Diagnostic help - Page 1
Forum section: electronics / beginners

author: svracer6724
author_rank: Newbie
timestamp: October 14, 2016, 12:00:07 pm
content: I'm a newbie when it comes to electronics and have recently put together a power supply kit- Elenco Model XP-15Khttp://www.elenco.com/product/productdetails/power_supply=ODU=/variable_voltage_power_supply_kit=NTcz. Unfortunately when I apply power the unit only outputs ~22 volts steady. The potentiometer has no effect at all. Since i have no practical experience with diagnosing the problem, I'm looking for direction as to what the problem might be. I have inspected the pcb looking for misplaced parts, parts installed backwards (diodes, caps), checked my soldering - all seems to be in order. I have determined that the potentiometer actually works with my dmm. I also replaced the LM317 regulator. Here is a link to the instruction manual; the schematic is on page 12.http://www.elenco.com/admin_data/pdffiles/XP-15K_REV-P.pdf.Any ideas as to my next step(s). Thanks in advance for your help.Bill

author: MosherIV
author_rank: Super Contributor
timestamp: October 14, 2016, 12:15:27 pm
content: Is D7 the right way round ?

author: svracer6724
author_rank: Newbie
timestamp: October 14, 2016, 04:23:28 pm
content: Yes, all the diodes are oriented per the pcb.

author: KeepItSimpleStupid
author_rank: Regular Contributor
timestamp: October 14, 2016, 04:49:54 pm
content: Look at ground and (2) of the LM317Look at ground and (3) of the LM317Look at difference between (1) and (2) of the regulator - should be 1.2 V or so.Look at difference between (3) and (2) of the regulator - If ~0.6 assume D7 is in incorrectly.Check the - end of C4 to ground. Should be -1.2V .Make sure D7 is oriented correctly. Remove D7. One side is fine. Check with diode function. You can operate the PS with D7 removed.

author: rstofer
author_rank: Super Contributor
timestamp: October 14, 2016, 10:10:09 pm
content: How did the Troubleshooting part of the manual work out. All the voltages correct?

author: svracer6724
author_rank: Newbie
timestamp: October 15, 2016, 01:51:37 am
content: Look at ground and (2) of the LM317 = 21.58vLook at ground and (3) of the LM317 = 22.14vLook at difference between (1) and (2) of the regulator - should be 1.2 V or so. = 0vLook at difference between (3) and (2) of the regulator - If ~0.6 assume D7 is in incorrectly. = .39vCheck the - end of C4 to ground. Should be -1.2V . = 0vI removed one side of D7 and checked with dmm, diode is good. Rechecked the above; all were similar except now the - end of C4 reads -1.36vVoltage still reads ~ 21.5 with D7 removed and pot has no effect.Still trying a few other things.Thank you for responding.

author: svracer6724
author_rank: Newbie
timestamp: October 15, 2016, 01:52:51 am
content: Thank you for responding.Per the troubleshooting guide I checked the following:cathode of D1 reads 22.18vAC voltage across P1 & P2 = 17.40vAC voltage across P2 & P3 = 17.28vVoltage at output of D1 = 22.25vVoltage at output of D3 = 22.23vUnder the heading "Poor Regulation"says to check AC ripple at the input of the regulator. Not sure how to do that.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 15, 2016, 02:10:52 am
content: Quote from: svracer6724 on October 15, 2016, 01:52:51 amIs that not one and the same node?No matter - it feels about right.What is the voltage at the output of D2-D4 ?QuoteIf there was significant ripple present the voltage you measured at the cathode of D1 would probably be lower. 22.2V is approximately the peak voltage you would expect after the rectifiers - so there is little or no ripple present.

author: KeepItSimpleStupid
author_rank: Regular Contributor
timestamp: October 15, 2016, 04:41:54 am
content: Remove a leg of R4 and short The ADJ terminal of the LM317 to ground. The output should be 1.2V.Still leave D7 out.There are then a couple of possibilities at this point since a lot of stuff is eliminated.1. Reduce the 150 ohm resistor to ~120 ohms. You can tack solder a 470 ohm resistor across the 150 ohm one. See if 1.2 V appears between 1 and 2 of the regulator. That voltage has to be there for the regulator to regulate. This 1.2 V must be there at all regulated voltages. i.e. . as built.2. The regulator needs a minimum load.Usually, you can do either one. Try like a 1K 1/2 W resistor for a load.

