Help fixing a variable power supply. - Page 1
Forum section: electronics / beginners

author: XOIIO
author_rank: Super Contributor
timestamp: January 03, 2014, 02:56:21 am
content: Alright hey everyone, second topic on this as the original one quite some time ago got out of control, my variable power supply has a problem where the amperage display does not work at all, and I am trying to fix it.I figured it may have been an IC on the display but those are actually just drivers, before the topic I checked all the diodes, they were all fine, I've since checked voltages and they all are in the rage the datasheets needed, I checked the voltages at 30v, and I was going to check more but I accidentally touched the hop left resistor (the high wattage one in this pic) and it was hot as hell, I checked and it was at 119 C whereas the other one is ice cold, I'm guessing that is a good place to start looking more into the problem but I am not quite sure where to go from here. I accidentally did touch a couple of the out pins on the LM 324n but that just tripped a relay or two. I'm letting it cool off and will see if it gets that hot with the power supply on a lower voltage but it still seems odd.I will note here that I only have a millimeter, no scope or other tools to check if any of the op amps are working improperly, and I have basically no working experience with op amps, but I figure the resistor is more likely linked to the problem than the op amps. I'm going to take out the second board to check the voltages on there as well.edit: On 10v or a bit higher the resistor seems fine but once you get to 20v it starts heating up to around 60 C, I'm not sure if this is a sign of the problem or just Chinese design not caring about how hot they drive it, though 119 seems excessive.The bottom right resistor is reading as 100 ohm which matches it's marking, the one getting hot is marked as 1k and it's also fine. (1.067k ohm)

author: XOIIO
author_rank: Super Contributor
timestamp: January 03, 2014, 03:16:16 am
content: The other board is all resistors and diodes and two CD4049UBCN, I believe this datasheet is correct, though I'm not sure where to probe on them to check if they have the correct voltage. I'm going to try a common ground and measure the other power rails I can see here and there.http://www.fairchildsemi.com/ds/CD/CD4049UBC.pdf

author: megajocke
author_rank: Regular Contributor
timestamp: January 03, 2014, 02:18:34 pm
content: A temperature of 119 degrees centigrade on a power resistor isn't much to be worried about. They are commonly rated for temperatures of over 200 degrees - I've even seen 350 degree ratings. They aren't made out of flesh and water, you know.As long as it doesn't heat up the nearby electrolytic capacitor it should be fine.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 03, 2014, 02:46:02 pm
content: Quote from: XOIIO on January 03, 2014, 02:56:21 amBecause you behaved like an idiot. Showing a junk video, wasting everybody's time. Claiming the 7106 is a microcontroller. Showing no willingness to listen to any of the recommendations.From your current posting we learn nothing has changed.QuoteYou figured nothing, we tried to tell you to look at that IC on the display, the 7106, you didn't listen. Now you claim the 7106 is just a driver. It isn't. At least two people told you last time what it is. And people told you to get the datasheet out, and start to verify the IC and surrounding components. You apparently didn't, not listening to suggestions. Somewhere to that board comes voltage, developed over a shunt, proportional to the current that should be displayed. That's where to continue if the 7106 checks out fine.You still aren't able or willing to provide a reasonable fault description:QuoteAnd you still don't know what you are doing. You are probing random stuff around the main board, in a live, mains powered instrument, and you don't listen.QuoteI can give you nine more millimeters, so you have a centimeter.

author: XOIIO
author_rank: Super Contributor
timestamp: January 03, 2014, 03:21:40 pm
content: Quote from: Bored@Work on January 03, 2014, 02:46:02 pmWell maybe if people didn't act like arrogant jerks off the bat there wouldn't be a problem, and guess what, the video showed the result of the problem, it wasn't a troubleshooting video, it was to show how the amperage display behaved. And, guess what, I never claimed it was a micro controller, I assumed that, because I didn't have time to look it up, and oh yes, like I said, most other ebay power and voltage meters are an all in one solution.Oh, and I don't know what I'm doing? where did you figure that out sherlock, when I said that I have not dealt with op amps before? I did take suggestions, I checked voltages, and they were all within the specifications, I couldn't find anything else wrong with any connections or anything on the other board with a current shunt, all connections and traces are fine, no cracked solder joints, I can't see anything that would be wrong that is why I am asking. Maybe someone helpful with some experience with a similar problem can offer useful advice,So, please enlighten me as to what I should be looking for instead of posting unhelpful and sarcastic crap like this and ignoring the whole "please be nice to beginners and do not deride them" part of the beginners forum.

