I sourced 100 LT3080 T0-220 Parts, will list on ebay. - Page 1
Forum section: electronics / general

author: minime72706
author_rank: Frequent Contributor
timestamp: July 03, 2012, 02:40:38 pm
content: NOTE: I'VE NOTICED THAT THERE IS A UK SELLER OF THESE; I'D PROBABLY BE MORE CONVENIENT FOR THOSE IN USA/CANADA.Hey y'all!I've been thinking about building an adjustable, CV/CC, computer-controlled, DC power supply for a long time and I've been a viewer of EEVblog off and on. I came across the power supply series last month and took an interest in it. I was most pleased to be introduced to a better alternative to the LM317, the LT3080. I sourced 100 parts in the TO-220 package directly from Linear Technologies because the price break at 100 and shipping costs made it impractical to buy something like only 10. They seem somewhat hard to get and surprisingly there's no one on eBay selling them, so I bought way more than I needed to justify (in my mind) buying them at all. I had hoped that some people would be happy to have a different route to purchase them as well, so hopefully they don't sit around!I'll likely sell the LT3080 in quantities of five (5) for something like $30 shipped.They're $3.14 a pop in quantities under 100 and shipping (for me in MA, USA) was about $11, so it seems fairly reasonable to me. I may drop the price and then charge actual postage to make it a bit more fair, but that's somewhat of a pain for me. After seeing Dave's video about anti-static and static-dissipative bags, I'll certainly use real 3M metal-in anti-static bags! I've not received the parts yet as I've only ordered them a few days ago (and they appear to be back-ordered); I'm posting preemptively to gauge interest! My eBay seller name is the same as my username on this forum, but again, you'll find nothing right now! I'll update when I receive parts and are ready to ship. I'll probably keep 20-25 for myself and put the rest up for sale.Thanks!

author: free_electron
author_rank: Super Contributor
timestamp: July 03, 2012, 03:20:45 pm
content: Please do explain why you think the LT3080 is better than an LM317....

author: minime72706
author_rank: Frequent Contributor
timestamp: July 03, 2012, 04:24:53 pm
content: Why don't you watch his video series on his power supply design? This is not MY idea. (when searching the episode list, beware, he changes the title 3 times in the series of like 8-9 episodes)Since it uses a current source and resistor as a reference, one can force a voltage into the control pin and achieve output voltages closely approaching 0V.With the LM317, a 1.25V voltage reference is used and any voltage you put on the ADJ pin is in SERIES with that 1.25V. In order to operate down close to 0V you would normally need a negative voltage.The LT3080 also boasts a much lower drop-out voltage.Furthermore, it is rather simple to safely put the LT3080 in parallel with more of its kind to get a higher output current. With some resistive ballasting you can design your circuit so they share the power load rather evenly.You can generally design a circuit using an LM317 to do what you want, but the LT3080 simplifies things in many ways. I might even be less noisy and more stable, but it's not important enough for me to compare datasheets.The use of an ellipse (....) signifies impatience; there's no need for that.

author: free_electron
author_rank: Super Contributor
timestamp: July 03, 2012, 05:26:11 pm
content: MY question was based on this :Quotehere is the problems with the LT3080 for that given setup:1) no current control... you'll have to create another regulation loop that will have to fight the LT3080 regulation2) max 1.1 ampere.. the LM317 does 1.53) low drop.. who cares.. a lab power supply burns off 90% of the energy anyway .. you start with 20 or 30 volt at the input to make 5 volt at the ouptut. low-drop is a non issue in this case4) controllable to 0 volt. so is an lm317 if you use a negative reference voltage.5) Neither the lt3080 or the lm317 are advisable for lab power supply usage. Don't get me wrong, they are great regulators , but they are intended to be put in a system where a set of resistors aligns them to make a specific voltage and the STAY there for the rest of their useful life. You do not slap a potentiometer on them and build a 'lab power supply with themthey are no where near robust enough , too many limitations , no protection , no controllable current limit no nothing. They are intended for a finished product. not a lab instrument.and then there is this whole 'computer control' stuff. we are going to spend 50$ in parts to slap on a fancy display and computer control but it is too cumbersome and 'expensive' to buld a decent regulator section. ( which would cost like 10$ ). Of course hammering software on a pic and pc is easier than making a good analog control loop. This is my gripe with these kinds of projects.Note : i am NOT attacking you. It's just that the internet is full of total crap designs made by people who have an affliction for writing code but have no clue how to properly use an opamp and transistor. The end result is always pure garbage. If you are really serious about making a good DC power supply , that is computer controllable , start with building a solid regulator system and provide a properly protected power section. and THEN slap on a pic if you want to.

