Pure Sine Wave UPSes for New Dell PCs

I had to give myself a basic education on alternating current, output
waveforms and active PFC power supplies just to be able to ask this
question.

It all started because I heard that new Dell PCs need a new kind of UPS
(uninterruptible power supply.)

It's a fact: To comply with regulations, Dell now includes something
called an active-PFC power supply with their PCs. I also understand that
users and power companies benefit from PFC, as does the environment.

The problem is, active PFC power supplies supposedly need current with a
pure sine wave output - and a Dell technical rep told me that all new
Dell computers need this - but virtually all UPSes for home use have a
stepped approximation of a sine wave.

Has anybody priced a UPS with pure sine wave output? Those buggers are
expensive!

On the other hand, according to APC: Starting in mid-2008, all APC
Back-UPS (home and small office) products were revised to better handle
load requirements for PFC devices...allowing the majority of Back-UPS
being manufactured now to handle most (not all) PFC loads within their
wattage range much better than older units. Their output is still
step-approximated, however, and APC still finds the occasional PFC
device that they have trouble with.

Here's where I throw up my hands. Do I really need to pay an
arm-and-a-leg for a pure sine UPS, or is most any properly sized UPS
'good enough'?

Daddy

Hi!

It all started because I heard that new Dell PCs need a new kind of UPS
(uninterruptible power supply.)

It's a fact: To comply with regulations, Dell now includes something
called an active-PFC power supply with their PCs. I also understand that
users and power companies benefit from PFC, as does the environment.

Active PFC is not a new idea. Many power supplies have had it for a long
time. The IBM PS/2 Model 95 had it in the early 90s.

It makes no big difference to most computer users, as most are not billed by
the power company based on how much they may be messing up the relationship
of voltage and current in each AC power cycle. Some industrial customers
are--and for them it is a good idea to try and clean up the mess they're
making with power factor correction circuits, because doing so will save
them money.

I think the truth is that Dell is cheap-skating on their power supplies. The
output of a UPS with a stepped approximation to a sinewave could conceivably
trip up a power factor correction circuit, but I'm unclear as to why this
might cause power cycling from the power supply. I think it more likely that
the amount of input filtering on the supply has been reduced to reduce
costs, exposing the power supply to more of the "nastiness" of the input
waveform when it's a stepped approximation to a sinewave. To my way of
thinking, that could trip things up. A half-decent switchmode power supply
really should not care too much about this.

Some electrical motors and simple transformers can "care" a great deal more
about this--and they may burn up or operate incorrectly when run from a
device that doesn't provide true sine wave output. This is why some UPS
units are made with a true sine wave output.

I would still recommend buying the best UPS you can afford. Get one that
offers more capacity than you need at the very least--it'll run cooler,
longer, give you room for future upgrades and in general be a better value
for the money than a unit just big enough to scrape by. In particular, I do
not recommend the APC "plugstrip" or similar units for computers. They are
the cheapest of the cheap, and the quality is just good enough. (They do
seem to work well with network switches, routers and such, however.) If you
can afford one with true sine wave output, so much the better--but I don't
think you *have* to for computer loads.

William

Here's where I throw up my hands. Do I really need to pay an
arm-and-a-leg for a pure sine UPS, or is most any properly sized UPS
'good enough'?

Not good enough. For the XPS 435T/9000, at least, it absolutely won't
work with a stepped approximation. You really do need a true sine wave.

Tom Lake

Tom Lake wrote:

Here's where I throw up my hands. Do I really need to pay an
arm-and-a-leg for a pure sine UPS, or is most any properly sized UPS
'good enough'?

Not good enough. For the XPS 435T/9000, at least, it absolutely won't
work with a stepped approximation. You really do need a true sine wave.

Tom Lake


In fact, I just finished a 'chat' with a tech from APC. They recommend a
pure sine wave UPS for the Studio XPS 9000, and I think my 8100 is
similar enough for the same recommendation.

Even APC's least expensive sine wave UPS (from their Smart UPS line) is
more than a third of what I paid for my PC in the first place! And
although I haven't looked at other makers, I can't imagine that their
sine wave UPSes are so much less expensive.

This sounds to me like a big consumer backlash that is waiting to
explode. Dell does not warn potential customers: "Attention - If you buy
this PC you will also need an expensive UPS to protect it." And it's not
only Dell's problem...any new PC with an active PFC power supply is
going to need an expensive UPS.

I can buy a new PC for the price of a properly-sized UPS for my system.
This whole thing is just too surreal. The UPS makers must be drooling.

Daddy

William R. Walsh wrote:
Hi!

It all started because I heard that new Dell PCs need a new kind of UPS
(uninterruptible power supply.)

It's a fact: To comply with regulations, Dell now includes something
called an active-PFC power supply with their PCs. I also understand that
users and power companies benefit from PFC, as does the environment.

