Odd output from Elenco XP-15K Power Supply - Page 1
Forum section: electronics / beginners

author: tomherrick
author_rank: Contributor
timestamp: January 20, 2019, 10:22:44 pm
content: Bought the kit and put it together yesterday. First step afterward was to check the VDC output. When the pot was turned all the way up to the 15V mark I got -0.53V. Yup, negative. When I turn it all the way back to zero Volts it reads -0.65. Voltage across P2/P1 reads 17VAC; same reading across P2/P3. When I unplug the unit, the LED slowly dims to off and the multimeter value dropped from -0.65 to -0.32 over the course of a minute or so and continues to drop very slowly.I've been over and over the instructions and rechecked the following: polarity of the diodes, caps and LED; output connections from the transformer to P1, P2 and P3; ensured that the regulator was oriented properly. I went through the test procedure and shorted across the output; the unit did not turn off.The error is likely mine and I keep looking past it when I QC my own work... Suggestions???Thanks.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: January 20, 2019, 11:33:22 pm
content: Hi Tom, welcome to the forum.Take some very crisp pictures of the board from both sides. Also, a shot of the schematic and post them in another post so that we can see your work and the schematic. You have a size limit for your jpg's (I can't remember what size), but the uploader will complain if they are too big.

author: tomherrick
author_rank: Contributor
timestamp: January 20, 2019, 11:57:26 pm
content: Here are photos of the circuit board and the schematic.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2019, 02:08:37 am
content: Easy enough to inline as well. Click the attachment, right-click the image, copy link. Paste into an img tag:Hah, looks like it's drawn in Multisim of all things.Tim

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2019, 09:02:54 am
content: As posted in the other thread:Quote from: rstofer on January 21, 2019, 02:16:00 amYes, this is a classic way of getting near 0V from the LM317, which can normally only go down to 1.25V.It could be a problem with the potentiometer.Quote from: tomherrick on January 20, 2019, 10:22:44 pmWhat are the voltages from pin 3 to 0V and pin 1 to 0V?

author: tomherrick
author_rank: Contributor
timestamp: January 21, 2019, 11:28:16 pm
content: "What are the voltages from pin 3 to 0V and pin 1 to 0V?"Pretty sure I know what Pin 1 and 3 refer to, but am lost at 0V unless it's Pin 2...I'm still boggled by getting a negative DCV reading at the output measuring around half a volt.

author: rstofer
author_rank: Super Contributor
timestamp: January 22, 2019, 02:46:53 pm
content: 0V refers to 'ground'. P2 on the transformer or the test jack (black) on P4.In general, any number we give you will be in reference to 'ground' because it is too hard to calculate from a different reference.Pin numbers are referring to the pins on the LM317Pin 1 = the adjustable control voltage - variable from about -1.4V to perhaps 15V - these aren't exact numbersPin 2 = output voltagePin 3 = the voltage going into the regulator. This might be on the order of 18VWe're trying to dissect the problem. Is there about 18V on pin 3? If so, everything to the left of the LM317 probably works. It is still possible the -1.4V (give or take) on the junction between R4 and D5 is not correct. So, there are two measurements. They are the first numbers required for troubleshooting.Next we're looking at pin 1 and expecting to be able to vary the voltage between some negative small value (like -1.4V) and some positive higher value - perhaps 15V. There's another set of measurements.Finally, we want to know if the regulator works by measuring pin 2. I assume it is not working because you have said it was stuck low.We don't know how you took your original measurements but would probably guess that you just measured the output across the jacks. Measuring a small negative value might be realistic if pin 1 is -1.4V. Just make your measurements with reference to the black output jack.So, make some measurements and post them.

author: tomherrick
author_rank: Contributor
timestamp: January 22, 2019, 03:13:05 pm
content: I've taken these measurements with photos of results:Voltage from P4 to P3 is 17.1VVoltage from P4 to P1 is 17.1VOutput voltage with pot turned to 0V is -0.65VOutput voltage with pot turned to 15V is -0.53V

author: rf+tech
author_rank: Frequent Contributor
timestamp: January 22, 2019, 03:25:28 pm
content: Why is an LM337 installed, when the schematic calls for an LM317?RF+ Tech

author: tomherrick
author_rank: Contributor
timestamp: January 22, 2019, 03:32:48 pm
content: I installed the components provided in the kit. It appears to be an LM317T.

author: mvs
author_rank: Frequent Contributor
timestamp: January 22, 2019, 04:05:43 pm
content: Quote from: tomherrick on January 22, 2019, 03:13:05 pmP4 is connected to P2, so you are measuring AC voltage from the transformer.Have you switched your DMM to DC voltage mode to measure DC voltage at the output?Check DC voltages across capacitors. You may post only values, no need to have them on photos.