author: BravoV
author_rank: Super Contributor
timestamp: July 03, 2012, 05:38:01 pm
content: Quote from: free_electron on July 03, 2012, 05:26:11 pmAssuming you've watched those videos, I guess you're politely saying that Dave's design is also total crap since he is using LT3080 for his upcoming psu that he is going to sell.Just correct me if I'm wrong.

author: jsoderquist
author_rank: Contributor
timestamp: July 03, 2012, 05:42:48 pm
content: Ellipse?"You keep using that word... I do not think it means what you think it means..." --Inigo Montoya

author: free_electron
author_rank: Super Contributor
timestamp: July 03, 2012, 05:48:45 pm
content: I have not watched those videos, and that is NOT what i am saying.Dave is using the 3080 to make a usb-powered , portable doohikey to use in the field as a stop-gap for not having to lug around a real lab power supply.If that is your intention the LT3080 will do just fine. IF i needed to design a usb powered adjustable supply i might do the same.Then again, i wouldn't be designing such a thing as i have zero interest in that. It is bad to use a computer usb port as power output. They are not intended for that purpose and i do not want to fry a 2500$ laptop. And it doesn't work on all usb ports. I can see a scenario will people with a laptop that has 4 usb ports will hook up a usb hub and connect 5 of these little power supplies and crank away... it'll all end up in tears .I'll lug around a real power supply. Actually one may be better off by buying a laptop power brick that gives 19 volts at 4 ampere and combine that with a circuit board with power transistor and regulator and use that as a low-weight transportable power supply. those power briks are robust, pack a lot of power , are light. add a little circuit board to that, two potmeters , 2 opamps a darlington power stage style TIP142 add a pic with an lcd display to show set voltage , current and real voltage and current. and you will have something that works way better, has more power , and doesn't need to be hooked up to a computer.

author: minime72706
author_rank: Frequent Contributor
timestamp: July 03, 2012, 06:08:17 pm
content: 1. Sorry, I meant "ellipsis". I goofed. I'm a bit out of it (mono/glandular fever).2. The USB power supply series, which I haven't watched yet, is a different design.3. I hate writing code; you have no idea how much I hate writing code. More experience in doing so, though painful, is still useful to me.My design is still evolving, but I'm looking at a 9VAC/18VAC or (10/20) transformer to switch between two ranges to save burning some energy in the linear regulation section.I've also considered a switching pre-regulator, but my experience in the past with the MC34063 is not stellar. (24VAC -> 3.3VDC = kaboom in my design)0-20VDC@2A adjustable section (two LT3080s in parallel is what's on my mind right now) and probably a 5V@1A fixed-output (I will use a SEPARATE regulator for the control circuitry, I don't want a short on the 5V output to shut down the regulation circuitry!)I have a fairly good BK Precision triple-output power supply. This is mostly for the heck of it, but if I end up with a semi-portable adjustable power supply, that's a bonus. A design LIKE IT might be useful at work to reduce the number of expensive DC supplies required when testing a chip (I make integrated circuits).PS - If anyone can find a ~40-50VA transformer with a 120VAC primary and something like a 6/12/24VAC secondary (three taps), I'd be RATHER grateful as it would improve my power dissipation. I've seen them before on surplus sites and on ebay, but I've otherwise come out empty-handed.EDIT - Heh, I just realized that my advertisement has turned into a debate about power supply design. I hope the thread doesn't get moved!EDIT 2 - Also, the computer control is meant to give one the kind of functionality that a high-end Agilent supply gives you - automated testing. It will also have some knobs to control the supply in lieu of a computer.I haven't really given it much thought yet, but has anyone a good idea for safely switching between the 9VAC and 18VAC secondaries while the PSU is in operation? I wouldn't want a spike on the output when I cross the threshold.

