Running Oscilloscopes and Bench DC Powersupplies off a UPS or Surge-protector - Page 1
Forum section: electronics / testgear

author: bsodmike
author_rank: Regular Contributor
timestamp: March 11, 2020, 10:27:41 am
content: Hi there,I live in an Asia where the mains power is terrible. I'm running about 7 or more UPSes in my apartment, and 3x 3000VA APC Online units for my homelab server equipment.My question is related to mains earth reference and want to check if there's any danger in running oscilloscopes or bench DC Powersupplies off a UPS. Of course, the UPS in question is 100% earthed. With the UPS I don't want to float my scope, as this could be dangerous.Does this mean:Connect UPS ----> Bench DC power-supply; this already has isolated outputsoscilloscopes - connect straight to mains, via Surge-protector powerstripThanks!!

author: bsodmike
author_rank: Regular Contributor
timestamp: March 11, 2020, 10:52:32 am
content: Having readhttps://electronics.stackexchange.com/a/73994this is a bad idea as I was thinking of using the UPS power as a form of 'conditioned' mains supply, but any time the battery kicks in, any connected equipment floats! This is dangerous even for the Bench DC power supply (even though the PSU outputs are mains-isolated? hmm!!).Thoughts??

author: nctnico
author_rank: Super Contributor
timestamp: March 11, 2020, 11:50:20 am
content: Every UPS I have seen passes the ground from the inlet on to the outlet. So for as long as your UPS is grounded, the connected equipment is grounded. If you worry that the grounding in the UPS isn't sufficient then you could run a seperate ground wire to the outlets of your bench.

author: David Hess
author_rank: Super Contributor
timestamp: March 12, 2020, 01:56:27 am
content: I have my whole workbench on an online UPS and have not had any problem in 20 years. For further isolation, I have a big isolation transformer after the UPS for special loads.Quote from: bsodmike on March 11, 2020, 10:52:32 amNone of my many UPSes float ground under any conditions. Some may float both neutral and hot which is irrelevant to equipment which provides its own isolation from neutral and hot anyway. The UL recommendation for floating neutral changed a few years back.

author: masterx81
author_rank: Frequent Contributor
timestamp: March 12, 2020, 07:11:19 am
content: I'm also interested on this.Some consideration:Some ups passes the neutral and recreate the live line, others provides full isolation (some online models).Working with a pwm generated sine wave, can made some problems to sensitive devices (like oscilloscopes, multimeters, etc) due to the noise?

author: tom66
author_rank: Super Contributor
timestamp: March 12, 2020, 10:25:02 am
content: We ran a Keithley SMU off a modified square wave UPS with no sign of ill effect. If a precision beast like that can run off such awful power I'd bet most oscilloscopes (with simple switchmode PSUs) will be fine.

author: Electro Detective
author_rank: Super Contributor
timestamp: March 12, 2020, 10:07:41 pm
content: Good luck running any equipment long term with a modified square wave UPS, and not all modified square waves are created equal,most are bad, some are worseThey work good enough for halogen and incandescent lighting backup apps and that's about itcompare the stresses on the modified square wave powered components with test gear versus powering from a decent UPS sine wave (that stays that way under load)modified square wave = heat and buzz and perhaps capacitor and rectifier strain, possibility of backup transition switching fails (Fault/Error),and square wave generated odd harmonic interference trash on the lines and or radiated everywhere, including some kick back to the power gridsine wave UPS or inverter = a few more dollars spent and business as usual

author: nctnico
author_rank: Super Contributor
timestamp: March 12, 2020, 10:26:11 pm
content: Typical UPSses are made for computer equipment

author: masterx81
author_rank: Frequent Contributor
timestamp: March 13, 2020, 08:51:36 am
content: and put an isolation transformer at the output of the ups? Must clean a lot the sine wave. But in any case, i would not use a modified square wave for anything

