From: William R. Walsh on

> 1- Dell technical support reps don't know what a PFC power
> supply is, and aren't sure whether any Dell computer has one.

I am not sure why that comes as a surprise. To them, a power supply is
a silver box with wires. They might know it has a fan or two inside.
They also know that when power supplies in computers fail, you send
out a new replacement.

Few people do component level repair on power supplies. I've been
incredibly frustrated in my attempts. The only reason I tried is
because the supplies in question aren't made any more, and I've been
losing them at a rate faster than I can replenish my supply. (In case
you're wondering, the supplies are the 335 watt Delta SMP-332AB units
from IBM PS/2 Model 95 computers. All I know about the failure is that
the controller is being told to shut down, and that was what someone
much better equipped than myself and more experienced had to say about

And it's dangerous--switching power supplies can supply enormous
amounts of electrical current, the high switching frequency can really
bite if you happen to come in contact with something operating at that
frequency and portions of the circuitry are directly line connected.
That last one makes for an extremely dangerous environment, as the AC
power in your home can deliver more than enough current to seriously
injure or kill you.

> 3- None of the big UPS makers will come out and say: You
> can't use our home and small office UPSes if your computer
> has a PFC power supply.

That's because it certainly *should* work. If I ever come into one of
these Dell machines where the power supply doesn't "like" a stepped
sinewave input, you can bet that I'll be doing a teardown.

I'm deadly serious:
although I usually don't go to the trouble of making web pages, or
even ones really intended for public consumption.

Anyone want to send me one of the PSUs for an exam? Private e-mail can
be found from the above pages or wct <atsign> walshcomptech <dot> com.
I'll return it to you fully reassembled, and it's unlikely to be
damaged in the process because I take extreme care when doing these
things. I will test it prior to returning it, and should it not work,
I'll front the cost for a replacement.

(Yes, that's only my word, but it's all you need as I take it quite

> 4- None of the formulas, online calculators or techniques to
> calculate or estimate the correct size UPS for your computer
> arrive at the same conclusion.

Couple of explantions: 1) there's more than one way to skin a cat, 2)
everyone has a different idea, 3) some are more truthful than others.
I'd put more faith in a calculator constructed by someone who *uses*
UPS units than I would someone who is *selling* them.

> 5- Nobody agrees on the correct method to calculate or estimate
> the correct size UPS.

See above.

> 6- No OEM will tell you: If you are thinking about purchasing this
> particular computer, consider this: It will require a UPS that will
> cost you half as much as the computer itself for adequate power
> protection.

It'd cost them the sale, and maybe in the worst case, even land them
in a court of law to defend their actions. When a computer from the
competition can be proven to operate perfectly on stepped sinewave
output, why won't theirs?

Not saying that's correct, but you can be sure it's how the average
consumer is going to view it.

> 7- No UPS maker will say whether they'll honor their warranty
> if you use a stepped approximation unit with a PFC power
> supply.

If the warranty to which you refer is the one that covers the UPS
itself against any failures or issues, they don't really have a choice
outside of snubbing the customer or coming up with an allegation that
you were abusing the unit. There are some pretty strict laws governing
what can and cannot be excluded or limited in a warranty, at least in
the US. It's unlikely that the UPS would be damaged even if the
computer failed to operate correctly from the power supplied by said

As for any such "connected equipment warranty", these are practically
worthless. You should not buy power protection equipment based on what
any such policy might claim. I suppose that someone, somewhere has
received a payout from one of these policies, but when you think about
it, only a fool provides a multi-thousand dollar warranty for the few
tens or hundreds of dollars that represent the profits from the sale.
If you haven't, look at the fine print in one of those things

> I apologize for ranting.

No problem. Sometimes you just gotta speak your mind. :-)

From: Brian K on

Great summary. I'll wait for your recommendations.

From: someone on
My XPS 420 has a 425 watt ps
Using an APC Back-Ups RS 1500
Under Power Chute Software it shows
121 watts of the available 865 watts @ normal usage

I live in an area with a flakey power grid
Power loss during a storm is pretty common

The computer works no different on battery.
No noises or any odd behavior

On Fri, 09 Apr 2010 20:04:31 -0400, Greg S <nospam(a)>

>On Fri, 09 Apr 2010 00:31:04 -0400, Daddy <daddy(a)invalid.invalid>
>>Tom Lake wrote:
>>>> I maintain that Dell is cheapskating on the quality of their power
>>>> supplies, and they may have a nasty backlash from doing so. (But then
>>>> again, I'm a pretty big believer in "do it right the first time" for
>>>> stuff like this.)
>>> Not necessarily. The PS in the 9000 is rated for 475 Watts and puts out
>>> a true 475 Watts. I've seen some name brand supplies put out less than
>>> their rating.
>>> Tom L
>>Well, I conducted my little experiment...pressed the power button on my
>>UPS to turn it off, computer and monitor immediately shut off.
>>That's not necessarily because my UPS doesn't produce a sine waveform.
>>For one thing, at 550 VA, my UPS is way under-powered. For another, this
>>sissy little UPS probably doesn't have a fast enough transfer time.
>>By the way, don't waste your time asking Dell technical support about
>>power supplies or UPSes. They don't have a clue.
>The specs for my 6-month old Precision 3400 included the sine wave UPS
>recommendation. While the 375W PS is smaller than many (yet more than
>sufficient for my needs), my BACK-UPS ES 750 has always worked
>flawlessly with this machine. Just one user's experience -- YMMV, of
From: Timothy Daniels on

"William R. Walsh" wrote:
> ..... The fuser demands an
> enormous inrush current whenever it fires, and this will overwhelm the
> inverter in all but the very largest UPS units, causing it to shut
> down (best case) or fail in an exciting way.
> William

What's a "fuser"?


From: Brian K on

A question about UPS. Say it's rated as 500 watts (watts not VA).

That means on battery it can run a 500 watt device for a period of time.

But, does it mean you can't plug a 1000 watt device into the non battery
side of the UPS? Or is that side of the UPS just like a power strip where
you can plug in a 2000 watt heater?