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What is VAC undervoltage shutdown level?

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Advanced Member level 5
Jun 13, 2021
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Neither LCS100US15 (100W) or LM25-23B15 (20W) Offline PSU’s tell in their datasheet what is their mains UVLO voltage.

Do you know?

LM25-23B15 (20W)

LCS100US15 (100W)

We want to put them in an enclosure with an inrush limiter up front…but need to know how low VAC input they operate down to? (if we cant switch the inrush relay on to short out the inrush resistor at low vac input, we're in trouble..the inrush limiter has its own wee mains txfmr based supply)

So you should pay attention at the input voltage derating curves. In this case, Mornsun starts to derate at 100Vac (100% efficiency) till 85Vac (80%), presumably at lower input level the efficiency drops to 0%.

For XP Power the values are 115 Vac (100%) and 85 Vac (80%)

To sum up, if you are ok with 80% of the output power, you can set 85 Vac as the UVLO, 115 Vac in the case you need the 100%

I've just opened the first datasheet.

On the first page, the very first feature says: 85 - 305 VAC.
Is this the information you are looking for?

For me it is. This is how I understand your question.


Thanks, thats the range where it is nominally operating......but for all we know, it may actually still run if one puts the mains down to , say 30VAC......?

We seek the UVLO, rather than the minimal spec'd voltage for operation.


my guess:
You won´t get this information, not even from the manufacturer.

* it may not be designed to be very accurate
* it may depend on waveform. (I guess it rather depends on sine peak than on RMS)
* it may depend on capcitor voltage, ripple voltage, load current, frequency....
* maybe even on temperarture.

So if the manufacturer gives you a value ... he is somehow responsible on this information.
And as it´s not specified - it may vary from production batch to the next (or revision) without notice.

Indeed I see it rather as an "internal function" than a user function.

But if you really need this information you should contact the manufacturer.

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