I have a 300W square wave inverter. Is it safe to use it with my laptop that
uses a switchmode power supply?
I have tried it with a lower power 1A 12V switchmode power supply and there
is a notiseable buzz coming from the switchmode power supply when it is on
the square wave. It is silent when used on a sine wave.
I don't want to damage the laptop or power supply. As I will be using it
only for 1 day, I don't want to buy extra equipment if I don't have to.
The are also called computer grade UPSes - because they can
output square waves (modified sine waves) with so much
harmonics as to even damage some small electric motors. But
since computers are so resilient, then that 'dirty' UPS
output (only when in battery backup mode) will not harm
properly constructed electronics.
As noted elsewhere, those square waves simply recharge front
end electrolytic capacitors typically to 320 volts DC. Then
the power supply sips electricity from those capacitors to
operate the computer. Those electrolytic capacitors will
remain charged whether the incoming AC is a sine wave or a
In addition, minimally sufficient power supplies will also
contain line filters. Although intended to keep RF
interference inside the machine, those filters also make that
square wave more sine like - just another layer between
electronics and the 'dirty' UPS output.
Meanwhile, a UPS may output 'dirty' square waves only during
battery backup mode. Most of the time, the UPS connects
cleaner sine waves, direct from the AC mains, into the load.
Those square waves only exist during rare battery backup mode.
Some third party power supplies are so inferior as to be
easily damaged. IOW essential features such as the line
filter, sufficiently sized electrolytic capacitors, and other
'cost cutting' measures could mean dirty UPS electricity
causes power supply failure. Its not a problem created by the
UPS. Its a problem created by bean counting computer
assemblers who don't even have basic electrical knowledge; who
purchase inferior power supplies only on price.
Again, they are called computer grade UPSes for good
reason. Electricity so dirty that it can even harm a power
strip surge protector attached to its output. Called
'computer grade' because computers must be a most resilient
appliance connected to AC mains.
An example: this plug-in UPS creates a modified 120 VAC sine
wave. IOW it creates two 200 volt square waves with a spike
of up to 270 volts between those square waves. Output not
destructive to computers as even required by Intel specs and
other industry standards. Just don't power other less
resilient appliances (ie small electric motor) with this UPS
when in battery backup mode.
Meanwhile some laptops have another problem with that so
dirty UPS output. Laptops monitor AC input and switch over to
internal batteries if AC power does not measure sufficient.
IOW some laptops running on a computer grade UPS will remain
powered from batteries and not recharge laptop's internal
batteries. This because of how AC voltage is monitored AND
because that UPS (in battery backup mode) outputs square
Heywood Jablome wrote:
There is quite a debate about this going in the aus.computers forum. I
suspect a square wave will be fine, unless someone could give me an exact
reason why any particular component would fail under a square wave. I'll
give it a go anyway.
The switching power supply in the computer generates square waves,
rectifies and filters them. You are using a low frequency square wave to
drive the power supply. The engineers on news:sci.electronics.design
and other sci.electronics newsgroups routinely tell people to use an
inverter to provide power to devices with switching power supplies.
Former professional electron wrangler.
Michael A. Terrell
How to read that hogpog discussion in aus.computers. Many
are just posting claims without providing technical reasons
why and without providing numbers. Heavily discount those
posts. Go back to that discussion and weed out the myths: no
supporting technical facts and no numbers. That entire
discussion will break down into only a few useful posts.
Heywood Jablome wrote:
I went through this a few months ago on behalf of a friend - he ended up
using a Jaycar square wave inverter. He used it to power an Apple powerbook
for about 10 hours continuously (it was doing time-lapse capture) with no
This is an example of the point I last posted. Provided was
an example that by itself does not constitute a fact. Alone,
it tells us nothing useful. However combined with underlying
concepts and numbers, only then would the post have provided
definitive information. For example, what was the inverter
output voltage? AND what does the Apple power supply claim as
its input voltage limits? Those numbers must be printed on
each appliances. What is its AC input voltage rating?
A Powerbook powered by a 120 volt inverter. The Powerbook
input voltage rating is what? 90 VAC to 265 VAC? That is the
point. Numbers are missing in so many posts in that other
newsgroup; making those posts useless. Numbers from that
Powerbook would make Poxy's post so more useful.
Poxy does provide one useful number. 10 hours without
failure. If ten hours from a UPS, then it tells us nothing
because the UPS only acts as an inverter when disconnected
from AC mains. However 10 hours from an inverter powered by
DC battery power does provide a useful number. Again,
underlying numbers AND the underlying concepts - the theory
behind the example - create useful facts. To know something,
one must have both fundamental theory and experimental
evidence. Poxy only provided one experiment that demonstrated
little due to missing facts and numbers.
However, previously posted was an example of a 120 VAC
inverter rated for computer use (computer grade). Two 200
volt square waves with a spike of up to 270 volts between
those square waves. A 120 volt computer should be powered by
this modified sine wave without damage. That 120 volt
computer rated to operate anywhere from 90 to 130 VAC.
Without these numbers, then my previous post provided little