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Working out voltage drop and current flowing through a circuit

  1. May 15, 2012 #1
    I have just wired up a LED circuit in series of 3 with a resistor of 1000 ohms 1.6W on each 3. Its being supplied via a 12 V battery. The circuit is wired up to a PIR which when detects movement the LED's will light up.

    I need some help working out how much current will be running throughout the circuit and its voltage drop?

    I have 84 White LED's connect. Each LED being 3.4 V @ 10mA
  2. jcsd
  3. May 15, 2012 #2


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    You have a 12 volt supply and across it you have 28 strings, each containg a resistor and 3 LEDs (all white, 3.4 volt ones) in series.

    The strings are in parallel. Is this all correct?

    If so, then the voltage across the LEDs is 3 times 3.4 volts or 10.2 volts. So the voltage across the resistor is 1.8 volts.

    The current you want, 10 mA, is flowing through this resistor, so its value must be V / I or 1.8 volts / 10 mA or 180 ohms.
    LEDs usually run at a maximum of 20 mA so you could do this by making the resistor 1.8 volts / 0.02 amps or 90 ohms. You could use 100 ohms since these are easy to find. 0.25 watt resistors would be OK.
    (1.8 volts times 0.02 A = 36 mW)

    If your 12 volt supply is really higher than 12 volts, you should recalculate the resistor sizes to get more accurate results.

    The total current would be 28 times the current through each string. So, 280 mA if it was 10 mA and 560 mA if it was 20 mA.
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