Dell desktop power supply no holdup time? - Page 1
Forum section: electronics / repair

author: jmelson
author_rank: Super Contributor
timestamp: October 14, 2021, 11:06:19 pm
content: I have a Dell Optiplex 980 computer that had VERY short holdup time during power fluctuations. I have replaced it with an eBay unit.Digging into it, I pulled the main storage capacitor (220 uF 400 V) and found it to be good. 235 uF and 0.3 Ohm ESR. Any ideas what else to check? I have an identical computer here that rides through small dips just fine. The PS is an L255EM-01.Thanks in advance for any suggestions.Jon

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: October 15, 2021, 02:43:18 am
content: What do you mean by "Hold up time" ? Define your meaning of "fluctuations" ?Why not put a battery backup on it if it's important?It could be a number of factors.....age of the components, video card, RAM, hard drives (using different levels of power to run)Surely there is more than one capacitor on that board. Capacitors other than the main large cap could cause similar.Did you know that chokes and other inductors can work similar to capacitors? Is there a large inductor in the power supply?

author: TimNJ
author_rank: Super Contributor
timestamp: October 15, 2021, 03:34:36 am
content: Most power supplies in the power range will have hold up times <50ms, for what it's worth. That said, I have a Dell tower at work and usually it stays on just fine if the power goes wonky for a moment. My monitor usually re-starts though.There is also "brown-out", i.e. AC line drops to 10 - 30% below nominal. Most power supplies are unbothered down to around 80Vac. Below this voltage, however, most power supplies will cut-out to protect themselves.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: October 15, 2021, 03:42:28 am
content: If we assume that the converter can work to zero voltage (which is actually far from the case), then 235 uF @ ≈300 V is just 0.07 C or A*s, or 21 W*s. For a typical PC load around 200 W it can provide only 0.1 seconds of working. Yes, there are more energy storages, asThermallyFrigidmentioned above, but we are still talking about fractions of a second. So I completely support this opinion: if this is critical to you, you need a battery backup.But if you are interested in exactly the question of why two identical configurations behave differently during mains power "fluctuactions" (provided that theyoperate under the same working load!), then I would recommend checking and comparing the output capacitors on the PC main power rails: +12, +5, and +3.3. But it may also be that the supervisor are simply more sensitive to undervoltage conditions, try to find its datasheet and check it.

author: jmelson
author_rank: Super Contributor
timestamp: October 16, 2021, 12:52:16 am
content: Quote from: ThermallyFrigid on October 15, 2021, 02:43:18 amThe time the computer can "ride through" a power dip. Most of the computers here can ride through about 1/4 to 1/5 second total power drop. This one would reboot after a short flicker of the lights, maybe 1/30th second (two mains cycles).QuoteThey are expensive, require maintenance and inefficient. And, it isn't so important, I've had many power fails and I've never had file system damage.QuoteThe 400 V cap is the main energy storage element in this power supply. But, it does have a standby supply for wake-on-LAN (which I don't use) and to power the system up when the button is pushed. It looks like the standby supply runs off the 400 V cap, though.Jon