Siemens Sinumerik 5V DC power source issues - Page 1
Forum section: electronics / beginners

author: mischike
author_rank: Newbie
timestamp: February 19, 2022, 12:13:34 pm
content: Dear all,First time poster here. We have two CNC lathes, identical, one of them is acting up. We traced the issues to the DC regulator of the main spindle drive, namely the 5V DC supply for the CPU control board. It has a strange dip (see attached trace). If that dip goes under 4.7V (and it does if the 230V supply line is under 226V), the board resets. The other lathe's 5V DC is clean, straight stable 5.0V DC.It is a regular non-switching 5V power supply.Is there a clean culprit for this pattern? The big electrolytic DC filter caps (still the originals from 1988)? The rectifying diode bridge? The 5V DC voltage regulator itself?Best regards,Mihaly

author: tunk
author_rank: Super Contributor
timestamp: February 19, 2022, 12:19:48 pm
content: Most likely the filter caps.Edit: I.e. they have lost much of their capacitance.

author: timenutgoblin
author_rank: Regular Contributor
timestamp: February 19, 2022, 12:50:45 pm
content: Quote from: mischike on February 19, 2022, 12:13:34 pmIf one of the PSU is failing (in one of the lathes), but the other is not then perhaps consider replacing the DC filter capacitors in both PSU as preventative maintenance.Judging by the phase of the mains AC it looks like the 5V regulator starts losing regulation just before the DC filter capacitors get recharged via the bridge rectifier as the mains AC starts approaching Vpeak. This is assuming the secondary AC voltage is in phase with mains AC.It might be worth checking the bridge rectifier, too.

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 19, 2022, 01:06:43 pm
content: If the rectifier is a single diode, then it looks fine. If it's supposed to be a full bridge: only half of it is working.

author: mischike
author_rank: Newbie
timestamp: February 19, 2022, 02:33:56 pm
content: Thank you guys! After taking the thing apart (btw, its a Simoreg 6RA2725). There are some definite telltale signs. The bridge rectifier is cracked (defo not a good sign). The filter caps do show signs of leakage on both their lateral, and undersides. Next week I'll replace them all, and report back!

author: wizard69
author_rank: Super Contributor
timestamp: February 19, 2022, 09:10:07 pm
content: Quote from: mischike on February 19, 2022, 02:33:56 pmCould be wrong here but that looks like a poor solder job from a previous repair. Doesn't really matter as this could very well be an issue of bad caps, especially on a supply of this age. On the old analog supplies used in automation I've found many that literally had caps that sounded like rattles. Sadly the place I work for doesn't offer the free time to fix things, we just pull the supply as fast as we can and stuff a new one in its place.

author: WatchfulEye
author_rank: Regular Contributor
timestamp: February 19, 2022, 11:19:47 pm
content: Watch the pinout on that bridge rectifier. The common pinout SIP bridge rectifiers is + AC AC - (including the same standard part number), this one appears to be different.

author: mischike
author_rank: Newbie
timestamp: February 22, 2022, 02:42:40 pm
content: Thank you all. Yes, the bridge rectifier had a wicked alternative pinout, but managed to find a replacement. I replaced all the caps as well.The dip is now gone from the 5V DC line, on the attached picture, the lower cursor was the bottom of the previous trace.Now we stress test the spindle drive during the night shift.