Help with multimeter to find watts

Discussion in 'General Electronics Chat' started by salai ezhil Mathi, Apr 21, 2017 at 1:28 AM.

  1. salai ezhil Mathi

    Thread Starter New Member

    Jan 24, 2017
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    Hello,

    I am just in the starting stage, please go easy on me.

    I have bought a multimeter like the one attached. I have connected 5 to 6 LED lights in series to a 12V lead acid battery. Please help me to measure the watts.

    when I check with the A at the right hand side, with the meter in 200m, the meter read 192. when I used an online calculator to find watts, it said it was 2.9 watts. will LEDs draw only that much power? I took the SMD leds with the board from a phillips LED bulb. 91XOGjq5RQL._SL1500_.jpg
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    1. How did you connect the meter to measure current?
    2. What formula are you using to calculate power?
     
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Measure the current drain from the battery, measure it's voltage under load, multiply them together Bingo! Wattage.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    Is there anything to limit the current through the LEDs besides the internal resistance of the battery?

    upload_2017-4-21_17-18-7.png
    If these are common LEDs such as those shown in the picture above, they should be connected with a resistor in series with them, otherwise you could have excessive current through the LEDs (Here, "excessive current" means enough current to significantly reduce the life of the LED, maybe down to zero.). With that, power measurement is easy -measure the voltage drop across the resistor and divide by the value of the resistor to get amps, then measure the voltage across the diodes to get the watts into the LEDs. Then just multiply them per Dodgydave.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Hello salai ezhil Mathi, welcome to the forums.

    Power used in some thing can be measured many ways. The basic way is to measure the voltage across the thing and the current thru the thing. Multiplying the voltage by the current gives the power:

    Code (Text):
    1. Power [Watts] = Voltage [Volts] x Current [Amps]
    Note the units of Volt, Amp and Watt. One Watt is one Volt times one Amp.

    The "200m" current scale means the maximum reading is 200 milliAmps. When you see 192 it means 192 mA, or .192 amps.

    You should also measure the voltage your 12 V battery is actually supplying. Let us assume the 12 V is accurate. In that case the power is:

    Code (Text):
    1. 12 Volts x .192 Amps = 2.304 Watts
    I do not know why this does not agree with your online tool. I tend not to use these, just work with the basic equations.

    The next equation you will need is Ohm's Law:

    Code (Text):
    1. Current [Amps] = Voltage [Volts] / Resistance [Ohms]
    (Edited to correct typo as Dick suggested)
     
    Last edited: Apr 21, 2017 at 6:50 AM
  6. DickCappels

    Moderator

    Aug 21, 2008
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    Slight typo perhaps.
    12 Volts x .192 Amps = 2.304 Watts
     
    ErnieM likes this.
  7. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    You stated the following:

    I totally agree as this is a basic rule. Then you additionally state the following:

    (Emphasis added)

    I may be obsessing over this? But this is not totally true. A better statement for me would be one Volt times one Amp is one Watt. This is an unambiguous statement, unlike the original.

    The first statement is not uniquely true. There are multiple solutions to the proposal, such as one Watt is two Volts times 500 milli-amps.
     
  8. salai ezhil Mathi

    Thread Starter New Member

    Jan 24, 2017
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    They are SMD LEDs. I cut them with the circuit board from a 4 feet led light battern.
     
  9. DickCappels

    Moderator

    Aug 21, 2008
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    That probably explains why they are drawing such a high current. Things should be ok if you put a resistor somewhere bear 470 ohms in the series string.
     
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