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| UPS voltage reading |
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| fixit7:
I have a UPS for my computer. With no power supplied to the battery backup so it operates only with it's internal battery, the output voltage showed around 80 volts when I was expecting around 115 volts AC. Is that because I do not have an RMS meter? |
| ejeffrey:
That also means your UPS has a square wave output rather than modified sine wave. Modified sine wave inverters use three levels (+V, 0, -V) to maintain the same RMS and peak value as a true sine wave, so they will read approximately correctly with either a peak detection or true-RMS multimeter. Where did you find that relic? I hvaen't seen a square wave inverter as part of a UPS... ever. I thought they only ever showed up in super janky "car camping" inverters, not any sort of UPS. |
| fixit7:
--- Quote from: blueskull on May 20, 2019, 12:25:13 am --- --- Quote from: fixit7 on May 20, 2019, 12:22:00 am ---Is that because I do not have an RMS meter? --- End quote --- You just answered your question. A peak-detection multimeter infers RMS voltage by its peak value, so if it detects 170V, it says 120V. Your UPS outputs 120V peak square wave, so the peak is only 120V, so the meter reads 120V*(120/170)=85V. --- End quote --- I do not understand your second sentence. When I measure my wall current it is around 115 volts. My computer runs on 115 volts. I don't think it can run on 85 Volts. |
| fixit7:
--- Quote from: ejeffrey on May 20, 2019, 01:32:18 am ---That also means your UPS has a square wave output rather than modified sine wave. Modified sine wave inverters use three levels (+V, 0, -V) to maintain the same RMS and peak value as a true sine wave, so they will read approximately correctly with either a peak detection or true-RMS multimeter. Where did you find that relic? I hvaen't seen a square wave inverter as part of a UPS... ever. I thought they only ever showed up in super janky "car camping" inverters, not any sort of UPS. --- End quote --- It is not a relic as it is only about 2 yrs. old. Are you saying that other UPSes would should 115 volts on my meter? Bought it a Frys. I am thinking of getting a better meter. How would I know if it is a true RMS meter. I was looking at this one. https://lygte-info.dk/review/DMMUNI-T%20UT191T%20UK.html |
| wraper:
--- Quote from: blueskull on May 20, 2019, 12:25:13 am --- --- Quote from: fixit7 on May 20, 2019, 12:22:00 am ---Is that because I do not have an RMS meter? --- End quote --- You just answered your question. A peak-detection multimeter infers RMS voltage by its peak value, so if it detects 170V, it says 120V. Your UPS outputs 120V peak square wave, so the peak is only 120V, so the meter reads 120V*(120/170)=85V. --- End quote --- Actually nope. Non true RMS multimeters measure average rectified value (ARV), not peak value. Nonetheless they don't properly measure modified sine. But relation between peak and measured value you wrote is wrong. --- Quote from: ejeffrey on May 20, 2019, 01:32:18 am ---That also means your UPS has a square wave output rather than modified sine wave. Modified sine wave inverters use three levels (+V, 0, -V) to maintain the same RMS and peak value as a true sine wave, so they will read approximately correctly with either a peak detection or true-RMS multimeter. --- End quote --- Nope it does not mean that. Because of what's written above. BTW modified sine UPS does not really maintain proper peak voltage. It drops a lot with increased load and battery discharge, so UPS increases duty cycle to compensate for that. With such waveform change, true RMS multimeter should maintain about the same output voltage readings but ARV multimeter readings should significantly change (given that modified sine UPS was properly designed). |
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