# Power ratings

Discussion in 'Electronic Design' started by David, Sep 17, 2006.

1. ### DavidGuest

On the back of PC monitors it shows a voltage and current rating. Are
these rms values? If I want to know the power consumption, can I
simply multiply the voltage byt the current?

Similarly, for energy star ratings, if they specify a max power, is
that simply P = Vrms * Irms?

Thanks,
David

2. ### John PopelishGuest

That is the correct formula for a resistive load. It is an upper

For instance, loads that have an inductive or capacitive component use
some of that current to store energy in magnetic or electric fields,
each half cycle, but return it to the power line, before storing it
the other way, the next half cycle. So the rms current may include
components that do not represent consumed power, but just borrowed power.

3. ### Homer J SimpsonGuest

Kind of.
Probably.

Why do you want to know?

4. ### Don LancasterGuest

P is virtually NEVER Vrms * Irms.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552

Please visit my GURU's LAIR web site at http://www.tinaja.com

5. ### Don LancasterGuest

P is virtually NEVER Vrms * Irms.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552

Please visit my GURU's LAIR web site at http://www.tinaja.com

6. ### DavidGuest

I would like to know the typical max current draw from a PC monitor.
If I new the energy starrating was 3W and I could use P = Vrms *
Irms, then I would know the typical max. Of course, this assumes that
most monitors are energy star compliant.

Thanks,
David

7. ### Homer J SimpsonGuest

Vrms * Irms is close enough for most purposes. Add 50% as a safety factor.

8. ### Phil AllisonGuest

"David"
** The voltage one is.

** No way.

Eg: I am using a fairly new, 17 inch CRT monitor made in China.

On the back is printed " 100 - 240VAC 60/50Hz 1.5A "

However, when tested, the current draw is only 0.36 A at 240 VAC.

At 100 VAC it rises to 0.72 A.

These two figures calculate to 86VA and 72 VA respectively.

However, when measured, the actual POWER consumption only 56 watts !!

The current draw from the AC supply is in the form of short pulses at
100Hz - due to the rectifier diodes and cap input filter in the SMPS.

At 240 VAC input, the peak value of these pulses is 1 amp.

At 100 VAC input, the peak value increases to 1.5 amps.

However, I reckon that " 1.5A " on the back alludes to the fuse fitted
inside.

Go figure......

........ Phil