author: madires
author_rank: Super Contributor
timestamp: July 03, 2012, 06:51:59 pm
content: Hi!Quote from: free_electron on July 03, 2012, 05:48:45 pmLOL! Using a PC or laptop (powered by the PSU) as power source requires some noise/EMI filtering. If you power an audio amp by the PCs PSU you'll be able to hear changing CPU loads. After adding some caps and chokes it's mostly gone. For cheap PSUs it's even more worse.QuoteDo you got a shortwave receiver? Put the laptop PSU and the shortware receiver in the same room and have some fun.Cheersmadires

author: minime72706
author_rank: Frequent Contributor
timestamp: July 03, 2012, 07:00:43 pm
content: Quote from: madires on July 03, 2012, 06:51:59 pmHeh, that sound of the changing CPU (and GPU) load drives me INSANE. You can hear it when you move the mouse...It's much less apparent if you use the rear port of a desktop computer (direct from the sound card) instead of the headphone port that might be located on the front

author: nctnico
author_rank: Super Contributor
timestamp: July 03, 2012, 07:20:04 pm
content: Quote from: free_electron on July 03, 2012, 05:48:45 pmIf you can spend the money, get a lightweight PSU like these:http://www.delta-elektronika.nl/en/products/es150-series.html

author: minime72706
author_rank: Frequent Contributor
timestamp: July 03, 2012, 07:30:19 pm
content: Those supplies are neat, but the purpose of this is mostly just to do a project with a practical purpose (one that I'll be apt to see to the end) and possibly to integrate the design into a test fixture at work.EDIT: By the way, unless I'm mistaken, the PSU I have is the BK Precision 1672.

author: Bored@Work
author_rank: Super Contributor
timestamp: July 03, 2012, 07:31:52 pm
content: The Lascar PSU 130 is nice, except that it doesn't have an adjustable current limit.

author: free_electron
author_rank: Super Contributor
timestamp: July 03, 2012, 07:33:39 pm
content: Quote from: minime72706 on July 03, 2012, 06:08:17 pmMC34063Motorola Crap... that thing is one of the worst switchers out there... I really don;t get why people seem to be stuck at LM317's , NE555's , LM324's and MC34063's...Almost anywhere you look on the internet, hobbyists are endlessly recycling that old tripe... All the while when there are hundreds if not thousands way better circuits out there.QuoteWhy not go to a real power transistor like a TIP142 ? my gut feelins still says the LT3080 is not intended for this kind of lab supply work. Reverse feed it and it will most certainly fry...Quotebetter make sure you have good readback capability then.. and that the supply is stable under impulse loads.Quotethat's a matter of having good line regulation. i would switch over using SCR's or mosfets. you can actually make a chopping system where you seamlessly go from one to another.BUt it's going to be hard getting the performance of , let's say a 3632 ...

author: minime72706
author_rank: Frequent Contributor
timestamp: July 03, 2012, 07:39:31 pm
content: I'm not averse to using a switcher, I was just making a joke of my past misfortunes with that chip. (My senior design partner almost lost his eye to a fragmenting device)Dave tested the LT3080's reverse protection, and as long as you don't short the input to ground and feed a voltage in backwards, it survives.Elaborate on what you're questioning with the PC-control, please.I'll look into your advice regarding my secondary-switching problem, thanks for that.

author: T4P
author_rank: Super Contributor
timestamp: July 03, 2012, 08:01:12 pm
content: The best regulator design is always a opamp + big fat chunky transistorFor switching the voltage taps use relays and some protection diodes to prevent the inductance from killing the circuitsOr for switching a not so complex regulation circuit and well tuned filters make the best out of a power supply

author: minime72706
author_rank: Frequent Contributor
timestamp: July 03, 2012, 08:04:42 pm
content: I have trouble arguing with that, heh.There's something to be said about tuning for stability and thermal protection, though.