author: tomherrick
author_rank: Contributor
timestamp: January 22, 2019, 04:11:03 pm
content: A previous respondent indicated wrote: "We don't know how you took your original measurements...", hence the photos to make it clear. I switched the DMM from VAC to VDC to take the output readings; guess I should have included the DMM selector switch in the photos.

author: rf+tech
author_rank: Frequent Contributor
timestamp: January 22, 2019, 04:12:07 pm
content: Thanks for the clear photo. That middle digit sure looked like a 3.What is the voltage on the LM317 adjust pin, at minimum and maximum?Please refer to the datasheet for correct pin identification.RF+ Tech

author: tomherrick
author_rank: Contributor
timestamp: January 22, 2019, 04:56:00 pm
content: MVS, not sure how to properly measure voltage across the caps. In VAC or VDC? The only steady reading I've gotten so far is across C1 at 49.5VAC. When I switch to VCD readings I get a myriad of results that keep changing. Am I looking for AC or DC values? Should I expect the values to change/degrade?rf+tech: On the LM317, Pin 1 is labeled ADJ. When pot is set to 0V the value is -0.57V; set to 15V it's -1.30V

author: rstofer
author_rank: Super Contributor
timestamp: January 22, 2019, 05:29:27 pm
content: Just to make sure everything is clear, measure AC voltage when looking at the transformer, DC voltage everywhere after the 4 diodes.For fun, measure between blue and yellow and blue and the other yellow. They should be the same but post these as the AC voltage results. I think you did this and got 17.1 VAC but I just wanted to be certain. Pretty sure that's valid.49.5 VAC on C1 is invalid because it is a DC voltage - it's after the rectifier diodes. I would expect a number less than 20 VDC.Voltage across ALL capacitors is DC. They are after the rectifier diodes.Is the regulator getting hot? One thing that will cause the DC output voltage to vary is the built in self protection from overcurrent. This will reduce the voltage substantially until overload is removed.

author: rf+tech
author_rank: Frequent Contributor
timestamp: January 22, 2019, 05:37:35 pm
content: What is the DC voltage across pins 1 and 2 of the LM317?To be clear, place the DMM black lead on pin 1 and the red lead on pin 2.This is expected to be 1.25 Volts DC, regardless of the potentiometer setting.Also, several requests have been made for the DC voltage on LM317 pin 3. Please measure this, with the DMM black lead connected to P4 common/ground/minus terminal. And for all future measurements, unless requested otherwise.

author: tomherrick
author_rank: Contributor
timestamp: January 22, 2019, 06:10:23 pm
content: DC voltage across pins 1 and 2 of the LM317 is -0.10.DC voltage across pin 4 and pin 3 is -0.83 @ 0V setting; -0.39 @ 15V setting.

author: rstofer
author_rank: Super Contributor
timestamp: January 22, 2019, 06:39:33 pm
content: Pin 3 to the black terminal should be 1.4 * 17.1V (assuming this is AC voltage between the blue and yellow or about 24VDC. This is really high for a 15V power supply. Make sure you measured 17.1VAC and check pin 3 as VDC.One other thought: The regulator takes between 3.5 and 10 mA of load current in order to regulate. At very low voltages, say 0.1V, if your device needs 10 mA, you need a 10 Ohm load. At very high voltage, say 15V, for 10 mA,, you need a 1500 Ohm load. Notice that the load resistor varies widely over voltage. You surely wouldn't want to use a 10 Ohm load on 15V as that would be 1.5 Amps.I used 10 mA for those numbers because that is worst case. Ordinarily I would expect your actual load to be adequate.

author: tomherrick
author_rank: Contributor
timestamp: January 22, 2019, 06:59:00 pm
content: Here are the measurements:P2 to P1: 17.0VACP2 to P3: 17.0VACP4 output negative to LM317 pin3: -0.51VDC

author: rf+tech
author_rank: Frequent Contributor
timestamp: January 22, 2019, 07:12:48 pm
content: Now we're getting somewhere. The AC input side looks as expected and the focus is now from D1, D2, D3, D4 and to the right side of the schematic.Please answer re is anything getting hot, especially the LM317.Measure DC voltage from P4 output to D1 cathode (banded end) and from P4 output to D3 cathode.Edit:A second look at the back side seems to show a suspicious looking solder joint at P4, as noted in attached photo.

