12V Car Inverter: Does Voltage Read Low Using Common Voltmeter?

[TomBrooklyn]

 When measuring the voltage of a 12V DC to 120V AC inverter typically used in motor vehicles, will the reading be low using a common voltmeter?

I was looking at reviews of the Traveler brand of inverters sold by Tractor Supply Company, and four of the five reviewers of the 1500W inverter and one reviewer of the 1000W inverter wrote that they found the voltage to be in the 90W to 102W range.    But another reviewer of the 1000W inverter stated that the output would be a square wave and a low reading of that would be typical with a "cheap meter."     I suspect he means a non-RMS meter.

Also, however, a Fluke Company training webpage says an averaging meter measurement of a square wave would read 10% high, not low.  


1000W Reviews: http://www.tractorsupply.com/tsc/product/traveller-1-000w-digital-power-inverter?cm_vc=IOPDP1

1500W Reviews: http://www.tractorsupply.com/tsc/product/traveller-1-500w-digital-power-inverter?cm_vc=IOPDP1

Fluke Webpage: http://en-us.fluke.com/training/training-library/measurements/electricity/what-is-true-rms.html

[Seekonk]

Does it really matter?  I would be more interested in the peak voltage.  Take a diode and run it into 10uF or more.  Should be 140V.  For mosy electronics it will not matter.  For motors more dead time (lower apparent voltage) it will ne a plus.

[Ian.M]

So-called 'modified' sine wave, typical for old or cheap&nasty inverters.  Its used to deliver equivalent power into a resistive load while still maintaining the same peak voltage for electronic devices that full-wave rectify the mains.

Cheap meters that aren't true RMS usually rectify the input on AC ranges and are scaled to read 11% high compared to DC because, for a pure sine input, the average voltage is 0.637 of the peak, but the RMS voltage is 0.707 of the peak.   Feed them a square wave and they read 11% high.  However if you feed them a 'modified' sine with 60% on time in each half cycle, they will read about 2/3 the correct voltage.

There are other more sophisticated waveforms that more closely approximate a sine wave but still underread on a cheap average reading meter.

[Seekonk]

I'm partial to the 2000W inverters HF used to sell for $129.  I should disclose that all (6) the ones I bought were purchased as defective.   

[Audioguru]

The manufacturer of the inverter does not know or refuses to say it is a cheap squarewave inverter that was used 30 years ago in third world countries. Since it has a squarewave then its peak voltage is the same as the RMS voltage of a mains sinewave so it does not blow up an incandescent light bulb or a heater. But a cheap meter measures its peak voltage then reduces it to the RMS voltage of a sinewave so it reads the voltage wrong.
Many modern electronic products do not work from a squarewave inverter because they rely on the 41% higher peak voltage of a sinewave.

[johansen]

The manufacturer of the inverter does not know or refuses to say it is a cheap squarewave inverter that was used 30 years ago in third world countries. Since it has a squarewave then its peak voltage is the same as the RMS voltage of a mains sinewave so it does not blow up an incandescent light bulb or a heater. But a cheap meter measures its peak voltage then reduces it to the RMS voltage of a sinewave so it reads the voltage wrong.
Many modern electronic products do not work from a squarewave inverter because they rely on the 41% higher peak voltage of a sinewave.

i suppose they use counterfeit 2n3055 transistors too eh?

btw, modern equipment should not care about the voltage or frequency.

[Audioguru]

A SMPS designed for 70V to 240VAC input does not care if much if its input is a squarewave with a low peak voltage. But a power supply with a low frequency transformer, a rectifier and a filter capacitor does care.

[Ian.M]

Square wave inverters have been obsolete for years, ever since the 'modified' sine inverter became common.

Older SMPSUs frequently had a much narrower input voltage range, and often required manual switching between 115V and 230V input.   It was possible to design a fully autoranging PSU, but that resulted less efficient magnetics, and needed a higher current switching transistor, which increased cost.  So back in the days that 'modified' sine was popular it was important for the peak voltage to be compatible.   

[dentaku]

I was just wondering about that yesterday. I plugged one of the small 75W ones into my car's 12V port and got 90V AC on my inexpensive Mastercraft meter.
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