author: T4P
author_rank: Super Contributor
timestamp: July 03, 2012, 08:10:56 pm
content: Quote from: minime72706 on July 03, 2012, 08:04:42 pmthe classic circuit still is far more stable

author: tom66
author_rank: Super Contributor
timestamp: July 03, 2012, 09:14:13 pm
content: My (purchased, not made) bench power supply uses SCRs to switch taps. It's quite a clever solution; the gates are biased such that when the output goes over a certain setpoint, it will switch taps. Because it uses SCRs (after the bridge) it has automatic delay (of one AC cycle), so won't jitter and it is a cheap, reliable solution.I would suggest one low voltage (3-4V) tap, so you can handle shorts easily, and then two middle of the range taps, so that total power dissipation is limited to around 10W.

author: minime72706
author_rank: Frequent Contributor
timestamp: July 03, 2012, 09:47:22 pm
content: I have not found such a transformer yet, but that would be nice. I'll be using a conservative circuit-breaker/fuse on the primary side, certainly.I wonder, what could I do to ensure that the control circuitry doesn't drop out under a fault condition? (short circuit leading to voltage droop, for instance)Thanks for the tip about SCRs; I knew those were useful for something!EDIT: There's a very strong chance that I'm going to be using a switching pre-regulator, but I haven't given it a ton of thought yet.

author: nctnico
author_rank: Super Contributor
timestamp: July 04, 2012, 12:21:19 am
content: Using a preregulator can be helpfull. About 15 to 20 years ago I designed (*) and built the PSU (2x60V 3A) in the picture below. It has a pre-regulator stage which 'disconnects' the secondary winding when the voltage on the capacitors is 5V above the required output voltage. Because the layout of the PCB isn't that good a lot of the 100Hz switching noise appears on the output. It has the size of a shoe-box but it can deliver a lot of power.(*) This is the first design I simulated and used MOSFETs for both switching and linear operation.

author: digsys
author_rank: Unknown
timestamp: July 04, 2012, 03:34:59 am
content: C'mon F_E, you're a better engineer than all that "bad mouthing" you're shelling out in this thread.The poor components never did anything wrong to you. In their defense -The LM317s are excellent regulators. So what, there's 100s much better now (depending on whatyou class as "better"), but it doesn't mean everything before is suddenly cr@p. Geez mate.I've spent parts of the last 2 yrs upgrading a huge inventory of Industrial P/Supplies that are ALLbased on the LM317s, mostly 5-20A. They've sold many 1000s in 30 yrs. As for the myths -They have built in Short CCT protection, the LM317HV T+K packages up to 60V IN-OUT !!They have Thermal protection, and (max) current limiting. It is VERY easy to add -Input protection (1 diode), O/Voltage (TVS and polyfuse), Adjustable current limiting and voltage.And YES, I've designed with DOZENS of other chips as well, including the LT3080 - ALL have apurpose in life. PS: The MC34063 was one of my favourite S/Ms years ago, poor thing.Sheesh, next someone will tell me that the ~100 XP PCs I look after are all KAKA :-)

author: David_AVD
author_rank: Super Contributor
timestamp: July 04, 2012, 04:01:37 am
content: As a new member, I find it curious (and a little disturbing) that a simple "I have these for sale" thread can turn out like this.

author: digsys
author_rank: Unknown
timestamp: July 04, 2012, 04:14:55 am
content: Quote from: David_AVD on July 04, 2012, 04:01:37 amAh yes, forums are a unique life form of their own :-) You never know WHAT they'll grow in to !!