author: tomherrick
author_rank: Contributor
timestamp: January 22, 2019, 07:26:54 pm
content: OK, now it's getting weird... DMM on VDC, neg to P4, pos to cathode side of D1, I get a momentary reading that differs each time then is replaced with the reading you see below. Momentary readings were 15.45, 17.24, 11.72, and 17.50 on D1. Momentary readings on D3 were 14.89, 19.82, and 19.21. All were shown for less than a second then the DMM display shows "1 . ". Golly, I hope that helps...

author: rstofer
author_rank: Super Contributor
timestamp: January 22, 2019, 07:35:02 pm
content: That measurement on pin 3 of the LM317 is very wrong. Unfortunately, troubleshooting beyond this point may require some disassembly and that is often problematic on single sided boards. The pad tends to lift off the board.If I were making the tests, I would remove the LM317 and then check the Input and Adjust pads. The Adjust won't change because it doesn't have the feedback from the output voltage but it should be about -1.4VDC. And the Input should be about 24VDC.Before we worry about regulating, we need to get the Input level correct and we need to know that we have that negative rail for the Adjust circuit.SolderWick is your friend and I would use the wider variety. Make sure there is plenty of solder on the pad before you apply the wick. I know, counterintuitive. If you have to buy SolderWick, buy some paste flux as well. Put that over the solderball on the pad to increase the amount of flux beyond what SolderWick supplies.Were it mine, I would cut the leads off the LM317 and remove them one by one. Less chance of damage that way. Of course, I would need to buy a couple of spare LM317s... I would absolutely do it by cutting the leads. I have destroyed too many pads...https://www.digikey.com/products/en/soldering-desoldering-rework-products/desoldering-braid-wick-pumps/265?k=solder+wick&k=&pkeyword=solder+wick&sv=0&v=473&pv1698=6&sf=0&FV=ffe00109&quantity=&ColumnSort=0&page=1&pageSize=25I would use the 0.1" braid for this operation.https://www.digikey.com/products/en/soldering-desoldering-rework-products/flux-flux-remover/266?k=paste+flux&k=&pkeyword=paste+flux&sv=0&v=473&pv183=3336&pv771=256&pv771=259&sf=0&FV=ffe0010a&quantity=&ColumnSort=0&page=1&pageSize=25LM317https://www.digikey.com/product-detail/en/texas-instruments/LM317T-LF01/LM317T-LF01-ND/3640769

author: rstofer
author_rank: Super Contributor
timestamp: January 22, 2019, 07:36:39 pm
content: Since the voltage should be 24VDC, I would expect it to overload a 20V scale.

author: rf+tech
author_rank: Frequent Contributor
timestamp: January 22, 2019, 07:45:59 pm
content: A less invasive technique would be to take a few more measurements. Up to this point, all DC measurements have been from the back side of the negative output binding post. And there appears to be a suspect solder joint where the wire from this binding post connects to the board.The next step would be to measure DC voltage across C1 (2200 uF), with the DMM black lead to the striped side of C1 and red lead to the other side of C1.Set the DMM to the 200 VDC range.

author: tomherrick
author_rank: Contributor
timestamp: January 22, 2019, 07:56:51 pm
content: These will have to wait a bit. I have a long list of tasks that I haven't gotten to yet. This 12V power source was simply intended for use with a marine GPS chartplotter to load maps and waypoints. This has turned into a stumbling block rather than an interesting intellectual exercise.That said, I hope to get back to it soon and thank everyone for their thoughtful input. Very nice of you all.I'll be back soon to pick up where we left off...

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 23, 2019, 04:15:21 am
content: Before taking it all apart, let's first make sure it's put together right in the first place!I'm sorry but there are more than just the one solder joint that look questionable to me. If this were on my bench the first thing I'd do is to go back over _every_ solder joint with some good leaded solder, flux and a properly heated and tinned iron. Then inspect carefully for solder bridges (I think I see one or two) and correct them.

author: ArthurDent
author_rank: Super Contributor
timestamp: January 23, 2019, 04:41:29 am
content: I believe alsetalokin4017 is correct that there is a problem with the soldering and you should reread the section of the assembly manual on soldering (shown below) and check each solder joint. Reheat with solder with flux and make sure that the solder freely flows to both the component lead and the pad on the circuit board. As previously mentioned, also check that there are no solder bridges shorting runs together. If you're lucky, nothing will have been damaged and resoldering will cure the problem.

