Electrical engineers needed

Wig

Can anyone explain the following power consumption figures?

Computer 'tower box' ONLY 16W in standby,
Monitor ONLY 17" TFT uses 9W standby, 35W in use
Epson printer ONLY uses 7W standby, 16W - 23W switched on.
Cable modem ONLY 9W (no standby)
110Volt speaker system 35W (no standby)

Combined individual units = 70W excluding printer


The following bit confused me and I need an explanation

With the computer switched on all peripherals attached and working, except printer (unplugged) = 132W
Remove the speaker power and the above changes to 149W

It goes up if I remove the speaker power (240V/110V transformer) when the computer is on.

[strike]With the power monitor on the mains plug -the plug which everything 'computer related' runs from......

Computer ONLY in standby 16W so far so good
plugged in monitor total increased to 25W so far so good
plugged in modem 33W
plugged in 110V speakers no increase

So that was weird the speakers on their own racked up 35W but in tandem with the other units (computer in standby mode) made no difference, but the other units (all 240V) all added up to their logical combined total 33W - 34W[/strike]
Ignore that part I have to do more tests

JasX

1- Part explanation could be inaccuracy in the meter at very low wattages?

2-Is the speaker thing consistent and repetable -computer sitting switched on and you can plug the speakers in and out several times and watch the power level go up and down?

If its not just a blip is something internal to the computer/else powering up when it detects the speakers are gone? CPU load change too?

victor2

I too would suspect whatever you are measuring the power consumption with - especially where a transformer is involved. It would be quite a speaker system that takes 35W. The stereo system I have plugged into my computer, which includes a CD/tuner, takes 15W when powered and 7W in standby.

Wig

JasX wrote: »

1- Part explanation could be inaccuracy in the meter at very low wattages?
I don't think so , because it does measure low watt lightbulbs (7W) exactly as they say they are rated. Also the total wattage increased with the addition of each item as expected according to it's previously measured individual rating. When the speakers were added it did not increase.


2-Is the speaker thing consistent and repetable -computer sitting switched on and you can plug the speakers in and out several times and watch the power level go up and down?
Absolutly repeatable, yes, it does it each and every time I plug in or unplug It goes up about 18W when they are unplugged.


If its not just a blip is something internal to the computer/else powering up when it detects the speakers are gone? CPU load change too?
I just ruled this out, disconnected the jack from the computer. Now the computer doesn't 'know' if the speakers are powered up or not they are totally separate. And it still does it.

>>>>>>>>>>>>.........

Wig

victor2 wrote: »

I too would suspect whatever you are measuring the power consumption with - especially where a transformer is involved. It would be quite a speaker system that takes 35W. The stereo system I have plugged into my computer, which includes a CD/tuner, takes 15W when powered and 7W in standby.

Maybe its the rather large (very large) transformer I have to convert into 110V and combined with the speakers' own power transformer which would be about 9W (same size as the modem transformer, which is 9W).

fwor

It's probably an artefact of the way the monitor attempts to measure a load which is not linear.

Something like a 70W bulb is a very simple load - when the mains AC cycle is at maximum voltage, the bulb is taking maximum power, and it takes correspondingly less as the AC voltage drops to zero in the cycle.

However, the switched mode power supplies in the PC and probably some of the other items you mention aren't like that all. The load with these is much less predictable and "spikey". It's quite possible that the voltage regulation circuit in your 240-110V converter is absorbing a part of the amplitude of the spikes, thus making the average consumption appear to be less.

stevemcol

Agree with FWOR, it may be a power factor issue.
As he suggests, voltage and current don't always increase and decrease in sync. By connecting different appliances is varying comibnation, you could be observing this phenom.
A bit nerdy but this guys does an interesting demo. http://www.youtube.com/watch?v=AI_e3dF14eg

Wig

fwor wrote: »

It's quite possible that the voltage regulation circuit in your 240-110V converter is absorbing a part of the amplitude of the spikes, thus making the average consumption appear to be less.

That's what I was thinking, honestly

But I wondered if the actual power consumed was actually less or just that it appeared to be less.?

paddyrg

I'm with the guys above - some loads are simple, some more complex, and all especially so at such low values. I suspect you may also have a point that the transformer itself will distort the load and indeed will not be anything approaching 100% efficient

victor2

stevemcol wrote: »

Agree with FWOR, it may be a power factor issue.
As he suggests, voltage and current don't always increase and decrease in sync. By connecting different appliances is varying comibnation, you could be observing this phenom.
A bit nerdy but this guys does an interesting demo. http://www.youtube.com/watch?v=AI_e3dF14eg

Nerdy indeed, but interesting nevertheless. Thanks!

stevemcol

Wig wrote: »

That's what I was thinking, honestly

But I wondered if the actual power consumed was actually less or just that it appeared to be less.?

If we are correct that it is the power factor that's being distorted then no, you can't influence the amount of power you are using (other than by switching off appliances). Simple power monitors just measure current which is an approximation of the power consumed (volts x amps). However, your main meter measures true power** (volts x amps x power factor).
So your simple power monitor may report higher or lower current but your meter would be ignoring these 'apparent power' changes.


**I'm only 90% certain of that fact and would be interested in other views.