author: free_electron
author_rank: Super Contributor
timestamp: July 04, 2012, 07:12:08 am
content: Quote from: digsys on July 04, 2012, 03:34:59 ami'm not bad-moutinhg the chips. i'm bad mouthing 'lab-grade' power supplies built with an lm317 and two resistors. That simply is NOT lab-grade.Yes LM317 will work perfectly in industrial applications. But they are all pre-set to a specific output voltage and will , during their lifetime , NEVER be set to something else..You can rip apart any test equipment from HP Agilent keithley and you will find heaps of lm317 and lm337. Nothing wrong, they perform even better than an 78xx , and you only need to keep one part in inventory, you buy them in larger quantity so overal it's cheaper. i fully agree with all that. I too make usage of those parts . no problem there.But all those applications have one thing in common : the lm317 is set to give a specific output voltage and it will stay there. FOREVER.None of those designs wil have potentiometers attached on them where someone changes the settings 50 times a day.None of those designs will have 30 different loads attached a day , some with shorts , some with overvoltages, some in reverse.They do their work hidden away in some system and the circuitry around them has been tuned for 1 operating point. for all means and purposes they only make 1 output voltage their whole life. it may have been trimmed in the factory or field, but it doesn't change every 2 seconds. The circuitry around them has been optimized for that operating point.That is a totally different scenario with making a lab power supply. Those things need to be built to take a beating , and stay stable under the most crazy conditions. So , yes , whenever someone exclaims that he is going to make a 'lab' power supply with an LM317 , my neck-hair bristles... and i will bad-mouth that. It is not a proper lab grade power supply. Period ! The LM317 , or any integrated regulator , is not robust enough, nor fulfills the requirements for that purpose. Trying to shoehorn the functionality in with trickery is just plain wrong. It is not meant to be used that way and was not designed for that.Get a big fat transsitor on a heatsink , make a control loop for voltage and current sensing ( properly done .. and not by using one lm317 as a pre-current limiter to feed another one to do voltage regulation.. your load regulation and impulse response goes to hell with that approach).All it really takes is two or three opamps and two transistors. Add some big fat protection diodes ( reverse output , and one across the pass element ) and you will have something that is virturally indestructible.A lab power supply needs proper current limiting with voltage foldback. Yes, The lm317 will protect itself when you hit 1.5 ampere , or you go over thermal but that is NOT adjustable current limit like you expect from a lab supply. And you'll never be able to burn off enough power in it before it thermally shuts down. no matter how big a heatink you use. the Rthjc of a to220 is not that stellar... yes you can use the TO-3 version but that is pricey , and still only 1.5 ampere max... you're better off slapping a 2n3055 (or something a bit more modern. there's all these beautiful transistors these days, yet we all seem to be stuck on the 2n3055) in then if you need a to-3 package... more bang for the buck , more power dissipation and more current handling.The LT3080 is a very nice design. you can put them in parallel and it is intended to replace an lm317 where you need more current (by paralleling) . The voltage control using a steering current is also very clever. It self-balances if you parallel them. It also has a wider working range since they split the regulation supply off the pass element supply. Perfect, great, brilliant , champagne and cigars all around ! i'll happily use it . But NOT as a LAB power supply ! i may use it in a lab supply to make the internal voltages for the opamps to run , but it will not be used as the regulating element , nor as the regulating loop !The reason i am so adamant about it is that i see a ton of these things coming from wingpangpong. And it invariably dies prematurely. it is not done properly Point two is that the web is full of 'arduino-style' engineers. Just because they know how to a tie a potmeter to the adj pin makes them believe they have made something that is nobel-prize material... this is just so wrong at so any levels. it is the easy way out. the lazy way out. the wrong way out.. i simply can't work that way. it just doesn't feel right and it goes against proper design. so yeah , when i see another one pop up... i get that itchy tingling feeling, and i just need my soapbox to 'vent'.Electronics is a passion, i want to do things the right way. At the same time i have no problem explaining how to do things. you can ask me anything , over and over. i will explain until you understand, but i will explain you how to do things the right way. I don't do "slap it togehter and see what happens", that's for junk. If it's worth doing, it's worth doing right. I do not want to be a 'duct-tape and super-glue engineer'.Me too i want to learn new things. The right things. we are supposed to move forward. not be stuck with 40 year old designs. "Because we've always done it that wy" is NOT the right answer if i have a question. Neither is "because it needs to be cheap" , "because we are lazy" , "because we can't be bothered" or "because we found it on the web , we don't understand it but we blindly copied it and introduced some problems of our own". You can't ever learn something, or move forward that way. you keep mucking about in mud ... you can't move forward if all you use is lm317 and 2n3055... why even bother to design newer parts. if everything can be slapped together with lm317's . let's stop making all those other products and designs. 640k ram ought to be enough for anyone ... right ?The goal of hobby electronics should be to learn something nad to make things better. Not to blindly solder some parts on a board to make an LED blink. That is not hobby electronics. That's just playing and trampling over a beaten path.As for the mc34063: i simply don't like that thing. It is too easy to destroy , the regulation loop is old fashioned , it needs big honking inductors and large filtercaps , has no thermal protection , fries itself for no sensible reason ( i've seen some that failed to start properly. they kicked in with the switch full on , without ever turning off.. you need an external current lim resistor.. too much junk needed.Give me something like an LT1616, LT1930 , that switches in the MHz range, monitors the coil current , can detect coil saturation and turn off and has all kinds of other protections in it. and if possible have synchronous rectification.I prefer any of the national semiconductor "Simpleswitchers" over that mc340xx. They have way better performance, need less stuff around them and are better protected.right. time to put the soapbox away. It's past midnight here, and tomorrow it's party time (4th of july)