author: XOIIO
author_rank: Super Contributor
timestamp: January 03, 2014, 03:22:57 pm
content: Quote from: megajocke on January 03, 2014, 02:18:34 pmIt does not, it's just beside the diodes they use as a rectifier, I'm surprised they run them that hot though, especially with the other one being stone cold.

author: EEVblog
author_rank: Unknown
timestamp: January 03, 2014, 10:09:51 pm
content: Quote from: Bored@Work on January 03, 2014, 02:46:02 pmB@W. This sort of attitude is not needed in the beginners section, please stop it.I'll be straight up here, because you say you don't read PM's. More reports are made about your posts than any other member (not many, but still, that's a fact). Any more and you will be banned from this section.

author: Alana
author_rank: Frequent Contributor
timestamp: January 03, 2014, 11:07:18 pm
content: Back to business:Have you checked power to your panel meters? If its based on ICL7107 then you should have +/- 5V supply. You mention boards with 4049 - from my experience with panel meters CD4949 is usually used as voltage inverter to get that -5V for the ICL without need for another transformer tap. In doubt post photos of boards in question.edited: mistaken LED and LCD versions of meter chip

author: Bored@Work
author_rank: Super Contributor
timestamp: January 03, 2014, 11:14:38 pm
content: Quote from: EEVblog on January 03, 2014, 10:09:51 pmIt has nothing to do with attitude, but telling the truth. That guy previously called uspricks,douchebagsandshit people. You can look it up in his postings in his first thread if you care. Let me guess, he didn't tell you that when he complained?You can also check that he showed a high clue resistance. Combine that with his activity to poke around in a live main circuit you might get the idea why I think a wakeup call was needed.QuoteI guess my fan club will see that as an invitation send more complaints.

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 01:01:51 am
content: Quote from: Bored@Work on January 03, 2014, 11:14:38 pmgosh golly you are right I guess I should have thought to check voltages with everything unplugged, how could I not have thought of that.News flash, if you don't behave like an arrogant jerk at the start when offering advice, then maybe people will be more inclined to take your advice.Quote from: Alana on January 03, 2014, 11:07:18 pmBoth modules (voltage and amperage) get a voltage of 9.08v, the chips get 9.04 volts,, they are actually ICL 7106's.I just noticed in my original post I explained it badly, (that's what I get for posting when tired), the amperage display does actually "work", it just does not display the proper amperage, at loe (sub 1v) levels it shows 20 amps, and higher it just goes to a 1 and nothing. I figured out which of the pairs is the variable input voltage for the voltage display so I guess I have to start cutting tape and follow the ones for the amperage display to try and figure out where they go to.

author: Alana
author_rank: Frequent Contributor
timestamp: January 04, 2014, 01:07:41 am
content: If i remember single 1 on ICL meter chip means "over mesurement range". It means chip itself gets too much voltage. Check what wires go between meter board and psu board and what voltage you have there. ICL chips have "native" range of 0-200mV that can be extended using voltage dividers.

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 01:41:55 am
content: Quote from: Alana on January 04, 2014, 01:07:41 amIt's not getting anything actually, it looks like it uses a 10k divider between the in+ and in- (not the power for the chip but the input reading, I'll have to cut the tape and see where those wires lead. It didn't show anything except at 30v I got .001 amps (miliamp range fuse is broken, but still it's basically nothing.)

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 01:55:32 am
content: Traced it back, one is just connected to ground, the other is connected to the current shunt, I tried using a clip lead to connect the solder joins in case the tiny wire they used crapped out somehow but that didn't fix anything. I'll have to figure out where the other wires from it lead to, but one side is just connected to ground of the whole thing. Not sure if that is common for these power supplies or variable ones in general.

