Facing a problem with Dave's power supply design - Page 1
Forum section: electronics / beginners

author: Asim
author_rank: Regular Contributor
timestamp: April 18, 2014, 10:47:00 pm
content: Hi guys, so i bread boarded the circuit Dave designed,I have a problem with the first op amp (1a in the schematic). The voltage across the 1 ohms sense resistor is 1.5mV which corresponds to 1.5mA ( expected because of the lm334+ some losses). the problem is with the output of that op amp, it is giving me 70mv while it should give me 1.5mv( is it a noise problem?), the output starts to change when the voltage across the sense resistor increases beyond 100mv( i don't have an exact value) I double checked the 10K ohms resistors and they are fine. while trouble shooting i disconnected the sense resistor and feeded 5V I was getting 5V at the output of the op amp as expected.is this normal? if not where do you think the problem is ?THANKS.

author: facumedica
author_rank: Contributor
timestamp: April 19, 2014, 12:22:44 am
content: Well, what you got there is called "the Rail-to-Rail limits"I've faced the same problems, if you put -2v on the VSS pin of the Op-Amp you will see that it will reach 0v without any trouble. I think this is your problem, If I'm wrong, excuse me, english is not my native language

author: Asim
author_rank: Regular Contributor
timestamp: April 19, 2014, 12:31:47 am
content: Quote from: facumedica on April 19, 2014, 12:22:44 amOUCH, I totally forgot that, yea that must be it. i will test it later

author: edavid
author_rank: Super Contributor
timestamp: April 19, 2014, 12:39:03 am
content: Quote from: facumedica on April 19, 2014, 12:22:44 amNo, that's not it, the TLC2272 is used specifically because it has rail-to-rail output.

author: facumedica
author_rank: Contributor
timestamp: April 19, 2014, 01:38:05 am
content: Did you use that Op-Amp??? Or did you replace it?

author: Asim
author_rank: Regular Contributor
timestamp: April 19, 2014, 07:44:50 am
content: I did use it

author: Asim
author_rank: Regular Contributor
timestamp: April 19, 2014, 08:36:09 am
content: i was trouble shooting it again, the current limiting works fine when the voltage across the sense resistor is greater than 300mV. I tested it with shorting the output of the power supply to the ground and by increasing the current limit pot from minimum(0V to 500mV). something is wrong with the differential amplifier when the input voltage is low !I am using TLC2272CP

author: mrkev
author_rank: Regular Contributor
timestamp: April 19, 2014, 11:28:02 am
content: I believe that facumedica is right... Even tho it is rail-to-rail, you can't expect it to go to 0V at the output. Keep in mind that inside, there are two mos-fets at the output. Even tho they are fully open, they will have some resistance and D-S voltage...Try to implement low negative voltage, one diode drop should be enoug...

author: Asim
author_rank: Regular Contributor
timestamp: April 19, 2014, 12:05:23 pm
content: Quote from: mrkev on April 19, 2014, 11:28:02 amI have no other choice but test it, the thing is i don't have a negative power supply. so i will build a simple one with lm337 / 7905 somewhere , I alse have a center tapped transformer already.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 19, 2014, 12:18:30 pm
content: Yes facumedica was right ,it's the output limits of this opamp .There's no true rail to rail opamps . looking at the data sheet for this particular opamp shows you minumum OL (output low with 500uA load) typicaly 90mV -150 mV ,so it's even poorer than a cheap lm324 (OL min ~ 20mV) . You got a few options ,you could supply a small neg rail for the opamp ,or add a small offset to the diff output .

author: mrkev
author_rank: Regular Contributor
timestamp: April 19, 2014, 12:25:57 pm
content: Quote from: Asim on April 19, 2014, 12:05:23 pmActually, you don't really need thatif you put one diode at ground and connect all opa directly to that gnd, rest of schematic directly after that diode, it should be enough. Even shottky should do.

author: Asim
author_rank: Regular Contributor
timestamp: April 19, 2014, 02:44:47 pm
content: That's a neat trick mrkev ,I will try to remember it next timeI tested it and it is behaving as expected.that's strange anyways, Dave didn't use a negative supply or a diode to lower the reference of the op amps grounds even though i am using the same op amp ?!

author: mrkev
author_rank: Regular Contributor
timestamp: April 19, 2014, 03:34:06 pm
content: Quote from: Asim on April 19, 2014, 02:44:47 pmWhen you design things, it never works as expected at first goAnyway, i don't think he missed that, he was probably sattisfied with current limitation starting at about 100mA...