author: nctnico
author_rank: Super Contributor
timestamp: July 04, 2012, 10:33:15 am
content: Nonsense. 99.99999% of the circuits don't really care about their input voltage. Within 0.1V is no problem. Using a LM317-like to built a simple lab supply is perfectly OK. You've got the whole regulation, short circuit current limiting, current fold back (thermal shutdown!) and thermal protection for very little money and effort. When I was a kid I built a triple output power supply with 3 LM317s and a couple of potmeters. I've used it for years.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: July 04, 2012, 11:15:26 am
content: Quote from: free_electron on July 04, 2012, 07:12:08 amI think you make the mistake of projecting YOUR view of hobby onto others too much.The goal of a hobby is just doing something you enjoy, or which results you enjoy.

author: free_electron
author_rank: Super Contributor
timestamp: July 04, 2012, 06:14:40 pm
content: Quote from: nctnico on July 04, 2012, 10:33:15 amMake that 'an adjustable supply' and i agree. Just becasue it is adjustable or thermal protection does not make it a lab supply.A lab-grade supply needs adjustable voltage and current limit, and bneeds to be able to work in both CV and CC mode. It needs to be robust , protected against all kinds of abuse. It also needs good load and line regulation and remain stable even under strange loads.Preferrably there is a kind of meter showing set and real values that is trustworthy.Here's food for thought : even the cheapest chinese 'bench-supply' ( i'm not even going to call it a lab supply ) doesn't use an lm317 ...Making a 'lab' supply with an lm317 is like bolting one of those fake ferarri or lamborghini body kits on a volkswagen beetle chassis and pretending its the real deal. It just isn't.

author: Tepe
author_rank: Frequent Contributor
timestamp: July 04, 2012, 07:28:25 pm
content: Quote from: nctnico on July 03, 2012, 07:20:04 pmThey appear to use the "special price for you, sir" principle. No thanks.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: July 05, 2012, 12:19:26 am
content: Quote from: Tepe on July 04, 2012, 07:28:25 pmKnow what you mean, but these are not that excessive actually;from the 2011 pricelist: 420 euro ex VAT for the 30V version, 430,- for 15V and 60V versions and 460,- for the 300V.

author: T4P
author_rank: Super Contributor
timestamp: July 05, 2012, 01:27:11 am
content: I fainted when i saw 420 Euro ...Okay ... Already so close to TTI

author: minime72706
author_rank: Frequent Contributor
timestamp: July 05, 2012, 04:11:47 am
content: free_electron, my supply design will have settable voltage AND current and it will use a high-side monitor to monitor the real current/voltage (I won't use a feedback control loop to actually try to correct the output, I can deal with the offset and component variability errors).As for robustness, the LT3080 is going to be hard to kill, especially if I add a reverse protection diode external to it. It's also supposed to be pretty damn stable, too.For safety, I'll probably have the microcontroller shut down the PSU if the output voltage exceeds like 10% of the set-point, but I won't be using any sort of control loop otherwise. Unless the DAC flips out, it should output what I tell it to output!EDIT: Also, yeah, I don't trust the MC34063 at all anymore. I think I saw a similar situation to what you describe, free_electron, where the switch turns on and stays on.My initial prototype that I made STILL WORKS, but pretty much every other one I built failed instantly, and spectacularly since I had a 24VAC transformer attached to it.I'm taking what you're saying into consideration. I would like to see just how poorly behaved the pre-current-limiter design is before I decide against it completely, though.Though I do like to joke about how older (40+) engineers all seem to rage about the overuse of micro-controllers, I completely agree with you. To this day I am unsure if the existence of Arduino is a good thing. I'm leading toward NO because I did my senior project a few years back using AVR GCC just fine and I am NOT a programmer. I would only use the Arduino IDE if I was doing something REALLY SIMPLE and/or was in a hurry for a good reason.I didn't initially appreciate your ranting and raving, but I'm seeing where you're coming from. Thanks.