Active PFC is not a new idea. Many power supplies have had it for a long
time. The IBM PS/2 Model 95 had it in the early 90s.

It makes no big difference to most computer users, as most are not billed by
the power company based on how much they may be messing up the relationship
of voltage and current in each AC power cycle. Some industrial customers
are--and for them it is a good idea to try and clean up the mess they're
making with power factor correction circuits, because doing so will save
them money.

I think the truth is that Dell is cheap-skating on their power supplies. The
output of a UPS with a stepped approximation to a sinewave could conceivably
trip up a power factor correction circuit, but I'm unclear as to why this
might cause power cycling from the power supply. I think it more likely that
the amount of input filtering on the supply has been reduced to reduce
costs, exposing the power supply to more of the "nastiness" of the input
waveform when it's a stepped approximation to a sinewave. To my way of
thinking, that could trip things up. A half-decent switchmode power supply
really should not care too much about this.

Some electrical motors and simple transformers can "care" a great deal more
about this--and they may burn up or operate incorrectly when run from a
device that doesn't provide true sine wave output. This is why some UPS
units are made with a true sine wave output.

I would still recommend buying the best UPS you can afford. Get one that
offers more capacity than you need at the very least--it'll run cooler,
longer, give you room for future upgrades and in general be a better value
for the money than a unit just big enough to scrape by. In particular, I do
not recommend the APC "plugstrip" or similar units for computers. They are
the cheapest of the cheap, and the quality is just good enough. (They do
seem to work well with network switches, routers and such, however.) If you
can afford one with true sine wave output, so much the better--but I don't
think you *have* to for computer loads.

William



Thank you once again, William, for your comprehensive and informative
reply. Please see my reply to Tom Lake. Like you, I find it hard to
swallow that new PCs *need* this expensive type of UPS.

Daddy

Hi!

And it's not only Dell's problem...any new PC with an active
PFC power supply is going to need an expensive UPS.

No. A *properly designed* power supply will work with a UPS that has a
stepped approximation to a sinewave output. I've done this often
enough with various power supplies that have active PFC circuits to
say with reasonable assurance that it will work.

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.)

There's also the PS/2s...I have a herd of those and some are hooked up
to UPS units with stepped approximation to a sinewave output.
Sometimes the power supply harmonizes a little bit when on battery
power, but they work. The late model 400 (actually 399.1!) watt power
supply has an active PFC circuit and it seems to get along fine on
power from the average UPS. (Some people do report power ups and power
downs, but I have never been able to duplicate this.)

I can buy a new PC for the price of a properly-sized UPS for
my system.

It's not cheap or particularly easy to design a UPS with true sine
wave output. What an AC generator does just by virtue of its
mechanical design a UPS must do electronically. The complexity of the
circuit is much greater--if you ever get a chance, compare the two
designs sometime.

I couldn't think of the term last night, but a lot of really old UPS
designs put out a simple squarewave and don't even bother trying to
make it look like a sinewave in any way.

There were some exceptions--I had a very nice (and old!) APC 1200VA
UPS with true sine wave output. A lot of BIG UPS units have true sine
wave outputs. Here comes your money saving tip--even today you can
find these and they are typically priced on a "if you can carry it you
can have it basis". That is because the batteries in them will be
totally dead. Good quality car batteries or deep cycle marine
batteries can usually serve as replacements, although you may have to
buy a battery box and wire up external cables.

That was exactly what I did with that APC unit. It took 48 volts worth
of input power to its inverter, so I wired up four big car batteries
in series. The thing would run a moderately sized 120 volt room air
conditioner or washing machine, although it had a hard time starting
either one.

The same thing works with many smaller UPS units as well, although
sometimes the charging circuit flips out and won't do its job. The
older APC UPS units sold from the mid to early 90s handle a battery
retrofit perfectly. I'm using several that have been converted to
operate from a lawn tractor battery. One of them is at the heart of a
backup lighting system--it is connected to several CFLs. These sit
idle until the power goes out--at which point the UPS goes to battery,
a contactor with a normally closed contact goes closed and the backup
lights come on. With a few 60W equivalent CFLs, the whole shebang runs
for at least two hours with ease.

Of course, if you do this you have to be wary of the fact that you're
dealing with high current circuitry that really won't understand how
you were just curious and wouldn't touch THAT again! Even one car
battery contains enough stored energy to weld, cauterize, start big
fires, etc. You must be sure that any wiring you do is safe and will
stand the load placed on it.

Oh, and charge sealed lead acid batteries in a decently ventilated
area only. Otherwise explosive hydrogen gas would build up.

William

William R. Walsh wrote:
Hi!

And it's not only Dell's problem...any new PC with an active
PFC power supply is going to need an expensive UPS.

No. A *properly designed* power supply will work with a UPS that has a
stepped approximation to a sinewave output. I've done this often
enough with various power supplies that have active PFC circuits to
say with reasonable assurance that it will work.

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.)

