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Way OT - what size inverter?


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Does anyone have any idea what size inverter would be needed ro run a dexskyop PC complete with lcd monitor, inkjet printer etc. ?

Part of an ongoing project that will pribably never work :lol:

David

A LCD uses less than 20 watts, I would allow around 200-300 watts for the PC plus approx 100watts for the printer - really needs more info though - what PC & what printer?

I would think a 500w inverter would be more than adequate, the only problem you may have is with cheaper square wave 'modified sine wave' inverters some may cause your PC problems,

Alternatives are - alot of LCD monitors use 12VDC from their power supply so supply them direct from the car or via a simple regulator,

get a 12VDC PC power supply - I got my opus one from http://linitx.com/index.php - they are alot cheaper now but they will not be big enough to run really power hungrey computers

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You need clean power for a PC, the PSU electronics will refuse to start up if the AC is carp. RS (used to) sell a cheap inverter that generated 240V DC as apparently switch mode PSU's used in computers can cope happily with this and it's easier to make than clean 240V AC. I have no idea how true this is across PC/printer/screen.

I'm not sure a cheap generator is going to be any "cleaner" than an inverter but await correction :ph34r:

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If you know anyone in IT then they may be able to get you a cheap, second-hand or discarded, Uniterruptable Power Supply (I got mine free from mate when they scrapped the server cabinets), these are obviously designed for use with IT and Comms equipment and the good ones convert AC to DC and back to AC, so you could just re-jig one of these in order to use a larger battery. <_<

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Following on from that....

You need a sine wave inverter if you are running a CRT. A modified sine wave one will work OK with LCD monitors. True sine wave inverters are expensive. Maplin used to sell some not too bad.

A sine wave is like a smooth wave you would get throwing a rock in a pond. A modified sine wave looks like a series of steps - two steps up two steps down two steps up and so on. It is the roughness of being dragged up and down the steps that upsets TV's Motors and things. The difference in roughness between a smooth wave and the steps is known as the Total Harmonic Distortion or THD. The better a piece of equipment, including inverters, the lower the THD figure will be.

For either, 350W will probably do. I base this on running a similar setup with an APC 350w UPS. The UPS only cost £80 from PC world in Crawley. It contains 2 x 10Ah 12v batteries in series to give 24v. The UPS will run happily from a pair of 12v car batteries though.

Before we lived in Crawley, the mains electricity was intermittent. We used a UPS there and connected loads of old truck batteries to make 24v The batteries were charged by a big transformer when the mains was on and the UPS ran the house when it was not. We didn't have a computer then but it did the TV, radio and a couple of lights.

Ed

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As in a nice clean sine wave for the reasons stated above. Butchering a UPS is not a bad idea, I have done the same with a ~350W one from work, it's huge compared to the average 350W inverter from Maplins but the price was right ;) and it produces pure AC sine-wave.

Here's a waveform picture from a pure sine wave inverter:

sinewave2.jpg

And here's a pseudo-sine wave (EG cheapo) type:

modifiedweb.gif

The cheap ones just make something that is roughly equal to 50Hz AC, it's easier to make it just bang up and down in one step or perhaps two/four steps instead of accurately making a proper smooth wave, hence why they're cheaper.

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An alternator does produce a pure sine wave.. so even a cheap generator would be clean...

Yes and no!

It is a sine wave, however, the kind of engine usually used to drive an alternator in a gen set is a single cylinder 2 or 4 stroke. In the combustion stroke it accelerates and on the compression it slows down - even with a big flywheel.

This causes the sine wave to be modulated by the acceleration of the engine which can produce a fairly lumpy result, worse as the load increases.

The best generators generate DC and use a sine wave inverter to generate the 230v AC. This means the voltage and frequency are rock steady and the engine can be revved to deliver the required power (so it can run slowly and quietly at low loads).

This modulation of the sine wave is used by some tachos which take a feed from the alternator. Each time a cylinder fires, there is a slight increase in the alternator speed and hence voltage before the regulator. So long as you know how many pistons there are, you can work out the RPM. It means that the pulley size and hence the ratio of engine to alternator speed does not matter.

Ed, I like your analogy comparing being dragged up & down stairs to riding the ripples on a pond as a measure of goodness!

Si

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