author: T4P
author_rank: Super Contributor
timestamp: July 05, 2012, 04:34:28 am
content: It's still wrong to shoehorn a LT3080 into a box with knobs and a high side monitor and pretend it's a lab PSUIt's ... JUST WRONG!

author: minime72706
author_rank: Frequent Contributor
timestamp: July 05, 2012, 04:41:16 am
content: I've repeatedly said that I don't expect the performance of a lab power supply out of this thing... In the very least I'm going to consider using a feedback loop. I'm not sure how to digitally control such a thing unless I use a digital potentiometer, which I get the impression are not terribly robust.EDIT: The instance I mentioned where I might use a similar design at work in a general IC test board speaks against this, but we're talking about low voltages and the only thing I need it to do is current limit. If you're drawing a lot of current, the chip is probably FUBAR anyways (silicon bug shorting it out, bond wires shorting out, bond wires incorrectly placed, etc.).

author: free_electron
author_rank: Super Contributor
timestamp: July 05, 2012, 05:24:49 am
content: One of the things that get my stomach in a knot is the classical design where they use an lm317 with a 3 ohm power resistor between its output pin and the input of another lm317.they put a potentiometer across this one ohm and connect the wiper to the adj pin of the first regulator.The current drawn creates a drop across the power resistor and they use that to force the first lm317 to scale back its output voltage.so the current limit works on the principle of denying the second regulator 'breathing room'. This creates all kinds of strange effects. the output can go into oscillaton , the second lm317 doesn't have enough forward drop to regulate properly and it all goes to hell.This works fine if you connect a quasi dc load but with modern electronics the crest factors can be quite high. That regulation system is not stable at all. Attempt to power something that does pwm speed control of a motor and that supply will go totally wonky.you can get away with this lt3080 if you are going to build a little thing that does 0.. 15 volts 0.500mA or so.For me a lab supply is something that can do 0..20 volts 0..3 ampere, has sense terminals , correct setting and readout of what it does , and can be put in series or parallel.there si a problem with putting two or three of these supplies in series. The output of these things cannot hold much in reverse . If a load dump occurs you will fry the regulator chip.if you use a big fat power transsitor , those holds 120 volts or more in reverse. The little lm317 or lt3080 can't hold that. 40 volts and it's fried...Making a decent power supply is not complicated. two opamps , one for voltage, one for current , a reference voltage ( which can be the lm317 ) to derive the setting. and a transistor to let the opamp pull on the base of the power transistor.Take a look at the schematics of an E3410 for example. the simplest design possible , and there's even two lm317 ' in it. they use one to create a constanct current source to drive the power transistor. all the regulator does is deviate current from the base of the power transistor.Anyway, to each his own. It just frustrates me that the internet is full of 'dumb' schematics being cloned over and over again without any progress. nobody goes through the effort of really working something out anymore. we are breeding a massive amount of 'recyclers' and 'duct tape ' engineers. ( i use the term 'engineer' to indicate someone who creates a new design . not in it's academical denotation). It's all so watered down these days. Very sad. Copy-paste without understanding what is going on. And with understanding i mean really understanding instinctively how something works. Not just being able to write it down in equasions. A trained monkey can write down equasions.It's like music almost. anyone can copy the sheet music , some can jot it down on a tone ladder , but only a real musician can 'hear in his mind' how it sounds by reading it. sheesh... i'm getting lyrical now.... time for my dosage of prozac or zoloft i guess. i must be 'down....I'm kicking and ranting and raving because i care...