author: Berni
author_rank: Super Contributor
timestamp: March 13, 2020, 10:55:32 am
content: Floating means disconnecting earth, it has nothing to do with live and neutral. For example if you live in Norway and likely have the 3phase delta system your live wire is going to have about 130V AC to ground, the neutral is also going to have 130V AC to ground while between them you might have 230V (does simply sum up due to phase difference). If your neighbor has faulty wiring and is sending significant current to earth you might instead have 30V on live and 200V on neutral when measured to earth.Any proper UPS will keep earth connected between input and output. So the ground on all of your instruments will remain connected. no matter if they are before or behind a UPS. However what you might loose is earth fault protection. Since the UPS is supplying power the RCD in your main electrical cabinet has no way to do anything about it or even detect it. So in the event that a fault in your equipment causes a live wire to touch the metal case there will be nothing shutting off the power unless the UPS itself has built in RCD functionality (But there would be a bang and lights out when the UPS switches back to mains power). So nothing really all that dangerous about test equipment on a UPS.But i certainly would be using a good quality sine wave output UPS since a cheap UPS could certainly spit out a lot of electrical interference.Use of isolation transformers with test equipment is not necessarily dangerous in itself. The danger is in the user becoming overly confident on that floating system and at some point connecting together the wrong two things that suddenly makes the whole system not floating anymore, sometimes accompanied by a loud band and lots of expensive damage or a shock hazard. So yeah floating is fine when you know what you are doing and are very careful, but is still to be avoided if possible because even the best engineers occasionally have a mind fart at times and do something stupid.

author: tom66
author_rank: Super Contributor
timestamp: March 13, 2020, 11:27:32 am
content: A modified square wave UPS is not really a big deal for most equipment.For an SMU the first thing that the mains will hit will be a big iron core transformer, which will have a pretty sharp rolloff after the 3rd harmonic. It might make the transformer hum, but that is about all. We couldn't see any evidence of 50/150/250Hz hum on the output when running on battery backup, nor did the instrument itself appear to have any degradation of accuracy or performance (beyond the typical crappy Keithley UI.) Anyway, the UPS is only to allow our test to continue for as long as possible in the event of a power cut.A typical oscilloscope with a flyback SMPS will just see a flatter than usual DC input. So if anything it is a little better. And while the rise time will be steeper, meaning higher inrush current on each cycle, this inrush will be dictated primarily by the input capacitor's ESR (1-2 ohms) and the UPS + power lead resistance (could be several ohms). That will limit the inrush current on each edge considerably, which shouldn't stress an appropriately sized bridge rectifier.The biggest problem is with devices that have active PFC that don't tend to like the square input and sometimes refuse to work or encounter oscillation on the output - I had a laptop charger that didn't work so well on such supplies. But, no damage was done. Anything with an AC motor or phase-control is also a potential concern, so don't try to run a typical soldering station off one.On the other hand, active PFC can be beneficial. I have run my Rigol DS1074Z off a 45V DC power supply, which it seems to be perfectly happy operating from with no apparent degradation in performance. I built a boost converter that runs from my car's cigarette lighter so I can probe the behaviour in the engine compartment without having to get an inverter for it. Power consumption is 20W or so.Mains devices are built to be hardy and withstand several kV impulses - a squarer-than-usual waveform is unlikely to cause harm for most equipment.

author: Fungus
author_rank: Super Contributor
timestamp: March 13, 2020, 04:17:39 pm
content: Quote from: bsodmike on March 11, 2020, 10:52:32 amNo, earth should be earth. There should be a wire straight through the UPS to mains ground.

