Salvaged 12 volt power supply from 12 volt UPS

[dclevy1]

I have a 12 volt UPS power supply that I salvaged from a working UPS unit and I am using it to deliver 12 volts to my various circuits but the problem is I don't know the amperage of the power supply and there is no labels nowhere on it as well as no fuse. So I also looked up a model number on the power supply and I got no result or data sheet. Is there a way I can find or figure out the amperage by looking at the components on the board? Yes I know it's dangerous to use a power supply without a fuse or built-in cut off mechanism I am planning to implement safety measures for this power supply in the near future. If I can use it


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[capt bullshot]

From the size of the components, I'd guess it's a 40W (maybe 60W) supply. For 12V that would be 3A ... 5A. The secondary rectifier diodes look more like 2A than 4A, so to be sure, I wouldn't draw more than 2 ... 3A from this supply. Usually they are overload and short circuit protected by the primary controller IC, so IMO there's no need for additional protection mechanisms. The overload protection may work only for gross overloads, so overloading by 20% or 40% will cook it slowly to death.
In general, you can guess the supply's power rating by comparing its transformer and other component sizes to power supplies of known ratings and similar age. Newer ones tend to be smaller at the same rating.

[danadak]

Another approach, albeit loosely defined, is to run the supply, use a IR
handheld thermometer, record key component temps, then load it
to desired load and rerecord heat rise in components. Not by any means
meant to substitute for design derived limits, but a crude representation
of its efficiency and currents.

Also any of the semi components marked, look at their datasheets. If
datasheet says diode is 5A and you want 50, that's a show stopper, 50A
that is.

If you are going to use this in a life support application throw this idea away.


Regards, Dana.

[dclevy1]

Another approach, albeit loosely defined, is to run the supply, use a IR
handheld thermometer, record key component temps, then load it
to desired load and rerecord heat rise in components. Not by any means
meant to substitute for design derived limits, but a crude representation
of its efficiency and currents.

Also any of the semi components marked, look at their datasheets. If
datasheet says diode is 5A and you want 50, that's a show stopper, 50A
that is.

If you are going to use this in a life support application throw this idea away.


Regards, Dana.

Thank you


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[dclevy1]

From the size of the components, I'd guess it's a 40W (maybe 60W) supply. For 12V that would be 3A ... 5A. The secondary rectifier diodes look more like 2A than 4A, so to be sure, I wouldn't draw more than 2 ... 3A from this supply. Usually they are overload and short circuit protected by the primary controller IC, so IMO there's no need for additional protection mechanisms. The overload protection may work only for gross overloads, so overloading by 20% or 40% will cook it slowly to death.
In general, you can guess the supply's power rating by comparing its transformer and other component sizes to power supplies of known ratings and similar age. Newer ones tend to be smaller at the same rating.

Hmmm appreciate the response. Thank you


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