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# When a UPS can't be used?

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Posted · Original PosterOP

There is a lot of different opinions about when you should use or not a UPS, and also a lot of unsupported ones saying it can damage your electronics. Manufactures says that you can't connect an AC motor to it and some people say you can't connect a computer with active power factor correction to it. What is true about it and what is completely bullshit?

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Ok.. So....

People say you shouldn't use a UPS with a PC when that UPS outputs a "square wave" or "stepped sine wave" or "simulated sine wave" (these are all the same things) as opposed to a "pure sine wave".

When you measure AC voltage from the wall using a DMM, you're seeing an RMS value.  That RMS value is 0.707 times the peak value.  The peak value is 1.41 times the value the DMM shows.  So, for example, the peak value for 115V mains would be 162.15V for 230V mains would be 325V.

That said, if you have 115V mains, you shouldn't have a problem.  Throwing that out there right now.  Because the peak of whatever sine wave coming out of your UPS is nowhere near the max capability of the APFC circuit of your PSU.

BUT... if you have 230V mains and a UPS that supports 230V, you may run into an issue.   Why?

So here's where it gets hard for me to explain... bare with me.

When the waveform is "square", it's like the peak is cut off.  To achieve the same RMS as a pure sine wave, they have longer peaks.  I wish I had the equation to determine the peak voltages handy.  It may also vary depending on how the sine wave is simulated.  But you can picture it like this:  Alternating current either alternates as a smooth wave (a "pure sine") or a fast oscillation with plateaus at the peaks.

So.. what happens with these "higher peak voltage plateaus".  A few things.... The PSU may not care at all.  If the PWM controller is programmed accordingly, and the bulk cap has a high enough voltage to accept the higher margin, the PSU just chugs along with no damage.  If the PWM controller lets the higher voltage peaks through and the bulk cap is undersized, then long term "abuse" could cause that bulk cap to get too hot and vent (fail).

Some PSUs I've used would just shut off because the PWM controller would see the higher voltage and trip a safety.  Unfortunately, I didn't keep track of which PWM controllers do this and which ones don't, but it's been a long time and I don't think they're in use anymore.

If you're going to use 400V bulk caps in your PSU, they better be 105°C rated.  Otherwise, they should be 420V.  Then I don't think you would have any issues.

As for motors:  The UPS could work with a motor... depending on the motor.  Motors don't care about the waveform.  But what could happen is the inrush current of firing up a motor plugged into a UPS would trip the protections in the UPS and shut it down.  Same is true with laser printers.  They use such a big "burst" of power on start up that the inrush current protection of the UPS trips.  So.. it's not like anything gets damaged.... it just doesn't work.

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1 hour ago, jonnyGURU said:

I wish I had the equation to determine the peak voltages handy

For sine wave its rated AC times squareroot of 2...

For that square wave, no idea or don't remember calculating that. Only sine wave to death.

"Hell is full of good meanings, but Heaven is full of good works"

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7 minutes ago, Stefan Payne said:

For sine wave its rated AC times squareroot of 2...

Which is 1.41... which is what I said.

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2 hours ago, jonnyGURU said:

in-rush current

you guys are smart, as an engineer i am proud

PC: Alienware 15 R3  Cpu: 7700hq  GPu : 1070 OC   Display: 1080p IPS Gsync panel 60hz  Storage: 970 evo 250 gb

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11 minutes ago, Tamesh16 said:

you guys are smart, as an engineer i am proud

It's one word, isn't it?   "Inrush".