power inverter gives 120V exact or not?

Even a tiny power drill is at the limit of yout tiny inverter. And you better be clipping it direct to the battery because you won't be pulling 400W out of the lighter receptacle. The no load voltage is not what counts so the 400V means little. The momentary fault light can be ignored. Have some fuses handy and try to drill a hole using the cigarette lighter plug.

[snip]

You have (at least) two problems. The first one is that cheap inverters don't even come close to putting out a sine wave. They call it a modified sine wave, but it is more like a modified square wave.

The second is that cheap Volt meters don't read RMS voltage. They assume when they are in AC mode that they are getting a sine wave, and so they don't give correct readings when the waveform is NOT a sine wave.

Most cheap inverters are very optimistically rated. Your 400 Watt inverter might only be able to put out 400 Watts for a minute or less before over-heating. And of course, it is difficult for the cigarette lighter outlet to deliver 400 Watts, as another poster said. After all, 400 / 12 =

33 Amps! And thats neglecting the inevitable losses in the inverter itself.

In general, ignore the power rating for the inverter. If you want to use it to power something, give it a try, and if it works, great. If not, then lesson learned. Of course, don't bother trying to power things which you know consume more than 400 Watts.

And as the other poster said, be sure to buy a bunch of fuses for the cigarette lighter circuit if you plan to use the inverter much.

--Mac

You might want to review Ohm's Law. I = E/R Either the unit is somewhat less than 400W or slightly greater than 2.6A.

2.6A at 120v is 26A at 12v plus the draw of the unit itself. You are probably pulling close to 30A from the cig lighter with the drill. It's a wonder your car did not burn up. Anyhow... The fault light... You did not say what kind of drill... VSR, standard??? A motor generates all sort of hysteresis currents when spinning down. They could severely damage your inverter. Voltages above desired is normal for circuits without good regulators when running no load. Its nothing to worry about. Put a load on it and it will be at spec. Your inverter really should be used for light bulbs and not much else.

I can't argue with what has been said. Since this is the first question I have noticed on this subject I have a related question.

Is it safe to run computers and other electronic equipment with the square wave inverters. My one experience running a lap top computer seemed to work fine until I noticed the next day that the battery on the computer was 'bad'. I subsequently determined that the small circuit board in the computer's battery had gone bad. I wonder if that was due to the inverter?

Mac wrote:

On Thu, 18 Aug 2005 10:40:56 -0500, Bill wrote: [corrected top-posting]

until

subsequently

bad. I

Could be. I'm not really sure about this, but I would probably use a sine wave inverter just to avoid problems.

Hopefully someone who knows more about it will chime in.

If not, maybe you can start a new thread in sci.electronics.design asking whether it is safe to run electronics from a modified square wave inverter.

--Mac

On my boat I run my notebook computer (which has a switching power supply) from a cheap "modified square wave" inverter with no problems.

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Peter Bennett, VE7CEI  
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Why would a switching power supply object to a square wave? It rectifies and chops the incoming AC before running it through a transformer. It doesn't know the waveform.

Electric motors are more likely to object to square waves.

I suppose one problem with a switching power supply is that the input voltage (at the first stage of rectification) might be wrong if the waveform is not a sine wave. Has anyone ever experienced a real problem?

No. Modern computer equipment uses switched power supplies, and they do not like square wave input. Pay a bit more, and get a sine wave inverter, and your computer will thank you. :-)

-tih

--
Don\'t ascribe to stupidity what can be adequately explained by ignorance.

A square wave ands sine wave with the same rms voltage have different peak voltages ( which is what the input reservoir cap will charge to ) for one thing.

Graham

This is why the 'modified sine-wave' inverter was invented. The waveform is far from sinusoidal, but the peak-to-RMS ratio is similar. Thus it gives an acceptable DC voltage on the filter capacitor (which is related to, but a little less than, the peak voltage of the supply waveform).

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
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