Inverter putting out 98 volts

[tealboy]

I'm using a power line cigarette lighter plugin style inverter for inside the cab, running my computer and periodically, for a 32 inch tv for tailgating. I have never checked the voltage until today and was surprised at how low it was.

Is this normal? Is it problematic for my tv, a med size fan or my computer? Worried about damaging the electronics but maybe it is fine. The inverter is a cheap one, maybe $20 or so.

Battery is new reading 12.5 volts and using a basic inexpensive meter to read output.

[Chum_lee]

You don't state the wattage of your plug in inverter. My RV has two types of cigarette lighter plug in sockets. The one on the dash with the cigarette lighter element in it is a 10 amp socket. (provided by Ford) The outlets under the dash and in the stateroom (provided by Fleetwood) are a 20 amp sockets. Using my junk inverter (150 watt MSW) in the dash socket ended up damaging it and the 120 volt rated output would vary from +- 90 volts up to about 118 volts depending on the load on it until it failed completely. Using the 20 amp plug in sockets works much better.

Chum lee

[DrewE]

wa8yxm wrote:

I have both Peak and TRUE RMS meters (RMS is the area "under the curve" that is the area between the sine wave and the base line (0 volts) and that will be the same with your inverter as with mine (True Sine Wave)..

RMS is "root mean square" and is computed as the square root of the average (mean) of the square of the voltage (over time), rather than the average of the voltage over time. The latter is what average responding voltmeters measure, though they're usually calibrated to show RMS voltages for sine waves.

A good many multimeters are average responding as that's pretty easy to do electronically or electromechanically. The AC voltage is rectified and fed through an integrator (which, for an analog meter, may just be the meter mechanism itself). By contract, properly measuring RMS voltage is typically done by either measuring the power dissipation in a fixed resistance (typically thermally) or by sampling and digitizing the voltage and then computing it mathematically with a little computer (the standard approach today now that little computers are dirt cheap).

The reason RMS values are used, rather than peak or average voltages and currents, is that it allows the power equations for a resistive load to be the same for AC and DC: the power dissipated is computed as the current times the voltage, or the square of the current times the resistance, or the square of the voltage divided by the resistance. Similarly, Ohm's law continues to hold without needing additional fudge factors or resorting to calculus.

[wa8yxm]

Most of the PLUG IN type inverters,, if not all, are MSW devices.. Now MSW depending on who you talk to means Modified Sine Wave, or Modified Square wave.

If you look at the waveform on a 'Scope it looks like a Stile (The steps a farmer uses to get over a fence.. also a name for a pile of dung).

On a sine wave the peak voltage is 1.414 times the RMS. so if you have a "Peak reading" voltmter.. they multiply by 1/1.414 (I think that's .707 but don't quote me) to give you an RMS reading of 120 volts.

BUT the Peak reading on the MSW is way lower so the peak reading voltmter gives you a lower reading since it does not know better.

I have both Peak and TRUE RMS meters (RMS is the area "under the curve" that is the area between the sine wave and the base line (0 volts) and that will be the same with your inverter as with mine (True Sine Wave)..

[Sam_Spade]

road-runner wrote:
This explains why peak-reading meters read low,

Where does one find a "peak reading" voltmeter ?

The effective voltage for AC is RMS (root mean square) and is NOT peak. Most meters will give a reasonably close number......unless the wave form is REALLY strange.

[DrewE]

road-runner wrote:

Most better MSW inverters are designed so that the peak voltage is roughly the same as for a sine wave (i.e. about 170 volts) and the duty cycle is such that the rms voltage, and hence power into a resistive load, is also the same.

I've looked at the output from about a dozen MSW inverters on a scope, and every one of them has a peak voltage in the 140 ballpark, directly proportional to the DC input voltage. This explains why peak-reading meters read low, and microwaves produce less cooking power.

I guess many MSW inverters are not "better" MSW inverters. Somehow that does not surprise me in the least.

Probably I should have said that it's possible to make an MSW inverter where the peak voltage and RMS voltage are the same as for a pure sine wave. Perhaps there are good reasons other than cost why this isn't typically done--such as attempting to minimize harmonic distortion as much as possible, although it's not possible to minimize it too much.

[road-runner]

Most better MSW inverters are designed so that the peak voltage is roughly the same as for a sine wave (i.e. about 170 volts) and the duty cycle is such that the rms voltage, and hence power into a resistive load, is also the same.

I've looked at the output from about a dozen MSW inverters on a scope, and every one of them has a peak voltage in the 140 ballpark, directly proportional to the DC input voltage. This explains why peak-reading meters read low, and microwaves produce less cooking power.

[road-runner]

If an RMS voltmeter isn't available, just plug an incandescent light bulb into the inverter, then into shore power, and compare the brightness. If the inverter's RMS voltage is different by more than a few volts, you should be able to see a difference in brightness.

[BFL13]

My various brand MSW inverters show mid to high 90s voltage with a regular multi-meter. So your figure indicates yours is ok, IMO. They also show funny things with the three-light tester, so don't worry about it.

MSW inverters run most things just fine. The few exceptions will be listed in their manuals. Especially watch out for electric blankets. They seem to fry with MSW according to folks on here.

We don't have anything in our rig that needs PSW, including the microwave--it runs ok, just slower than with PSW, but not enough to matter any.

[DrewE]

time2roll wrote:
If the tv works all is fine. Just the flat MSW registering low on the pear-to-peak voltmeter.

Get sine wave or a true RMS meter to read the volts.

It may also be an average responding voltmeter, rather than a peak responding one.

Most better MSW inverters are designed so that the peak voltage is roughly the same as for a sine wave (i.e. about 170 volts) and the duty cycle is such that the rms voltage, and hence power into a resistive load, is also the same. Often the peak voltage is none too well regulated so it's all somewhat approximate.

Assuming it's a modern TV, it's probably rated to work with a 100V nominal mains voltage anyhow. Many electronic devices have international power supplies that accept any voltage between 100V (used in Japan) and 240V nominal.

[Almot]

If TV works and it is old, stop worrying. I believe the cheapest PSW inverter under 300W will cost several times of what you paid for that one.

Your laptop doesn't really "run" on 110V, it runs on DC. In the worst case you could damage AC-DC brick of the laptop though not likely, and again, a new brick would cost less than PSW inverter. You can also plug laptop directly in DC.

[time2roll]

If the tv works all is fine. Just the flat MSW registering low on the pear-to-peak voltmeter.

Get sine wave or a true RMS meter to read the volts.

[SidecarFlip]

Considering the cost of a PSW inverter versus a MSW unit. No point is being that cheap in my view.

[Sam_Spade]

tealboy wrote:

I added a light load and it went to 110 and up to 118 with a medium load so maybe the no load situation was the problem?

YES.
And that also indicates that you do NOT need to throw away your meter. 😉

[shastagary]

a Modified Sine Wave inverter will not show the proper voltage when using a standard Multi-Meter Volt meter Differences