author: minime72706
author_rank: Frequent Contributor
timestamp: July 05, 2012, 05:41:04 am
content: I understand; there are a lot of pretty incompetent people out there that call themselves engineers. I guess I didn't think much about what I was doing, but I'm not trying to design anything amazing anyways.I don't expect to be driving a highly inductive load with this supply and why not protect it with a zener connected from output back to the input? (to prevent the output switch transistor from breaking down)I intend to output 20-30V @ 2A for the adjustable side and probably a fixed 5V supply @ 1A. I still haven't found a transformer that makes me happy, but I'll settle for a 9V/18V secondary - it's a lot better than a single 24V secondary, which is a common voltage.

author: minime72706
author_rank: Frequent Contributor
timestamp: July 06, 2012, 01:51:45 pm
content: As a random aside, I just sampled a bunch of parts to play with from Maxim-IC (DAC,ADC,current-shunt monitor, opamps, some other crap) and then it dawned on me when I was looking through my random development boards. The ATXMEGA chips pretty much have everything I want integrated into one chip (assuming I can tolerate however crappy the internal DAC/ADC might be). I've been looking for an excuse to play with those chips for a long time.It even has a battery backup system, which I thought might be useful because if there's a fault in the power supply, such as the output being shorted, I don't want the voltage droop (from the short) to shut down all the regulation and control circuits.Using a MCU like the xmega with a couple op-amps and a current-shunt monitor seems like a more cost effective and practical way to do this, but meh, if I have some high performance parts from Maxim, I might as well use those too.I might even be able to find a use for the xmega's internal comparators.

author: minime72706
author_rank: Frequent Contributor
timestamp: July 08, 2012, 03:37:03 am
content: I have a feeling that I'll be dead and buried by the time I receive those parts...

author: bingo600
author_rank: Super Contributor
timestamp: July 08, 2012, 02:52:36 pm
content: QuoteFrom Maxim no ...From Atmel yes ../Bingo

author: minime72706
author_rank: Frequent Contributor
timestamp: July 09, 2012, 12:22:18 am
content: Yeah I knew as much about Atmel, but I'm talking about the LT3080s from Linear Technology - I think they might not have them on hand right now.

author: minime72706
author_rank: Frequent Contributor
timestamp: July 10, 2012, 01:44:46 pm
content: Oh neat; looks like they're being delivered today! I can finally build something... maybe I should make a different thread since free_electron and I derailed my thread... thanks for the advice, though!EDIT: They faked me out yesterday, but I just signed for them at 9:15am EST (my time).

author: minime72706
author_rank: Frequent Contributor
timestamp: July 15, 2012, 06:36:14 pm
content: THREAD SERIOUSLY DERAILED (with useful banter, actually), BUT THEY'RE LISTED ON EBAY NOW.I'LL CONSIDER BEST OFFERS SINCE $25 ISN'T A HUGE SAVINGS, BUT IT DEPENDS ON HOW MUCH SHIPPING MIGHT BE.I'm selling them in 5-unit quantities.I hope I can get rid of at least some of these... if not they're good regulators to have in my gigantic hobby kit I guess!

author: minime72706
author_rank: Frequent Contributor
timestamp: July 15, 2012, 10:24:46 pm
content: Yeah, I guess I'll have to generate a product and sell it to make it worth it.

author: minime72706
author_rank: Frequent Contributor
timestamp: July 17, 2012, 03:28:45 pm
content: I'm going to use an external pass transistor with the LT3080 just to stick it to you guys. It'll basically be an op-amp at that point... if you ignore the weak current source.

author: minime72706
author_rank: Frequent Contributor
timestamp: July 23, 2012, 11:29:37 pm
content: This thread was severely derailed long ago, but I just wanted to bring it back to the top in case there's any interest.http://www.ebay.com/itm/Lot-of-FIVE-LT3080-LDO-Adjustable-Linear-Voltage-Regulator-TO-220-1-1A-LM317-/200792817644?pt=LH_DefaultDomain_0&hash=item2ec02f3fecI'm not sure why such a long link is necessary, but there it is. I'm selling them in quantities of 5. MAKE AN OFFER IF YOU DON'T LIKE THE PRICE AS IT IS.Thanks.