 3 of them). Alternatively a 1.5degC/W and an 80mm fan would give the same result... at about the same price...
Oh and just in case you're thinking "I could make a heatsink"... thats equivalent to around a 300mm length of 100mm x 50mm U channel with forced air down its length...
£21.50! The ones I found in a quick search were £110 each. lol. Of course you could DIY... http://sound.westhost.com/articles/diy-heatsink.htm . Beauty isn't it?
The lm338 is quite a bullet proof design, at Vin - Vout = 20v for example, the internal current limit will kick in at around 2.5A. This protects the chip but makes a nonsense of this designs claims. As always on the internet, if something takes your fancy do your best to verify everything about it, including the credibility of the author. When it comes to electronics, the link at the top of this post is good place to start.
Last edited by ecat; 06-08-2010 at 05:31 PM.
You didn't mention just how many amps at what voltage you actually need. I built about a year ago a design from Elektor that works in a similar way to a Switched Mode Power Supply, but with most of the dangers to the user removed.....also, with the chips noted, you can have 10 amps at up to 40 volts. With other chips, you can have even higher voltages......if you want full details, I can post them here for you, just let me know......it was easy to build, easy to set up and is around 85% efficient, almost as efficient as a top SM Power Supply....and with the plus that I know EXACTLY how it works and I can easily repair it if ever needed.....all the clever stuff is in two identical chips.....
Sounds interesting, tho if it was in Elektor its a copyrighted design... but you could post up the gist of it for information.... personally, with the cost of PSUs on eBay in the £20 region its hardly worth building you own in many cases... I have a 0 - 20v bench supply @ 5A I built 20y ago, when I had access to unlimited MIL spec parts lol, but hardly ever use it, since a fixed 5v and 12v supply suffices for most things PIC-based these days. Rarely do any discrete analogue stuff now....
A good source for heatsinks are old blown inverters, they are usually mounted on a large cast custom heatsink, often with an inbuilt fan.
.John S -
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