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650VA UPS runs hot

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I think it's just that much error in the output; it only controls that in averages over a couple of seconds.

I wouldn't run an AC clock off it, lol.
 
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How is the voltage conversion done? By just the transformer? If i see the input voltage fluctuate the output voltage changes directly with that as well. So my assumption is that its a simple 'analog' voltage conversion done by the transformer.
"Automatic Voltage Regulator" function is typically done using specific design multi tap transformer.
Normally mains current to load just "goes through" its high/mains voltage side, while low voltage side provides power for keeping battery charged etc.
If mains voltage drops significant amount then load is switched to "higher" tap of mains voltage winding boosting voltage.
Again in case of mains voltage rising it can connect load to lower tap. ("buck" function)

Depending on price level there could be multiple taps for multiple levels of voltage boost/buck without need to switch onto battery power.
Again during blackout or when mains voltage goes too much haywire, UPS disconnects completely from mains and runs that transformer the other way using low voltage side to power high voltage side and load.
You'll recognize those UPSes from scarcity of power semiconductors and actual old fashioned transformer.
Higher end UPSes again can lack such "dumb" transformer and use more complex circuitry with whole rows of power semiconductors. With what might look like transformer being just big inductor to filter high frequency crud/smooth output.

I'm talking about the Input voltage vs Output voltage, this could differ from 232V input to 235V output. It's compensating with at least 3 volts or so. It flutucates greatly as well (the output voltage together with the input voltage). Worst condition i saw was 231V > 235V, which is 4 volts. The amount of Hz also varies from 48 in worst condition to 50Hz at best. Could the UPS be compensating that as well?
Those few volt fluctuations are down to local power grid voltage varying depending on load and even voltage losses caused in wiring of your house.
Also frequency can well vary some depending on quality of local power grid.
Wouldn't expect very accurate measuring of input/output from cheap UPS.
 
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That level of tech is way out of date, and way too expensive for today.

I'd bet it's a ~150-200kHz PWM power supply, that is regulating the output to a level that approximates a 50-60Hz waveform, and filtering for that output frequency, like a 600W class D amplifier outputting 50-60Hz.

Those are really cheap these days, and the multitapped transformer you describe would be 10x the cost.

Post a pic of the transformer, and that will tell; a 600W 60Hz transformer is about 6"x6"x6"; a 600w 150kHz transformer is half the size of my fist.
A 50-60Hz transformer will be iron, a switcher will be ferrite.
 

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Starts to beep like crazy once you throw Furmark + CB together on it. I think i need a bigger one here. :D
Your assessment is correct. It's overheating because it's overloaded.
 
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I'm not going to screw open my UPS now, but the transformer is'nt that 'big'. Most of the weight of the unit is caused by the battery and not transformer. If there's heat it's caused by the transformer as well.



There's 2x black mass and the rest of the wiring that comes from the transformer.

Your assessment is correct. It's overheating because it's overloaded.
Yes, in a worst case situation it is. But when playing SOME games it's not beeping and well within the 100% load range. Once i start PUBG and in intensive scenes it start to beep.
 
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