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 02:07:28 am
content: well, it all looks fine, though I've never really looked into current shunts, it's kind of setup like this it seems.GND to output, in- of amp meter --||||||||-- GND from main power board, in+ of amp meterThere doesn't seem to be anything else in that circuit, I'd kind of think you would need a diode so it wouldn't read the current you put into it, I did notice it did that actually when I was working on something some time ago but that was before it messed up.

author: Alana
author_rank: Frequent Contributor
timestamp: January 04, 2014, 02:30:51 am
content: Disconect those wires and check if meter is behaving ok. What value is current shunt resistor?

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 03:08:24 am
content: Quote from: Alana on January 04, 2014, 02:30:51 amwill I need to disconnect the shunt resistor from everything else, because It's reading as nothing, just a direct connection, no resistance at all. The wires leading to it disconnected still read no current or voltage.The meter does appear to work normally without the connections to the shunt, it just seems odd that it's not reading as anything. It's the usual Chinese coil of thick wireAlso the meter does not use a 10k divider, it's a 75.5 ohm one or whatever the closes actual value should be

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 03:22:21 am
content: hmm even with the ground on one side disconnected it's measuring no resistance, so it can't be finding some other path unless it's through the meter but I doubt that, here's the shunt I'm talking about. (not sure why it has that bit of soldered wire on it, but it always did.

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 04:46:10 am
content: I'm really not sure about this I mean, it worked fine earlier, and now it seems that the amperage display is just getting no signal but I'm really not sure why the shunt resistor would have no resistance, how would that even happen? It doesn't really seem possible, but I can't see anything else that would be used as a shunt, I'll take a photo of the circuit board and just photoshop test showing the connections, this just seems weird.I have two 10w 5.1k resistors, two 5w 50 ohm resistors and I'm guessing another 5w 68 ohm resistor, I don't think any of those would work to replace a current shunt.

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 05:02:08 am
content: Alright the blue just shows where the meter in+ is jumpered across. I was wrong the output ground has no connection to the main power board that doesn't go through the current shunt, both meters get their ground connection through it as well.

author: Anks
author_rank: Frequent Contributor
timestamp: January 04, 2014, 05:11:24 am
content: The shunt is most likely either 1ohm or less.

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 06:58:08 am
content: Quote from: Anks on January 04, 2014, 05:11:24 amOK, I figured they would be a bit more.Anyways, anyone have an idea as to why I'm getting no votage or current from the wires that lead to the display or the problem in general?

author: Monkeh
author_rank: Super Contributor
timestamp: January 04, 2014, 12:41:00 pm
content: Quote from: XOIIO on January 04, 2014, 06:58:08 amLook into Ohm's law some time.QuoteDrive a current of 1A out of the supply, measure the voltage between in+ and in-.

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 12:50:01 pm
content: Quote from: Monkeh on January 04, 2014, 12:41:00 pmAlright with it shorted out and set to one amp I'm getting .029v (29 millivolts? just a 4 digit multimeter, never measured millivolts before. That seems right but I always doubt myself.)The amperage display itself is reading .81 amps, but it's pretty random, when you are around 1.5v it climbs up from 10, going up to 15 or 20, and then any higher it maxes out.

author: Monkeh
author_rank: Super Contributor
timestamp: January 04, 2014, 12:53:16 pm
content: And how do you know that 1A is correct?Assuming it is, at any rate, you have a 29 milliohm shunt. Which is why you thought it was a dead short.And now, we turn back to the ammeter board..Here's thedatasheet. Read it, understand the theory of operation, and figure out what's happening.

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 01:14:20 pm
content: Quote from: Monkeh on January 04, 2014, 12:53:16 pmBecause, I used my multimeter to check what the current being put out was.

author: XOIIO
author_rank: Super Contributor
timestamp: January 04, 2014, 01:52:44 pm
content: Alright the middle of the divider leads to a meg ohm resistor, and that actually is connected to the In HI of the chip, I guess I just didn't have a good connection before, I checked though and all the traces between connect fine so it has nothing to do with that part of it, it looks like they are just using the first schematic from page 11 of the datasheet.

author: Monkeh
author_rank: Super Contributor
timestamp: January 04, 2014, 03:24:09 pm
content: Quote from: XOIIO on January 04, 2014, 01:52:44 pmSo, check it out..

