Advice for power supply design - Page 1
Forum section: electronics / projects

author: Scotty
author_rank: Newbie
timestamp: February 13, 2015, 05:40:09 pm
content: Hi! I'm a mechanical engineering student, but I've been doing hobby electronics for a couple years now.I, like most people here, am building a power supply from scratch mostly because I have a few of the components already lying around and I don't have a super great power supply to begin with and I can build it to my specifications and features that I want, like switched outputs on every voltage rail(not something I see much on power supplies these days)so here are my main design goals:--3-4A output current--adjustable to 1.5-20V give or take. This is fairly loose--achieving 0V isn't a high priority--very low ripple at max current output--low voltage drift--most of all: SIMPLEContraints:--I have 3 multi tap transformers from old cash registers they each have a 25V,15V and 12V secondaries ( I think... its been a while since I measured. it's 25V for sure though) , no idea what the current limit is, its not labeled, so I'm giving it a conservative value of 4A, I can include pics if requested. The fuse on in the socket is labeled 3A on the 120V primary side. so not sure if that would help deducting the output current of said transformer.My final design will have fixed 12V, 5V, and 3.3V output rails. What I will be focusing on here is the variable voltage rail.So I did some research and settled on the LM138K as my voltage regulator and the circuit is taken almost directly from the datasheet. Super simple.I modeled it up in Multisim and played around with the filter capacitor values and such to achieve a low to no voltage ripple at 4A at max and min voltage of the regulator. Im not sure how well the virtual scope in Multisim is but I'm thinking it gave me a pretty good ballpark. This seems like a great solution for me and my budget, only problem is navigating around all the fake LM138k chips. First question: Whats the average going price for one of those and where could I reliably find a genuine one? I'm finding them from $8-$60 USD so it's quite confusing.I recently came across this circuit :(not sure if I'm allowed to post links like this so be kind and give a guy a break its my first real post)http://electronics-lab.com/projects/power/001//This circuit meets my requirements, though current limiting is not essential it would be nice to have.My only concern is the stability and voltage ripple at max load. It is also not nearly as simple as the LM138k design.So on to my main question.I am wondering which of the two power supplies would be the most stable, accurate,and most of all easy to implement. I don't have ready access to board etching supplies/equipment so I'm mostly stuck doing strip board or point to point.Also I have a pair of very large Mallory type CGS electrolytic capacitorslabeled "10000MFD 100VDCPOS + 85degrees Celsius "These caps have been sitting in a box brand new, never used for over 10 years.They look brand new to this day so I am planning to use them in my design for the main filter cap after the bridge rectifier.My only concern is that if these caps need to be reformed.I have read that electrolytic caps need reforming if they have been sitting for long periods, unused, to work at peak performance.I personally do not have a power supply that goes to a high enough voltage to properly reform them for my purposes.Would any of you know how I cold get this done? Is reforming even necessary?

author: edavid
author_rank: Super Contributor
timestamp: February 13, 2015, 06:11:58 pm
content: Quote from: Scotty on February 13, 2015, 05:40:09 pmBuy from a US seller that promises genuine parts, and test them when you get them.(BTW, why wouldn't you use an LM338K instead of an LM138K?)QuoteNo, they might have a little extra leakage current at first, but it won't cause any problems.P.S. If I were you, I'd buy a variable bench supply (even a cheap Chinese one), and build your fixed supplies. Current limiting is pretty handy.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 13, 2015, 07:19:30 pm
content: Why don't you fill out your location info so we know where you're located?I have drawers full of useless (to me) junk like TO-3 linear regulators. They have to be genuine because I've had them longer than China has been in the business.

author: dannyf
author_rank: Super Contributor
timestamp: February 13, 2015, 07:36:28 pm
content: I would look into LT1083 for its current capabilities.You can let the output voltage to control switching the taps or to use a switching mode pre-regulator - I did one a while ago on one of those threads I started.

author: rdl
author_rank: Super Contributor
timestamp: February 13, 2015, 07:41:45 pm
content: Wow, I just looked and at Mouser there's only a 1 cent difference in cost between the LM138K and LM338K, but both are over $50!!!!I know I have at least one in my parts cabinet along with an LM317HVK that I bought back around 1990 or so. I didn't know I was sitting on a gold mine. I understand they're more expensive because of the TO-3 case, but 50 bucks seems like an awful lot of up charge.I'd probably just use the TO-220 version, or an LM317 with a pass transistor.

