Uninterruptible power supply - Page 1
Forum section: electronics / beginners

author: jheatac
author_rank: Regular Contributor
timestamp: August 05, 2024, 12:18:25 pm
content: Hello,I have a small 500va uninterruptible power supply labelled IBM OP500. I am wayyy over my head but if I let the smoke out it is not a big deal.I've had this thing 20 years or so. It's on its 4th battery, I believe. The battery light started flashing a few months back so I ordered a replacement.I put it in and gave it a couple days and the battery light still flashes bad battery. It does not carry a load when line voltage is removed.I threw it on the bench yesterday and determined that it is charging - which I assumed because it got acceptably warm when I changed the battery.My question is if anyone can give me a direction to a chip to look around.I am certainly charging. How do these charge and sense battery voltage at the same time?It does switch to battery when line voltage is removed but it isn't enough to carry a load. I had a 200w 12v power supply on the bench (idle) I used for a makeshift load and it was getting 24vac, I believe. So it is switching and trying, but not getting any current. I did not test output without a load.My thinking is the same pathway that senses the battery voltage has a considerable voltage drop.I don't have the board out to get pictures yet.

author: indeterminate
author_rank: Regular Contributor
timestamp: August 05, 2024, 10:50:15 pm
content: You need to get yourself a multimeter without one you are blind to whats going onhttps://eevblog.store/collections/test-equipmentonce you have a meter you can see if the battery is being charged at the wright voltageand go hunting for the fault if it is not

author: jheatac
author_rank: Regular Contributor
timestamp: August 06, 2024, 12:30:01 am
content: I'm going to file this under "Even a blind squirrel finds a nut every once in a while"I separated the case and inspected the board. Everything looked perfect. Nothing hot. Solder connections all good. I checked the battery while it was charging and it was 13.6v to 14vBeing overwhelmed at where to start, I checked ohms on the power switch - SPDT slide. Really not on/off... More charge/and active UPS. It has a green LED regardless. When off, the charge LED blinks.One way was 0.12ohm, roughly the test leads. When I checked the other, it was in the 100 ohms then kohms then if I wiggled the switch I got it down to 22 ohm. I took the switch apart and cleaned the contacts and the unit works again.If only they were all so easy.

author: MrAl
author_rank: Super Contributor
timestamp: August 06, 2024, 02:23:34 pm
content: Quote from: jheatac on August 06, 2024, 12:30:01 amSo then maybe the old battery still works too.

author: jheatac
author_rank: Regular Contributor
timestamp: August 08, 2024, 01:42:59 am
content: Yes, it does.I ordered a new battery (~$30) without checking. Old one was 2018 so I just changed it out anyways

author: AG6QR
author_rank: Frequent Contributor
timestamp: August 08, 2024, 02:32:03 am
content: Quote from: jheatac on August 06, 2024, 12:30:01 amThey aren'tALLso easy, but quite a few are. You've learned a valuable lesson here: switches, connectors, and contacts are frequent sources of trouble in older electronics. A bit of corrosion builds up, and then the pieces of metal no longer make a solid electrical connection when they touch. A lot of failing older devices can be fixed by a generous spraying of deoxit contact cleaner, combined with working the switches back and forth to try and remove the corrosion from mating surfaces. A device with card edge connectors can often be "fixed" by removing the boards from the connectors, cleaning them a bit, and reseating the cleaned boards into the connectors.Cables that flex are another common source of problems.Electrolytic capacitors of a certain age will dry out, develop high ESR, and make devices behave badly.After many decades, old carbon composition resistors increase their resistance by a lot, particularly if they have been running hot.A good, experienced, repair technician has the skill to troubleshoot in detail, but also has the experience to have a pretty good intuitive feel for the most likely components to fail. Depending on the situation, they might start with a quick and inexpensive fix like spraying deoxit on the switches before spending a lot of time with detailed troubleshooting.