author: Bored@Work
author_rank: Super Contributor
timestamp: April 19, 2014, 07:10:13 pm
content: Quote from: Asim on April 19, 2014, 02:44:47 pmYou are aware that Dave never finished the design because it has at least one fatal flaw? And the diode trick? Well, it creates all sorts of funny problems, e.g. not having a common ground any more.

author: Asim
author_rank: Regular Contributor
timestamp: April 20, 2014, 12:23:58 am
content: Ummmmm, no I am not aware. What fatal flows ?!!

author: mrkev
author_rank: Regular Contributor
timestamp: April 20, 2014, 12:52:31 am
content: Quote from: Bored@Work on April 19, 2014, 07:10:13 pmNo, actually he builded exactly this design with few changes (so it could be controled from uC)... I don't know what you're talking about...http://youtu.be/Lg6oYFerUlAQuote from: Bored@Work on April 19, 2014, 07:10:13 pmNah, not really; even for bigger current than 1A... I can't think of any funny problems you may see there, nor how it changed "common ground".Simple negative source is better, sure, but that diode is't usually a problem, more so if Asim just wanted to find the problem on bench supply while considering negative supply later...

author: Bored@Work
author_rank: Super Contributor
timestamp: April 20, 2014, 07:12:57 am
content: I am talking about the LT3080 having a tendency to blow up. Just because Dave once build something doesn't make it correct or perfect.And you can argue all the way you want, you didn't think the rubbish with the diode through.

author: Rerouter
author_rank: Super Contributor
timestamp: April 20, 2014, 07:46:49 am
content: Bored is right there, you essentially need to diode clamp the set pin to the output pin, exceeding one from the other by (i believe) 1V can cause it to die internally, or just act funny,This was only later found out after harassing linear for some weeks, as they where unwilling to admit to it, they did make a newer alternative with a 10V differential which makes things more manageable,

author: Asim
author_rank: Regular Contributor
timestamp: April 20, 2014, 10:10:22 am
content: after reading Rerouter comment, I searched the forum until i found thishttps://www.eevblog.com/forum/projects/lt3080-wierdness-dave's-power-supply-(eev224)-gone-mad/LM317 here I come , though i will need to generate a negative rail from the battery to get to 0V output.any idea of an easy way to do it ?

author: Rerouter
author_rank: Super Contributor
timestamp: April 20, 2014, 10:28:17 am
content: generic 3.3/5V isolated switcher, you hook the positive side of the output to ground and away you go with -5V,

author: mrkev
author_rank: Regular Contributor
timestamp: April 20, 2014, 01:01:27 pm
content: Quote from: Bored@Work on April 20, 2014, 07:12:57 amOk, I didn't know that. Btw. you were the one who told us, that Dave never build it. Next time, just say what the problem is right away and safe yourself (and everybody else) some time, more so when it's not an obvious design flaw...Quote from: Bored@Work on April 20, 2014, 07:12:57 amAs I wrote, I wouldn't recomend it in final version. However it could be usefull in some cases.Anyway, I can't really "argue", since you didn't use any arguments ("it's rubbish" doesn't count even if you write it twice...).

author: rdl
author_rank: Super Contributor
timestamp: April 20, 2014, 05:37:24 pm
content: QuoteIt costs a couple of bucks, but I once used an LM385-1.2 and a 7660. I think used a voltage regulator too, I'd have to dig up the schematic to be sure. It might not be a great solution for battery power though. A CMOS 555 inverter may be better. The LM385 is pretty low power though.

author: gxti
author_rank: Frequent Contributor
timestamp: April 20, 2014, 05:55:49 pm
content: LM7705 is a nice, inexpensive switched-capacitor regulator that puts out a stable -0.23V, just enough to make any "rail to rail" op-amp actually go rail-to-rail. Will work for some non-RR opamps too but not the older designs like TL072 with overhead big enough to kick a football through.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: April 20, 2014, 06:23:15 pm
content: Quote from: Bored@Work on April 20, 2014, 07:12:57 amIn my search here I found it was the entire LT308x series that was flawed. But what I don't understand is why the issue seemed to die a silent death. Is LT held in some venerable position?And yes a big clue was the fact that Dave abandonded the project. From my naive point of view it was a good learning experience. i.e. Don't put all your eggs into a single-chip-basket and stick with what you know works.