author: tomherrick
author_rank: Contributor
timestamp: January 23, 2019, 12:51:39 pm
content: I suspect your suspicions are correct. I had one heck of a time soldering this project. I picked up a 25W Weller soldering iron and it took forever to heat the wire and board. I used a 250W gun on the pot leads and those worked well, but it's a tad clunky for the small stuff. Plus, it's been over 50 years since I did any electronic soldering...

author: tomherrick
author_rank: Contributor
timestamp: January 23, 2019, 01:37:58 pm
content: Well, whaddya know??? 'Twas the soldering - or lack of. The output is, however, a tad on the high side. The range is supposed to be from 0 to 15VDC. Perhaps a resistor is a little toasty?

author: rf+tech
author_rank: Frequent Contributor
timestamp: January 23, 2019, 01:59:34 pm
content: CongratulationsAs for the few extra volts, you deserve themLimiting the output to 15 Volts would require adding a 10 kOhm resistor in parallel with the pot, to reduce the effective resistance to 1.65 kOhms.That's up to you, whether to pursue the extra effort.RF+ Tech

author: ArthurDent
author_rank: Super Contributor
timestamp: January 23, 2019, 03:20:12 pm
content: Great that you got your power supply kit working! Think of this minor problem as a positive learning experience. Future kits you assemble will seem easier now that you've had success with your first one.One change that has been made from when you (and I) soldered all those years ago is the use of lead free solder. If you read the construction section of your power supply manual on soldering they mention how the new lead free solder requires higher temperatures and doesn't flow as well. This obviously made your soldering job more difficult. There is a possibility that your small 25W iron doesn't get quite hot enough for lead free solder and that's why the higher temperature of your soldering gun melted the solder better even though it is physically way too big. Even if the iron gets to the required temperature you probably have to leave the tip on the spot you're soldering a few seconds longer to allow the solder to flow properly.Here is a link to a short video that compares soldering with lead free vs lead solder and has good information. Just remember that there are videos on everything you can think of and some can be very helpful so Google is your friend. Members on this forum are always ready and willing to help as well. One thing that might help is to get some old scrap circuit board to practice soldering on. sometimes you will find a thin layer of oxide or crud on a circuit board and lightly cleaning the board with a Scotch Brite pad will help the solder adhere.

author: tomherrick
author_rank: Contributor
timestamp: January 23, 2019, 04:42:49 pm
content: I think somebody else needs to own that 25W iron... I'll find another higher wattage unit that is more nimble around a circuit board than the heavy-watt gun. Oh, and I'll watch that video at lunch today. Thanks.I do want to limit the output to 15VDC, however, as the reason for acquiring the power supply is to test and set up marine electronics off the boat in the shop. 15VDC is the top end of the range for the Garmin chartplotter we acquired, and I'd expect that's about it for most nominal 12V equipment.I tested all the resistors: R1-2.1k, R2-147, R4-2.2. R3, the 2K pot, when turned all the way to the right at the 15V setting reads 2.38k. When turned to the left at the 0V setting it reads 0.00 ohms. I'm not getting something here. Why would a value of 0 ohms totally block the voltage output and 2.38k ohms allow almost 20VDC?rf+tech mentioned:"Limiting the output to 15 Volts would require adding a 10 kOhm resistor in parallel with the pot, to reduce the effective resistance to 1.65 kOhms."Looking at the schematic I'm not sure where such a resistor would go to be parallel with the pot.

author: rf+tech
author_rank: Frequent Contributor
timestamp: January 23, 2019, 05:57:08 pm
content: The extra resistor would be connected across the two ends of the potentiometer. As supplied, a 2 kOhm pot has too much resistance to limit the output to 15 Volts.The total value of the paralleled resistor/potentiometer is derived from the LM317 datasheet formula:Vout= 1.25V x (1 + (Rpot/R1)) + Iadjx RpotThe last term compensates for the current flow from the adjust terminal, which is small enough to ignore.So the simplified formula reduces to:Vout= 1.25V x (1 + (Rpot/R1))Substituting 147 Ohms for R1 and solving for Rpot, we arrive at 1610 Ohms, which limits Voutto 15 Volts.By adding a resistance in parallel with the potentiometer, we can "trim" the maximum value.Take the reciprocal of 1610 Ohms, subtract the reciprocal of 2380 Ohms, then find the reciprocal of the result.That value represents the amount of parallel resistance to reduce the total to the calculated value of 1610 Ohms.As for the result of that calculation, I'll leave that as an exercise for the student.Do post your result.RF+ TechEdited for better clarity

author: tomherrick
author_rank: Contributor
timestamp: January 23, 2019, 07:21:00 pm
content: Which would be the two ends of the pot? There are three contacts, two appear to be from P1 and P3 by various routes, and one is an output leading to P5.