author: Electro Detective
author_rank: Super Contributor
timestamp: March 13, 2020, 11:43:11 pm
content: Quote from: Fungus on March 13, 2020, 04:17:39 pmIf not, perhaps a loose chassis connection somewhere?or an idiot mod ?Check all possible scenarios with the wiring in normal and backup mode and note them down,BEFORE assuming isolationand or contemplating a cable hack death wish job@ tom66: glad it works for youimho modified square wave UPS are just not worth the hassle, or gamble that most gear might work or refuse to kick inor weaken or cook some components eventuallySine wave UPS = pay a bit more, cry and think a bit less

author: PKTKS
author_rank: Super Contributor
timestamp: March 14, 2020, 01:51:20 pm
content: Very interesting topic. BTW I have spent significant time and effortin my own bench setup with these issues.For the record this what I have being using in the attempt to protectmy hardware as well as to do the right choices and decisionsbased on my own needs - which may vary per case of course.- I DO RUN ALL MY BENCH VIA ISOLATION TRANSFORMERS- Yes I am aware of the risks and benefits.- Yes that includes scopes DMMs and FGens- Yes some bench PSU as well - not all because the transformers are VA limited- The isolation units are dedicated and spread forming isolated networks- I do connect them very carefully - no GROUND - ALL ISOLATED- I do use a high impedance always at least 100x sometimes 10x never 1x- I do use gloves when needed- i HAVE A SEPARATED DEDICATED TRANSFORMER VARIAC BULBED for loads.- Some isolated nets are UPS based - semi-sinus - or non sinus inverters.- the combos ISOLATION+UPS works in any mains fault - tripped nicely by surges- or events outside a good sane quality mains line (we pay for that..)- as far as I am capable so far I AVOID ALL THOSE CRAPPY PFC PSUs.- IMHO they were pushed by legislators driven by the lobby - to spare investments in mains grid- PFC are bad in semi-sinus UPS - they do fail - they do have unpredictable results.- NORMAL SMPS - e.g. forward/half-bridge/full-bridge flybacks are PERFECT w/non sinus UPS- HALF BRIDGE FLYBACKS WORK OUT OF THE BOX FROM non-sinus UPS- in case they have the handy selector 120/240 volts just use it to adjust your transformers.- A PERFECT VERY GOOD SOLUTION.The BEST thing outside this scope is a COMPLETE DITCH of all PFC thingsavoiding to finance the corporation lobby to improve mains grid and purchasea much more expensive but bullet proof MAINS CONDITIONERwhich do that in a rack unit - (ISOLATED) LINE CONDITION - with protectionand UPS servicePlease refer to the best option to my opinion so farhttps://www.furmanpower.com/product/1500va-2ru-uninterruptible-power-supply-and-voltage-regulator-F1500-UPSa 230 3KVA or even 5KVA is already available.Professional equipo for power amp and digital hardware.PFC free. ISOLATED - dedicated line controllerswith true sine inverters.So my PSUs are focused doing their job of LOAD REGULATIONand not doing mains PFC for the mains providers.we save time and money avoiding crappy PFCswith required more expensive true sine inverters.P.S: Yes a last worth mention is that- the non-sinus UPS service DO GENERATE A WIDE SPREAD VISIBLE INTERFERENCE- all sensitive things (like scopes/funcgens/PCs based devices and MONITORS as well)WILL BE AFFECTED WHEN RUNNING FROM UPS..so far i can live with that much better than having a faulty bricked deviceon mains faults and wide variationsPaul

author: David Hess
author_rank: Super Contributor
timestamp: March 15, 2020, 01:18:05 am
content: Quote from: masterx81 on March 12, 2020, 07:11:19 amI have only used online UPSes which produce a sine wave output for my workbench and have never noticed a problem. The steps are visible on my oscilloscope but the high frequency content is pretty low.

author: Electro Detective
author_rank: Super Contributor
timestamp: March 15, 2020, 08:46:53 am
content: The 'steps' aren't that bad with a decent sine wave UPS running under it's rated output, even with nasty inductive loadsIf there's no sine distortion, buzz or heat, there's no problemIf you need to lose the 'steps', run the UPS output through a decent isolation transformer that's half or less what the UPS can supply, and on to the load/sand check the proceedings from input to load output/s on an oscilloscope (preferably old school type is my preference) to ensure all is wellBonus option: a good opportunity to find out how far you can push your luck before the UPS complains