NicoD
Posts: 8
Joined: Mon May 20, 2013 7:18 pm

lcd power consumption

Sat Dec 26, 2015 8:09 pm

According to the specs the lcd should consume less than 500mA, at 5V that means less than 2.5W. When I have the powersupply plugged into a power-meter (or whatever that's called in English), I see power dropping from 9.6W to 5.15W when I turn off the backlight of the display. That's a difference of 4.45W. Does that mean my powerbrick is wasting about 2W?

I did some measurements with 3 USB powerbricks (>2A) that I had laying around and measured the following

Note: I'm using a Pi 2 B, with wired network plugged in, no USB devices, official 7" touch. The Pi is a clean install of Raspbian with no extra services running

PowerAdd PR-M525 (2 USB sockets, max 2.1A)
powerbrick idle, no Pi connected: 0.4W
Pi booted, running idle, lcd turned on: 10.3W
Pi booted, running idle, backlight turned off: 5.4W
Pi in 'poweroff', backlight turned off: 2.3W

Official Raspberry Pi powerbrick
powerbrick idle, no Pi connected: 0.3W
Pi booted, running idle, lcd turned on: 9.6W
Pi booted, running idle, backlight turned off: 5.15W
Pi in 'poweroff', backlight turned off: 2.2W

Apple ipad powerbrick (5.1V, 2.3A)
powerbrick idle, no Pi connected: 0.1W
Pi booted, running idle, lcd turned on: 9.1W
Pi booted, running idle, backlight turned off: 4.75W
Pi in 'poweroff', backlight turned off: 2.0W

About 9.5W for an idle Pi with turned on lcd, that's a bit more than I expected :-(

Nico

karlkiste
Posts: 184
Joined: Tue Jan 22, 2013 8:50 am
Location: berlin, germany

Re: lcd power consumption

Sun Dec 27, 2015 7:00 am

Here's what I measured with only the display connected to the power meter:

viewtopic.php?f=108&t=128234&p=857722#p857722

Greets,
Kiste

JimmyN
Posts: 1109
Joined: Wed Mar 18, 2015 7:05 pm
Location: Virginia, USA

Re: lcd power consumption

Sun Dec 27, 2015 9:34 am

Something doesn't seem right in the wattage measurement. It's not just the display power that appears to be doubled, you're showing 5.4w, 5.15w, and 4.75w just for the RPi, which would indicate that it's also drawing around an amp with no USB or other appreciable load on it.

The wattmeter may not be accurate at those low values. Either that or your efficiency is only 50% since the wattage is about double what you would expect for either the RPi or the display.

NicoD
Posts: 8
Joined: Mon May 20, 2013 7:18 pm

Re: lcd power consumption

Sun Dec 27, 2015 1:54 pm

@Kiste: are those numbers measured on at the 230V power socket or at the power supply exit?

@JimmyN: it does look suspicious indeed. It would expect/hope the efficiency would be at least somewhere around 80%.
Unfortunately my other powermeter doesn't go below 10W at all.

Hmm, if the power supply would indeed be wasting 50% it should get rather hot. The Raspberry Pi power supply doesn't feel warm at all so I guess it is the powermeter that is not being accurate.

It would be interesting to see some numbers from other people measuring power consumption at the high-voltage side.

karlkiste
Posts: 184
Joined: Tue Jan 22, 2013 8:50 am
Location: berlin, germany

Re: lcd power consumption

Sun Dec 27, 2015 9:21 pm

NicoD wrote:@Kiste: are those numbers measured on at the 230V power socket or at the power supply exit?
It's measured at the input of the power supply. What kind of power meter do you use? Does it display VoltAmperes along with Watts? If so, Watts is the number to read. VoltAmperes is "Apparent Power" and in general of no concern to you. With switching power supplies, apparent power is usually significantly higher than "real" (or "active") power which is measured in Watts.

Greets,
Kiste

NicoD
Posts: 8
Joined: Mon May 20, 2013 7:18 pm

Re: lcd power consumption

Mon Dec 28, 2015 2:12 pm

The power meter I'm using is a Perel.eu E305EM5 (http://www.perel.eu/products/view/?id=382030). It is supposed to be accurate up to .2W according to the specs.

The display says Watt, not VA. According to the manual this is the 'real power'.

Nico

karlkiste
Posts: 184
Joined: Tue Jan 22, 2013 8:50 am
Location: berlin, germany

Re: lcd power consumption

Mon Dec 28, 2015 5:25 pm

NicoD wrote:The display says Watt, not VA. According to the manual this is the 'real power'.Nico
Yes, as it can also display power factor it really should be able to display real power. It's just, your numbers would resemble what I had on my display in VA... Maybe one day I'll measure on the DC side using a calibrated meter. I probably won't have time this year, however. ;-)

Greets,
Kiste

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