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How do I calculate power drain for a FPV transmitter?

Discussion in 'General Discussion' started by VoicOfReason, Sep 1, 2014.

  1. VoicOfReason

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    Using the Phantom 2's battery as reference (5100mah), how does one calculate the power drain IN MaH from a FPV transmitter? Mine is 600mw, so how do I figure this out?
    I'm probably wrong about this, but at 600mw, is that 600/60 = drain per minute = 10mw per minute.
    10mw per minute x appx. 20 minute flight time = 200mah.

    Is this right?
     
  2. OI Photography

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    Your vid Tx draws a very small amount of current, less than 1A. Impact on flight time on a Phantom battery is negligible.

    The best way to judge your real-world results is to fly a few test flights with the Tx powered on, a few with it powered off, and then a few without it mounted. You'll get more of a difference in flight time from the weight of the Tx than you do from it's power drain.
     
  3. VoicOfReason

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    What about a 2000mw Tx? If 600mw is negligible, than wouldn't 2000mw be not too bad either?
    Also, how exactly can I calculate the power drain?
     
  4. noiseboy72

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    You cannot calculate current draw on output power - unless you know the efficiency of the TX. Watts = Volts x Amps and Amps = Watts / Volts, so 2W@10V should be 200mA, but a 2W TX will actually pull about 1.6-2A, which is not negligible. This will decrease flight time and increase current draw, so the batteries work harder. It knocks about 2 minutes off, compared to a 200mW TX in my experience.
     
  5. Goldmagnet

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    Hi

    The immersion RC draws 3 watts of electrical power to generate 600mw of RF.

    3 Watts of power at 11 volts is a current draw of 273 ma.

    A battery rated at 5100mah would run the transmitter (alone) for 18.7 hours.

    The effect on a typical P2 flight time would be small, if you were getting 20 minutes before, my maths (might be wrong) shows that reducing by 19 seconds.

    I hope this helps.

    Huw
     
  6. Goldmagnet

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    Hi

    Using the same maths, an TX drawing 1.6 amps would reduce the flight time by 112 seconds, so ties in with observations.

    I am using:

    New Flight time = Old Flight time x (1+(TX current draw/15.5))

    All times in seconds.

    Cheers

    Huw
     
  7. VoicOfReason

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    Are you referring to a 2000mw Tx?
     
  8. noiseboy72

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    I used the 2W TX as a worse case example, but if the Immersion manages much better efficiency, then as you say, this will have a small effect on flight time.
     
  9. Goldmagnet

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    Hi

    Yes I realized the 1.6 amps was a worse case, its nice to have a formula to use now.

    Cheers

    Huw