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hmy: I want to buy a new UPS, but cannot find any concrete info on my PC's power requirements. I have a Dell OptiPlex 9020, purchased 02/2014. Dell's tech support disconnected twice today, & I have had enough of that. I searched my user's manual, but found nothing about the requirements for an AC power source. I could assume it should work fine with a PWM (pulse width modulated) UPS, but do not want to learn the hard way that it might not.

I have been browsing Amazon's pages for various brands and models of UPS, & had seen 1 review of a standard UPS in which the guy was furious about having bought one that would not keep his new DELL PC running during a power outage. He had not known about his own PC's power requirements, which is irrelevant to me, but having read that his new DELL required a pure sine wave AC source, inspired me to wonder about my own new DELL's power requirements.

Granted, it did work fine when the power went out last week, but I am wondering if that was supposed to happen. Were the other loads on the UPS just right to make it work? In other words, was the standard UPS' output just barely within tolerance of the PC's input requirements, even though the published or official specs for that UPS did not meet the requirements of the PC. As an electronic tech for some 10 years, I have seen my employer use parts for applications that exceeded those parts published specs, because the parts had been tested & found to have been better than their specs stated. At the time, I thought that was taking a chance that the manufacturer of that part might decide that it was a waste to make such great parts even though their specs did not require it.

Perhaps my UPS is one in a thousand that just happens to work under certain conditions with a new DELL? Yes, it would save hassle but not money to simply buy a PURE SINEWAVE UPS, but I would rather not, if it is not necessary.

Most PC's use a switching power supply to save weight. These have a rectifier directly connected to the AC line input to develop a high voltage DC output which is then used to drive a solid state inverter at high frequency (~ 40 Khz). This high frequency AC signal is isolated from the DC output by a light weight transformer using a ferrite (powdered iron) core. By running the inverter at high frequency the supply is light weight.

Such switching supplies generally can tolerate rough AC inputs. A linear supply, using a 60hz input transformer would require a purer sine wave to avoid heating of the input transformer. I doubt that any PC's made these days use such supplies the cost and weight would be prohibitive.

Now I did have a UPS that would not work well during a power outage, but the problem was that IT would NOT tolerate the unstable AC input from my emergency backup propane powered generator! IE: The ups would not pass the generator output through to the computer and would only run off the batteries! Maybe this is what the guy was complaining about?

for the most part computers don't care that much about what the wave looks like coming into there ac power inputs as long as the peak voltages don't get higher then what the parts can handle, when I am home again I will take some pictures of my scope showing some waves from ac outlet and my inverter.

even the sine wave in your wall outlets is not perfect, not even close but it works.

thanks, guys.

as promised I am home and got some waveforms


here is a nice sine wave, generated by an iphone app


my wall power wave, told you it was not perfect.


and the inverters modified sine wave, looks more like a square wave to me.

these waves will run a computer just fine, I ran my old laptop on that inverter in the car more then once.

"diehard67" wrote:[attachment=]3477[/attachment]
and the inverters modified sine wave, looks more like a square wave to me.

Did they actually rounded the wave to the nearest integer? hmy:

That's a nice classic analog CRT scope. What model is it? I have both a Tek 454 and a Tek 465.

What brand & model of inverter?

the scope is a heathkit io-14


the inverter is an xpower 300, got it for $20 on sale at Canadian tire the day before going on a long road trip, so I got it to power my laptop in the car.

so it runs on the car battery & makes 120 VAC. Still, I think the PWM output of a UPS may be much closer to a sine wave. I will check with CYBERPOWER to see if they have ocillographs of the actual output.