author: rstofer
author_rank: Super Contributor
timestamp: January 23, 2019, 07:30:20 pm
content: The center contact is usually the wiper.Connect your Ohmmeter across the two outer pins and note that the value doesn't change when you turn the shaft. Disconnected from the PCB, of course.https://components101.com/potentiometer

author: tomherrick
author_rank: Contributor
timestamp: January 23, 2019, 07:44:36 pm
content: It appears that on this pot the "wiper" is in position 1 as shown in the linked diagram. When I put a DMM lead on 1 and 2 or 1 and 3 I got the same kind of variable readings.

author: ArthurDent
author_rank: Super Contributor
timestamp: January 23, 2019, 08:38:51 pm
content: The pot ends are the outside terminals and the center terminal is the wiper as shown in the attached diagram. If you measure the resistance between the outside terminalswhen the pot isn't connected to the board, the resistance will be always be 2K no matter how the shaft or knob is turned. However, the way this pot is wired in the circuit it is as a two terminal variable resistor so when it is on the board the resistance between the middle and bottom terminal (on the schematic) will vary from zero to 2K as the shaft and knob is turned. The middle and top terminal are wired together and the resistance between those two terminals will always be zero.If you haven't cut the protruding pins off the pot, wire the 10K resistor to the outside pins.

author: rf+tech
author_rank: Frequent Contributor
timestamp: January 23, 2019, 08:49:47 pm
content: With the wiper externally connected to one end of the pot via a copper track on the circuit board, as shown in the Elenco schematic, it is easy to come to an incorrect pin identification.Visualize the wiper rotating with the shaft, through about 300° or rotation. One should see that the simplest construction technique would have the wiper connected to the middle terminal.For clarity, please refer to this Wikipedia entry:https://en.wikipedia.org/wiki/PotentiometerOn the right side of the page, an interior drawing and photo show how this type of potentiometer is constructed.For the application in this power supply, connecting the extra resistor across the two outer-most pins is the foolproof method.Attempting to connect the resistor between the wiper and one end has a 50% chance of not working as expected - because the resistor would be shorted out by the external connection between the wiper one end.And by the way, if the wiper were to be externally connected to the opposite end, zero Volts would be full CW and maximum output would be at full CCW.@ArthurDent:With the pot measuring 2.38 k and 147 Ω, the parallel resistor value ends up less than 10 k. It's a bit far from a standard value, so the OP will likely ask, and learn more by the exercise of solving for the optimum value. Please do check me on the math.RF+ Tech

author: ArthurDent
author_rank: Super Contributor
timestamp: January 23, 2019, 09:10:39 pm
content: One thing that hasn't been mentioned is that your intended use of this power supply is for testing marine electronics. I randomly checked the specs on a Hummingbird ChartPlotter and that draws 615Ma. I suspect that other pieces of equipment might require a power supply with higher current capacity. The power supply you've put together is rated at 200Ma (.2A) at 15 volts so it could not be used for testing this particular chartplotter.What would be a good idea is to check the manuals on the pieces of equipment you want to test power on your bench to find what amount of current they need and get a power supply with an output current capacity that is greater than the highest current drawn. There are some supplies that output 13.8VDC at several amps that are designed to power mobile equipment.

author: tomherrick
author_rank: Contributor
timestamp: January 23, 2019, 09:40:47 pm
content: OK, well, thanks to all for your time and assistance. I've spent too much time on this already. Apparently misread the spec as I thought it was 2A at 15VDC.

author: ArthurDent
author_rank: Super Contributor
timestamp: January 23, 2019, 10:24:46 pm
content: tomherrick - You should still look at this as a positive experience as you did learn from it (and no one got injured!). We all have these little oversights and a while ago I was working on a voltage reference that I had powered on for months allowing it to burn-in when I went to make a change and forgot to remove the power and destroyed a $60 IC.A good place to find a suitable supply would be Craigslist or Facebook market place and look for a supply that hams might use for powering mobile equipment. Most of these have 13.8VDC output and some even have a cigarette lighter socket as well as binding posts to power equipment. They generally range from a few amps up to 20 amps or more and could start at $25 and up to $100 for really big ones.

author: tomherrick
author_rank: Contributor
timestamp: January 24, 2019, 12:18:49 am
content: The thing that really bugs me is that I did determine the amperage of the equipment this was supposed to power. So I looked for a 12VDC 2A power supply. I probably misread or there was probably-not a typo. Simple goof on my part. Thanks again to everyone who tried to help me work out this issue. Damn nice forum you have here.T