author: Unknown
author_rank: Unknown
timestamp: January 04, 2014, 06:46:59 pm
content: Quote from: XOIIO on January 04, 2014, 02:07:28 amPardon?

author: XOIIO
author_rank: Super Contributor
timestamp: January 05, 2014, 01:35:26 am
content: Quote from: Lurch on January 04, 2014, 06:46:59 pmWell, it would seem you would want a diode so that any voltage/current from whatever you are powering wouldn't go back in to the meters in case it could possibly damage them.

author: XOIIO
author_rank: Super Contributor
timestamp: January 05, 2014, 02:56:25 am
content: Quote from: Monkeh on January 04, 2014, 03:24:09 pmWell, on in- and in+ I'm getting a max of 148 millivolts, however if I measure between in- and the divider I'm only getting .037v ( microvolts?), if I measure between in+ and the divider I get 111 millivolts maximum.I took in+ and out it in the middle of the divider and the amperage still slowly climbs but it started at a higher level than when it is on the other side, so I don't know if maybe it relates to part of the divider circuit? The only other thing in it though is a 1 megohm resistor and a capacitor but all the caps are ceramic, I don't think one of them could be causing this.

author: XOIIO
author_rank: Super Contributor
timestamp: January 06, 2014, 06:00:12 pm
content: Well, I've given up, I have no clue what could be causing this, everything seems fine, all the connections are good, I have no clue. Only thing I can think of is some inrush current or something somehow messing up the chip.

author: Monkeh
author_rank: Super Contributor
timestamp: January 06, 2014, 06:11:54 pm
content: Quote from: XOIIO on January 06, 2014, 06:00:12 pmGiven up? You haven't even started.

author: XOIIO
author_rank: Super Contributor
timestamp: January 06, 2014, 06:25:47 pm
content: Quote from: Monkeh on January 06, 2014, 06:11:54 pmHow so? I've checked all the components that are part of the circuitry the chip uses to measure the input with, all the traces are intact, no cracked solder, and actual input it receives changes when I tweak the current limit yet does not exceed the specified 200mv maximum, the only thing left that could be causing this is the chip itself, and I can't find anywhere that has them, and there does not seem to be any compatible modern replacement. I also think it's the chip because before it maxes out the amperage steadily climbs with no changes being made to the voltage or amperage adjustments.

author: Monkeh
author_rank: Super Contributor
timestamp: January 06, 2014, 06:31:18 pm
content: Quote from: XOIIO on January 06, 2014, 06:25:47 pmSo.. all of them, then? How about you draw up the circuit and precise values they've used.QuoteYou mean other than Mouser, Digikey, Arrow, Newark/Farnell, RS, Tayda, Jameco, Futurlec, Future Electronics, Avnet.. It seems to be a very hard part to locate! Digikey only have 1022 in stock, you'd better order fast!QuoteThis sounds like a floating input.

author: XOIIO
author_rank: Super Contributor
timestamp: January 06, 2014, 06:38:52 pm
content: Quote from: Monkeh on January 06, 2014, 06:31:18 pmAh, I figured something old like this would only be on ebay or some chinese site since looking for other info on the chip never even brought up digikey or mouser or any similar sites.I guess I could do a schematic, I'll have to pull the thing apart again :/ I'll do it when I'm not dead tired tommorow.

author: XOIIO
author_rank: Super Contributor
timestamp: January 06, 2014, 06:41:11 pm
content: I also probably had trouble finding it since I had a totally wrong number it. Yup, I need sleep.

author: grumpydoc
author_rank: Super Contributor
timestamp: January 06, 2014, 07:50:13 pm
content: QuoteJust Googling a part number is often a good way to get data-sheets but not necessarily the best way to find suppliers.In the UK, at least, using the "Search Tools" to select UK only sites yields some links to places to buy - the same might be true if you choose the country-specific search for where-ever you live (which you haven't set in your profile).The best way is to search the larger suppliers' web sites - Farnell and Mouser are both goods placesThat saidthe 5th link produced by Google is for the copy of the data-sheet hosted by Futurelec - now opinions vary as howgooda supplier Futurelec is but it is a supplier.QuoteBut did youlookon ebay?It can be reasonably useful for older stuff.QuoteThat doesn't help.