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565 posts

Ultimate Geek

# 183704 27-Oct-2015 12:36
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I have a Japanese device which lists a power rating of 100V 500VA.

The device is plugged into a to a 240v to 100v 1000VA step down transformer (which comes with the device from the manufacturer).

A power meter measures the device power consumption at <200W (~130W) at the wall (inc transofrmer).

I'm looking to protect the device with a UPS for 3-5min and want to confirm size of the UPS I should be looking at.

Should I go by the measured consumption of 200W, the device rating of 500VA or transformer rating of 1000VA?

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195 posts

Master Geek


  # 1415070 27-Oct-2015 20:35
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What kind of device is it? If it's motorised, you may need a UPS capable of delivering much more current than a simple P=EI calculation would suggest.

Does your power meter tell you the power factor of your device?

22920 posts

Uber Geek


  # 1415095 27-Oct-2015 21:30
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UPS's usually are a pretty bad inverter in them, and that makes transformers unhappy. The 24V AC one for my sprinkler controller cooked itself on the last power outage when it was running on the UPS. Not sure how the stepdown would get on with that.

What on earth needs a stepdown transfromer and a UPS tho?



1031 posts

Uber Geek

  # 1415101 27-Oct-2015 21:35
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If you have measured the total power consumption at ~200 W at source then that includes all conversions in the transformer.

Does the consuming device have the ability to operate in any other modes that draw more power ? What is it?
For example , was it a thing that had already heated up? If the load device goes into a mode and draws 500VA while on UPS, the backup time will drop to 40% of the time you think you're going to get at 200W(~VA).

The transformer rating is irrelevant to your UPS requirement as it only determines the maximum transformer load. Load is maximum 500 W(~VA), typical 200 W.

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