Low Voltage Reading From a MSW inverter.

[RJsfishin]

I just put a really neat looking LCD Blue back lit ac voltmeter on the output of my MSW inverter,.....not because I needed it, only because I thought it looked good.
Well it looks really good, except w/ an input voltage of 13.2 dc it is reading 107 volts ac, tv on or off, makes no difference. There is no problem, everything work great, and always has, so its just curiosity, is it normal to get a low.....or false reading on MSW ? It has been plugged in my shop for several days reading 124 volts. I have never checked ac voltage on an inverter before.

[wa8yxm]

Ok, Voltmeters come in two flavors.
One is a TRUE RMS volt meter.. This one does some fairly heavy math (only takes a few very simple parts to do it but its fairly heavy math) to calculate the actual RMS voltage present. I will explain RMS later

The cheaper meter is a PEAK READING meter.

Warning: 40+ years ago when I was in school, the following was part of a COLLEGE course (Electronics, AC Theory) I took. Yes, I aced the course.

Now with a True Sine wave RMS is .707 times Peak, so if the meter sees say
(169 volts (I am rounding here) it displays 120 volts.

BUT with a MSW inverter, due to the "Square wave Like" Or Stile like nature of the wave form,, You will get a lower reading.. Possibly as low as 89 volts (120 * .707)

So: What exactly is RMS (Root Mean Square) Well.... Draw a sine wave (Either true or MSW) on a piece of paper.. Now there are two halves to this wave, A Positive and a Negative half, MIRROR the negative half so it's all positive (Due to the use of the word Square this is not needed but it makes the visual easier)

now, measure the AREA under the the wave line (Between the base line or the ZERO line)

Now, calculate the STRAIGHT DC-Voltage line that would give you the same area.

Because the MSW steps up suddenly, the peak is much lower than on a standard Sine wave, so a non-integrating meter (Peak reading) will give you a false (low) voltage reading.

[Wayne_Dohnal]

Plug an ordinary light bulb into the inverter output, and compare it with the bulb plugged into shore power. If it's the same brightness, your MSW inverter's output voltage is pretty close on an RMS basis.

[RJsfishin]

Something like that rings a bell from way back.
True RMS probably means more than 6 bucks ? 🙂

[ken_white]

Unless its a true RMS (root-mean-square) meter it is probably designed for a sine wave input and is reading the wrong value due the the different waveform shape.