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Need help measuring TV power consumption

Discussion in 'General Electronics Discussion' started by RussellSydneyOz, Feb 17, 2012.

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  1. RussellSydneyOz

    RussellSydneyOz

    2
    0
    Feb 17, 2012
    I am not sure that I am being bamboozled by science.
    I recently purchased a Panasonic plasma TV and when I place my old multimeter in the power circuit to measure the AC current, I get a reading of 0.10 amps when the TV is in standby.
    245V x 0.1A = 24.5 Watts in my book.
    The TV spec quotes 0.3W
    I contacted the Panasonic Help desk and got this reply:-
    ----------------------------------------------------------------------------------------------------------------------------------
    We have consulted with one of our product technicians, who has advised it seems likely the power meter you are using is designed for resistive loads. As you may be aware such meters output erroneous readings when measuring complex capacitive or inductive loads as presented by switch mode power supplies.
    The TH-P50ST30A like all Panasonic televisions has three power modes:
    · On Mode (153W): measured in Normal (Home Mode) picture setting using an IEC62087 test video disk.
    Active Standby (11W): when set is initially switched to standby via remote, the TV DVB-T tuner PCB performs an RF scan looking for new services / TV software updates etc. This normally takes less than 60 seconds.
    . Passive Standby(0.3W): When active standby functions are completed, TV goes into ‘deep sleep’.
    These measurements have been confirmed as accurate by our technician.
    ----------------------------------------------------------------------------------------------------------------------------------
    OK - my test is a resistive test, but would I not get a low reading rather than a high one?
    Do I need to update my equipment such as the Fluke 175 that reads "True-rms" ?
    (I am looking at around $A350 for such a meter)
    Any advise is welcome
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
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    Jan 21, 2010
    No, their tech support is correct and a true RMS reading ammeter would not help.

    The issue here is that the voltage and current are out of phase due to the load (the TV's standby PSU) being highly capacitive or inductive.

    Google "power factor" for more information.
     
  3. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    Steve, do you think ac current clamp meter will be much cheaper and accurate option for testing current consumption of the Plasma TV?
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
    2,779
    Jan 21, 2010
    A current clamp won't help.

    You need something that can calculate the power usage and which knows about power factor.

    There's plenty about.

    Here's one. It's not the cheapest I've seen, but it gives you an idea of what I'm on about.
     
  5. RussellSydneyOz

    RussellSydneyOz

    2
    0
    Feb 17, 2012
    Thanks Steve - I have immediately ordered one of these Steplight units and will fill you in when I test it.
     
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