Cyberpower UPS awful output - Page 1
Forum section: electronics / beginners

author: boblatino
author_rank: Newbie
timestamp: December 24, 2021, 06:25:56 pm
content: Hi, I purchased an ups at Costco that I planned to use in my home server / routers, primarily for graceful shutdowns. I wanted to see the output first on the scope and I was surprised on how terrible it looks. Is this normal for a non pure sine wave ups? Would it be safe to use on a nas / server? The form looks literally as a square wave with 140v peak to peak and just 43v rms. It has a long zero period as well. Attached the imageThanks!

author: james_s
author_rank: Super Contributor
timestamp: December 24, 2021, 06:36:25 pm
content: That looks normal, they call that a "modified sine wave" which is really just a square wave with deadtime, that's how almost all inverters work that are not specifically billed as "pure sine". It causes no problems at all on most gear but if you have something like a desktop PC with active PFC then you need a pure sine UPS. These often have "PFC" in the model number of state they are "compatible with active PFC".

author: fordem
author_rank: Regular Contributor
timestamp: December 24, 2021, 06:37:02 pm
content: Waveshape wise, that's about what I'd expect - and yes, it'll be fine for a NAS or a server - as a general rule, switch mode power supplies aren't that fussy about the wave form shape, and most of them don't care about the voltage either.Over the years I've seen the move from square wave, through "modified sine wave" (which is what you're looking at), to "pure sine wave" - the worst I've seen was a "Minuteman" back in the early 90's, that had some wicked spikes off of the leading edge when the waveform went positive.

author: boblatino
author_rank: Newbie
timestamp: December 24, 2021, 06:49:14 pm
content: My power supply does seem to have active PFChttps://www.newegg.com/evga-100-w1-500-kr-500w/p/N82E16817438016?Item=N82E16817438016&nm_mc=TEMC-RMA-Approvel&cm_mmc=TEMC-RMA-Approvel-_-Content-_-text-_-Is it mandatory to use pure sine wave?Thanks

author: CaptDon
author_rank: Super Contributor
timestamp: December 24, 2021, 08:32:35 pm
content: I have several UPS units from about 300 watts to over 1kw. The outputs all look similar to yours. I was also surprised at how ugly the output was. I thought these units did some sort of PWM to build a partial sinewave or at least something looking like a staircase. Since I had access to thousands of surplus inductors and capacitors in the engineering lab I decided to try to build an L/C filter similar to what the phone company used years ago to make squarewave inverter outputs into a reasonable sinewave shape. My outcome wasn't very good. That wave isn't a squarewave coming from UPS's like it was at the phone company and my filters did very little to improve the output waveshape but did create some really scary spikes reflected back toward the UPS. It is a wonder I didn't kill the UPS?? I am using my homebuilt filter on the commercial power coming to my workbench. With 12 130L20 MOV's in 4 groups of three along the L/C sections and the series inductors and shunting capacitors I think my bench power is well filtered and sensibly protected.

author: james_s
author_rank: Super Contributor
timestamp: December 24, 2021, 11:47:59 pm
content: Quote from: boblatino on December 24, 2021, 06:49:14 pmI don't know if it's necessarily mandatory but it's probably a good idea if you want to avoid letting the magic smoke out. You might try contacting support from the PSU manufacture and ask if it's acceptable to run it from a modified sine wave UPS.I can check the model number of the sine wave UPS I have when I'm back home, I think it's a Cyberpower also. I got it because I have a lamp on a dimmer connected to the UPS that my TV and stuff is on and with the square wave the light would go out every time the UPS kicked in which is obviously not helpful.

author: David Hess
author_rank: Super Contributor
timestamp: December 25, 2021, 07:37:14 pm
content: Unless the power factor correction is implemented incorrectly, it will work fine with a modified sine inverter. The only problems I have seen with modified sine outputs is driving motors and some audio equipment where the fast edges may result in audio buzz.The idea behind a modified sine output is that it has the same peak and RMS levels as a sine wave.

author: nfmax
author_rank: Super Contributor
timestamp: December 25, 2021, 10:41:45 pm
content: When I investigated UPSs earlier this year I found this not to be true. In several cases The RMS value was right but the peak was much too high. To the extent of exceeding the peak value of a 264V sine wave. This risks damaging anything with a switch mode supply, as that is the specified limit. For some reason people thought this wasn’t a big deal - it probably won’t blow up. 🤞Of course in 110V land with universal input supplies there isn’t a problem.

author: BrianHG
author_rank: Super Contributor
timestamp: December 26, 2021, 12:53:07 pm
content: I know the waveform looks correct for a modified sine wave output inverter/UPS, however, Why is the voltage half of what it should read? For 120v RMS, you should read around 280vp-p, IE +140, then 0, then -140, then 0.Unless you are not using a proper differential probe with connections to the to AC output pins to avoid blowing up your scope probe's frame GND, that output has the correct shape for a cheap UPS, but only half voltage. This is dangerous as you server's PSU will run it's HV side transistor at higher current and duty cycle than what it was designed for, which may kill it depending on it's design.Read the output with a volt-meter to be sure it is 120v RMS and not 60v RMS.If you do not know what you are doing with a scope measuring mains voltage or have the right type of HV differential probe to do so,stop nowor get an isolated battery powered scope.One slip up and say goodbye to your scope or your life.

author: David Hess
author_rank: Super Contributor
timestamp: December 26, 2021, 09:29:38 pm
content: 120 volts RMS sine is multiplied by Sqrt(2) to make 170 volts peak or 340 volts peak-to-peak, which is why universal power supplies without power factor correction internally operate at 340 volts, and power factor correction boost stages produce a little more than 340 volts.

author: james_s
author_rank: Super Contributor
timestamp: December 26, 2021, 10:50:56 pm
content: I looked at the sine wave UPS I have, it's a CyberPower CP850PFCLCD, seems to work just fine. It draws only about 2W quiescent once the battery is charged up, much less than some of the older units.

author: BrianHG
author_rank: Super Contributor
timestamp: December 27, 2021, 03:09:12 am
content: Quote from: David Hess on December 26, 2021, 09:29:38 pmYes, true sine wave. Not modified sine wave as a stepped square wave.If the output was a full square wave, the peak would be down at 120v peak, or 240vp-p.

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2021, 03:57:04 am
content: Quote from: BrianHG on December 27, 2021, 03:09:12 amThat is right, but a "modified sine" has the same RMS and peak values as a sine wave by using a 170 volt peak square wave with a long dead time where the output is zero.

author: SunnyDay
author_rank: Contributor
timestamp: December 27, 2021, 05:38:17 pm
content: This site -https://www.hardwareinsights.com/database-of-ups-output-waveforms/5/has a database of many UPS waveforms for models sold in 220V markets, but the results for 120V models are probably similar. Assuming his test method is valid, it seems that most consumer grade modified sine wave UPSs have poor output quality, and don't adhere to the ideal peak and RMS voltage values that they should be producing.

author: james_s
author_rank: Super Contributor
timestamp: December 27, 2021, 10:36:24 pm
content: They probably don't. A UPS is intended to power computer equipment (so SMPS) for just long enough to ride out a blip or shut down the system properly in an emergency. Most computer equipment doesn't really care much about the voltage, frequency or waveform so long as it's in the right ballpark.