author: Asim
author_rank: Regular Contributor
timestamp: April 20, 2014, 07:07:17 pm
content: guys, I have an idea feel free to correct me if i am wrong. I want the power supply to be portable that's why I want it to be battery powered. I will be using portable(3 or 4) 5V phone chargers( it has a lithium ion with a charging and step up circuit ).here is the idea: connect two batteries in series to get 10V and the 3rd one connect it backward so I will have 10V_ground_ -5V . i will be using the -5V to power the op amps. I will generate a -1.25V from the +2.5V reference with a inverting amplifier and use that as a reference voltage to the set pin, when the current limit kicks in i will short the set pin with a transistor to -1.25V instead of the ground.any problems with this ?

author: Asim
author_rank: Regular Contributor
timestamp: April 20, 2014, 07:16:51 pm
content: Quote from: rdl on April 20, 2014, 05:37:24 pmI already have some lm317, if things worked out with it i will look for a better regulator.

author: Asim
author_rank: Regular Contributor
timestamp: April 20, 2014, 07:18:32 pm
content: Quote from: Rerouter on April 20, 2014, 10:28:17 amI can't find such thing where I live( Dubai) and ordering it will take time

author: rdl
author_rank: Super Contributor
timestamp: April 20, 2014, 07:28:46 pm
content: I took the power for the negative side from the unregulated input, which was about 22 volts. So I had to use a little TO-92 LM337 to regulate the input voltage to the 7660 down below 10 volts ( I think I set it to 8 volts).Then an LM385-1.2 generated the the -1.25 volt reference for the LM317. The reference was a little bit too negative though. When turned all the way down, the LM317 actually put out about -0.05 volts.

author: facumedica
author_rank: Contributor
timestamp: April 20, 2014, 10:38:27 pm
content: Remember that Rail-to-Rail Op-Amps are ideal in theory, but as you may know they are made from transistors and resistors inside, and the transistors are like diodes and diodes are PN junctions of semiconductor materials and these materials aren't "perfect" they obviously will have a loss (just like every material, even copper, silver, aluminium).So, in brief, Op-Amps aren't perfect because transistors aren't perfect because diodes aren't perfect because the PN junction isn't perfect because the materials and methods of construction aren't perfect. (Nobody's perfect)I am not an engineer or anything (just a student) but something I learnt is that you will need to use some "practical tricks" to get what you expected to do in theory. So, test for yourself every option.And the voltage supply does not need to be symmetrical (I asked that question few weeks ago) I'm working in a microcontrolled power supply so I'm struggling with the same problems as you.Good luck! Keep the good work!

author: Asim
author_rank: Regular Contributor
timestamp: April 20, 2014, 10:50:02 pm
content: Quote from: facumedica on April 20, 2014, 10:38:27 pmI know it doesn't need to be symmetrical, what I am asking is, is their something I am missing on the idea I posted earlier? it seems to me as valid solution

author: Rerouter
author_rank: Super Contributor
timestamp: April 20, 2014, 11:21:49 pm
content: Quote from: Asim on April 20, 2014, 07:07:17 pmYou dont connect the last one up backwards, else you will just get +5V where you where expecting -5V,The other issue you face is dissimilar rates of discharge, as the -5V only effects the low current control circuitry, it will last much longer than the other 2, resulting in you replacing the top 2 much more often, not actually a big issue but something to be aware of,other than that, it should work, just keep track of where the current path is flowing, as with -5V being your op amps negative, any current they sink will discharge the battery, if you minimize this you could well get away with a CR2032 coin cell, and 2 honking big batteries for the main supply,Edit: thinking further i should warn you that a battery can be discharged to a negative potential, if there is a greater positive potential (the other 2 batteries) driving current through it while flat, so it would probably be smart to make up some form of low battery indicator as most chemistry get very unhappy when they go over a volt in the negative,

author: Asim
author_rank: Regular Contributor
timestamp: April 20, 2014, 11:39:04 pm
content: Quote from: Rerouter on April 20, 2014, 11:21:49 pmI had that in mind, i will need to recharge the main batteries more often than the other one(recharging two is better than 3)about the batteries running flat, I will be using 2 additional op amps for the battery indicators.here is the schematic( I draw it in 5 min i am sure something wrong will turn up xD)EDIT: I noticed the battery, I will fix it

author: edavid
author_rank: Super Contributor
timestamp: April 21, 2014, 12:23:41 am
content: I don't think this design is a good starting point for a battery powered supply. I think you would be much better off with an adjustable buck-boost converter module... maybe something like this:http://www.ebay.com/itm/Buck-Boost-Voltage-Converter-Voltmeter-Control-Start-up-Display-Blue-Color-/261373900003

author: facumedica
author_rank: Contributor
timestamp: April 21, 2014, 12:39:24 am
content: Quote from: edavid on April 21, 2014, 12:23:41 amLet him experiment with this kind of stuff! Let him suffer the steps of a design!Good luck!