There's also the PS/2s...I have a herd of those and some are hooked up
to UPS units with stepped approximation to a sinewave output.
Sometimes the power supply harmonizes a little bit when on battery
power, but they work. The late model 400 (actually 399.1!) watt power
supply has an active PFC circuit and it seems to get along fine on
power from the average UPS. (Some people do report power ups and power
downs, but I have never been able to duplicate this.)

I can buy a new PC for the price of a properly-sized UPS for
my system.

It's not cheap or particularly easy to design a UPS with true sine
wave output. What an AC generator does just by virtue of its
mechanical design a UPS must do electronically. The complexity of the
circuit is much greater--if you ever get a chance, compare the two
designs sometime.

I couldn't think of the term last night, but a lot of really old UPS
designs put out a simple squarewave and don't even bother trying to
make it look like a sinewave in any way.

There were some exceptions--I had a very nice (and old!) APC 1200VA
UPS with true sine wave output. A lot of BIG UPS units have true sine
wave outputs. Here comes your money saving tip--even today you can
find these and they are typically priced on a "if you can carry it you
can have it basis". That is because the batteries in them will be
totally dead. Good quality car batteries or deep cycle marine
batteries can usually serve as replacements, although you may have to
buy a battery box and wire up external cables.

That was exactly what I did with that APC unit. It took 48 volts worth
of input power to its inverter, so I wired up four big car batteries
in series. The thing would run a moderately sized 120 volt room air
conditioner or washing machine, although it had a hard time starting
either one.

The same thing works with many smaller UPS units as well, although
sometimes the charging circuit flips out and won't do its job. The
older APC UPS units sold from the mid to early 90s handle a battery
retrofit perfectly. I'm using several that have been converted to
operate from a lawn tractor battery. One of them is at the heart of a
backup lighting system--it is connected to several CFLs. These sit
idle until the power goes out--at which point the UPS goes to battery,
a contactor with a normally closed contact goes closed and the backup
lights come on. With a few 60W equivalent CFLs, the whole shebang runs
for at least two hours with ease.

Of course, if you do this you have to be wary of the fact that you're
dealing with high current circuitry that really won't understand how
you were just curious and wouldn't touch THAT again! Even one car
battery contains enough stored energy to weld, cauterize, start big
fires, etc. You must be sure that any wiring you do is safe and will
stand the load placed on it.

Oh, and charge sealed lead acid batteries in a decently ventilated
area only. Otherwise explosive hydrogen gas would build up.

William

Very interesting, William, as usual. But a lawn tractor battery is not
an option for me. Nor can I imagine how I would explain it to my wife.

Daddy

Daddy wrote:
I had to give myself a basic education on alternating current, output
waveforms and active PFC power supplies just to be able to ask this
question.

It all started because I heard that new Dell PCs need a new kind of UPS
(uninterruptible power supply.)

It's a fact: To comply with regulations, Dell now includes something
called an active-PFC power supply with their PCs. I also understand that
users and power companies benefit from PFC, as does the environment.

The problem is, active PFC power supplies supposedly need current with a
pure sine wave output - and a Dell technical rep told me that all new
Dell computers need this - but virtually all UPSes for home use have a
stepped approximation of a sine wave.

Has anybody priced a UPS with pure sine wave output? Those buggers are
expensive!

On the other hand, according to APC: Starting in mid-2008, all APC
Back-UPS (home and small office) products were revised to better handle
load requirements for PFC devices...allowing the majority of Back-UPS
being manufactured now to handle most (not all) PFC loads within their
wattage range much better than older units. Their output is still
step-approximated, however, and APC still finds the occasional PFC
device that they have trouble with.

Here's where I throw up my hands. Do I really need to pay an
arm-and-a-leg for a pure sine UPS, or is most any properly sized UPS
'good enough'?

Daddy

I have made my decision, based on further research.

The issue concerning a UPS and a power factor corrected power supply
arises when the power supply recovers from a temporary power shortage.

A PFC power supply can have a high "inrush current" -- the amount of
current it draws when first switched on. A highly efficient power supply
(with apparent power approx. equal to actual power) can draw as much as
100% of its full power for an extended period of time (approx. 50 ms.)

A UPS that delivers its output power in a sine waveform is more likely
to provide power for long enough vs. a UPS with a stepped approximation
of a sine wave.

Another factor in UPS selection is the transfer time. A PFC power supply
can request power from the UPS in as little as 4 milliseconds. If the
UPS has a transfer time longer than 4 ms, it may not be able to provide
the power supply with power in time. (This entire discussions does not
apply to an "online" UPS.)

It's probably impossible to come up with accurate measurements for the
actual inrush current and minimum transfer time of a given power supply.
So, practically speaking, the only way to know for sure whether your
computer is adequately protected is to 'pull the plug' from the wall
socket. That's what I plan to do tonight.

Daddy

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

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, and...my 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.

Daddy