author: jitter
author_rank: Frequent Contributor
timestamp: October 15, 2016, 06:22:11 am
content: Could you please post a photo of your circuit?You may think you have checked everything but I can tell you, Murphy gets you every time...

author: singapol
author_rank: Frequent Contributor
timestamp: October 15, 2016, 07:21:04 am
content: Quote from: jitter on October 15, 2016, 06:22:11 amsvracer6724,it's not your fault but with elenco. This is not the usual way a LM317 is connected ie. output starts at 1.2V to whatever the transformer voltage. Elecon states 0-15V and for that it pumps a negative voltage into the adj. pin. the elenco schematic is wrong I think about the orientation of D5 and D6. they should be reversed as in TI's LM317 datasheet.http://www.ti.com/lit/ds/slvs044x/slvs044x.pdfSee page 11 of the diode...cathode to ground as elenco's version will shunt negative voltage to ground.

author: KeepItSimpleStupid
author_rank: Regular Contributor
timestamp: October 15, 2016, 09:13:02 am
content: singapol:The datasheet shows a slightly different design. Elenco is supplying a -1.2 or (approximately 2 diode drops) reference to the ground of the regulator. The OP/TS measured -1.36 V with D7 removed.TI's design allows that (-1.36 V) voltage to be trimmed so you can get 0V out.

author: svracer6724
author_rank: Newbie
timestamp: October 16, 2016, 05:31:14 am
content: It's going to be a couple days before I can get back into the shop. I'll try the additional suggestions and post a photo as soon as I can. Thank you.Bill

author: svracer6724
author_rank: Newbie
timestamp: October 20, 2016, 12:55:15 am
content: Quote from: jitter on October 15, 2016, 06:22:11 amHere are the pics you asked for. Let me know if you see something that I'm missing here.Thank you

author: svracer6724
author_rank: Newbie
timestamp: October 20, 2016, 01:05:12 am
content: Quote from: KeepItSimpleStupid on October 15, 2016, 04:41:54 amNot sure what to do at this point... maybe take up another hobby? lol

author: jitter
author_rank: Frequent Contributor
timestamp: October 20, 2016, 04:57:17 am
content: Thanks for the photos.The solder joints look reasonably neat, but some have only partial wetting. I would suggest you reflow all those and try again.

author: singapol
author_rank: Frequent Contributor
timestamp: October 20, 2016, 05:50:00 am
content: Quote from: svracer6724 on October 20, 2016, 01:05:12 amLook at similar circuit :http://www.eleccircuit.com/wp-content/uploads/2013/01/0-60-volt-dc-variable-power-supply-using-lm317-lm337.jpgCompare with your elenco. I would remove C4 and see what happens.

author: KeepItSimpleStupid
author_rank: Regular Contributor
timestamp: October 21, 2016, 08:17:24 am
content: Quote from:leave the 470 ohm resistor in. Put R4 back and remove the short. It should work.If it works, put D7 back.

author: DimitriP
author_rank: Super Contributor
timestamp: October 21, 2016, 08:39:45 am
content: I'm almost convinced that pins 2 & 3 are shorted together. Or at least the picture didn't convince me they are not

author: svracer6724
author_rank: Newbie
timestamp: October 26, 2016, 10:03:26 pm
content: OK, so I figured out what was wrong. When I first put this kit together, the nut holding the 2kOhm potentiometer seemed to be damaged and I had to really torque it on. In doing so it appears that I pulled one of the legs enough to cause a break in its trace.I soldered a wire from the leg past the damage on the trace and it works as designed now. Don't know if its really usable now but I'm happy to have located the problem.Thank you all for your help and guidance.Bill

author: KeepItSimpleStupid
author_rank: Regular Contributor
timestamp: October 26, 2016, 10:07:36 pm
content: Good job!