author: Scotty
author_rank: Newbie
timestamp: February 13, 2015, 08:03:43 pm
content: Quote from: Alex Eisenhut on February 13, 2015, 07:19:30 pmupdated my location infoQuote from: edavid on February 13, 2015, 06:11:58 pmI have a Elenco XP-720so I'm just building this as a learning experience and because I am needing something that can handle more than 1A. I did do a bunch of looking for an inexpensive variable bench supply but they were either cheap junk thats not worth my time or a bit out of my price range for what I wanted.

author: Scotty
author_rank: Newbie
timestamp: February 13, 2015, 09:41:51 pm
content: From what I can tell there very little difference between the LM338k and the LM138kI found a LM338k on Jameco for $7. It kinda seems a bit suspicious considering digikey and Mouser sell the same thing for $50+I've dealt with Jameco in the past, but not with chips.What do y'all think?I'm thinking that price is a bit too good to be true. Or maybe it's the first realistic price.

author: edavid
author_rank: Super Contributor
timestamp: February 13, 2015, 10:00:51 pm
content: Quote from: Scotty on February 13, 2015, 09:41:51 pmJameco usually sells good surplus, NOS parts, but they aren't technical and don't really know what they are selling, so be sure to test the parts if you buy from them.This eBay seller tells a good story:http://www.ebay.com/itm/New-LM338K-STEEL-P-GENUINE-REAL-OEM-NATIONAL-SEMICONDUCTOR-5A-VOLTAGE-REGULATOR-/121562175958

author: rdl
author_rank: Super Contributor
timestamp: February 13, 2015, 11:01:59 pm
content: The LM138 has a little bit tighter specs and a wider operating temperature range. For building a basic power supply the 3 version is probably fine. I've heard/read where people say the metal can TO-3 package might be less prone to pick up noise than the tab types, but I doubt it's a big difference.Quote from: Scotty on February 13, 2015, 09:41:51 pm

author: digsys
author_rank: Unknown
timestamp: February 13, 2015, 11:38:42 pm
content: Quote from: ScottyThe LM338K is officially out of production. Up to 3 months ago, we bought our last 250+ from named suppliers for app $7-8 eaScrounging around found a few after that for $20+, The STEEL version is still available for app $40-80 !! ea and Climbing !!You're better off going for the Linear Tech LT1083 = 7.5A, or LT1084 = 5A. It's a much nicer package as well.

author: Paul Price
author_rank: Super Contributor
timestamp: February 13, 2015, 11:42:07 pm
content: "Things are not always what they seem..sometimes skim milk masquerades as cream." ..Gilbert and SullivanNon-metal cased LM338 regulators are not very likely to pick up noise, just don't pick up any noise short of a nearby nuclear bomb detonation.http://www.ebay.com/itm/1x-NEW-LM138-LM138K-LM138-883C-MIL-TO-3-/261770286102?pt=LH_DefaultDomain_0&hash=item3cf2b99416Don't believe everything you read. Much that tries to pass for truth is only hysteria.Some or most of the LM338K on ebay are probably the real thing..and at the prices offered (a buck or two) what do you have to lose to try one or two? The LM338 is probably not at all likely being used in new designs in this age of energy conscious switchers, so there must be thousands if not tons of real ones left over, obsoleted and never used.For 15 to $20 USD you can get the whole power supply, even including digital voltage readout..sans case.http://www.ebay.com/itm/LM338K-Step-Down-Power-Supply-Board-Module-3A-DC-AC-Kit-Voltage-Regulator-/291379969345?pt=LH_DefaultDomain_3&hash=item43d7997d41And if turns out to be less than promised, you'd still end up with all the parts needed to make your own neato-keno P/S.Here's the whole $20 USD shebang with digital readouts see pic below.Try to beat a deal like that with Mouser. You would pay more with shipping just for the screws and nuts.Online skivvy, many have bought LM338J TO3 (in stock unicornelectronics.com) for $4 ea ($25 min order) but have bought many obsoleted parts from them, no problem.