Visualizing a UPS output wave

[pqass]

Can you switch your scope to DC coupling?  It shouldn't matter at 60Hz but maybe something weird is going on here.

Bingo!  It was the AC coupling that introduced the wavy weirdness.  See attached.

1st shot: no load, DC coupling, 10X probe across 10K resistor in the L-100K-10K-100K-N divider,
2nd shot: same as above but with 120W load,
3rd shot: 120W load, DC coupling, 10X probe across transformer open secondary.

Note:
(a) alternating square wave seen as expected; see red trace in attachment from my reply #14,
(b) ringing when under load,
(c) duty cycle increases under load,
(d) slightly rounded tops in 2nd but may not be noticeable in 3rd, and
(e) transformer doesn't appear to mangle the 60Hz waveform.

Okay then. I think I've beat this topic to death enough. 

[5U4GB]

Okay then. I think I've beat this topic to death enough.

I think you've created the definitive answer to any question of "should I pay extra for a proper sinewave UPS", thanks for putting in the effort.

Having said that, it'd be interesting to know which UPS produced that output, not for name-and-shame but just to know that product X from vendor Y behaves like this.  In particular it's to counter the obvious response "well it must have come from a cheap no-name UPS, no true Scotsman... ah, UPS would do that".

[Vovk_Z]

Perhaps transformers (or some transformers) aren't as suitable for this purpose as I'd assumed. 

Most transformers are typically suitable (at least those <= 100..200 VA, unloaded or lightly loaded with a pure resistive load) . 

[pqass]

Having said that, it'd be interesting to know which UPS produced that output, not for name-and-shame but just to know that product X from vendor Y behaves like this.  In particular it's to counter the obvious response "well it must have come from a cheap no-name UPS, no true Scotsman... ah, UPS would do that".

The product is from APC: Back-UPS XS 1300 LCD
Back-UPS is their economy line and is probably no different than other brands in this category.
The square wave output is not unexpected and has been perfectly fine these last 16 years.
Especially since I've replaced the internal 7Ah batteries with an external 18Ah pack, it should be cheaper since I won't be replacing batteries as often.

[MrAl]

Hi,

The waves being shown now look much more typical.  The duty cycle should vary a little depending on load, but they should look more rectangular than anything else, and they appear to be looking like that now.

This is very typical with the pseudo sine converter which seem to go by other names also such as "modified" sine converters.
These seem to work for some things, but if the device being powered uses a power factor correction circuit it may not work very well.

I have a little converter that puts out that kind of rectangular wave.  I tried it with an old style wall wart and it worked, but I did not have it running for long.

Rectifier circuit draw an unusually short current pulse from the line anyway so if the device being powered just has a full wave rectifier inside it may work ok.  With an automatic power factor correction circuit though it would expect a real sine wave to power it.

[5U4GB]

Especially since I've replaced the internal 7Ah batteries with an external 18Ah pack, it should be cheaper since I won't be replacing batteries as often.

If the UPS isn't set up for higher-capacity external batteries then you may want to be careful with that, a lot of them aren't designed to run at full load for extended periods of time, if you extend the runtime from (quick Google) the rated 5, to 15 minutes, you may end up cooking the electronics.  The ability to add external batteries on the more expensive models isn't purely a value-add, it's because the electronics aren't up to handling longer runtimes.

[pqass]

If the UPS isn't set up for higher-capacity external batteries ...

I'm well aware of that.  The time until shutdown hasn't changed. 
The larger external pack was used because the normal 7Ahr pack gets massacred with >10A draw for just a 170W load (>1C).
It's like they're designed to fail every 3 years.  Hmmmm.
With the 18Ahr pack, the same load will be just 0.55C which should translate into far fewer replacements and <$$$.