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Comment by nuker | Hacker News Reader

The main point against seems to be that downstream UPS will not like "simulated or modified sine wave" if upstream UPS is not producing "Pure sine wave". To be tested :)

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Another important point is that an UPS can produce huge surge currents, for example when switching from battery to utility. The upstream load goes from zero to the sum of both the protected load and the UPS charging its battery.

When daisy-chaining an UPS to another, the downstream UPS can easily overload and trip the upstream UPS because of that.

Wow. Your post is much more informative and clearer than the article. And it's only three sentences.

> Another important point is that an UPS can produce huge surge currents, for example when switching from battery to utility.

It has not happened yet. If downstream UPS likes the source, it is not going to switch to battery, and battery charging will Not be happening because downstream UPS had it charged already long time ago.

An easy fix for that would be to put a surge protector between the UPSs. (Joke)

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I bought three UPSs, all them used independently; but I had to stop using all of them. They all had a capacitor whine that was driving me crazy.

I've tested it before. TFA is correct, at least with the APC UPS I tried it with once. The apc ups went to battery and then turned off when out of battery.

Thanks! But why they don't like it? UPS, as a black box, should harvest whatever energy is in the source, regardless is it sine wave or freaking square mess. Even DC or RF is present :) Why so picky?

That's true to an extent for online UPSes, which converts from AC to DC and then from DC to AC, completely isolating the input current from the output, but these are the most expensive types of UPSes.

Your run-of-the-mill UPS is likely to either be offline, which forwards the input to the output, or line-interactive, which can compensate to an extent under or over voltage conditions with a regulator. If the input current characteristics are outside allowable tolerances, they can't compensate and must switch the load to battery to continue powering it.

> If the input current characteristics are outside allowable tolerances

This is the key to this whole discussion. I guess it boils down to existing line-interactive designs, why they can't work with simulated sine as a source.

A transfer-switch based UPS still needs to protect against brownouts, so it too has line behavior parameters outside of which it will switch to battery. It's an interesting question if/why a modified sine wave input would trigger that or not. Probably one of those "it depends on the design" things. (modified sine -> approximately an oscillating square wave.)

A dual-conversion (online) UPS is almost certainly more robust as far as what kinds of inputs it can accept (though as GP noted, they're more expensive, and they're also less efficient due to the additional rectification->inverter).

> Your run-of-the-mill UPS is likely to either be offline, which forwards the input to the output,

This one surely should "like" simulated sine wave as a source and don't drop to battery?

An offline UPS will switch to battery if it doesn't like the input. What qualifies as acceptable input depends on the design and specifications of the UPS. If it expects a real grid-like sine wave and doesn't see one it will reject it, regardless if the load would like it or not.

> If it expects a real grid-like sine wave and doesn't see one it will reject it, regardless if the load would like it or not.

But why would it require grid-like sine wave and not go along with whatever is the source, provided source can still be used to charge its batteries? I saw no answers yet, and this "why" is the very key to the discussion.

There's a lot of compromise in UPS design.

For example, a common topology in offline UPS is that the inverter and the charger are the same circuit driven differently[0] - so you can't charge the battery while the inverter is carrying load. This is popular at the low-end because you have literally half as much UPS, but makes what you're describing impossible.

Another common issue at the low end is that the inverter isn't thermally sized to run non-stop, they know they can cut a corner because your battery presents a finite and known duty cycle.

There are ways around this, but at some point you end up fixing the wrong problem - eg, it's cheaper, safer, and more resilient to buy a transfer switch instead of uprating two UPS to be capable of daisy-chaining.

[0] https://patents.google.com/patent/US5302858A/en

I think you're being gatekept. It's inconceivable that just no one knows why square and triangular waves don't pass as AC.

I cannot tell without /s, but yes, who designs like this, lol

It's not sarcasm. When nobody tells you a why item and you think that's because literally everyone else is dumber than you, it's likely there are odd numbers of sign errors in your heuristics.

In other words. I asked if A should lead to B. You answered "A leads to Not B" :)

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