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Found this circuit on youtube

Discussion in 'General Electronics Discussion' started by mrel, Sep 7, 2019.

  1. mrel


    Jul 2, 2008
    Found this circuit on youtube this circuit turn on Led using photocell .
    Anyone know what the 100,000 ohm resistor in the circuit for?
    Is this to protect the led or mosfet or the photocell?
    Looking at circuit look like there no resistor to limit current to led.
    Led rating is:12 volts ,watts 20 ,current 1800ma..
    If i run led at 11.50 volts and current at 1.500 ma would my resistor be 7.6 ohm?
    should i put the resistor in series with the led?
    In the youtube the guy use 10 watts ,12 volts led,i want to change 20 watts 12 volts led.
    Thank for the help mrel IMG_20190906_020737.jpg
  2. AnalogKid


    Jun 10, 2015
    The 100K resistor is what turns on the MOSFET when it is dark. It forms a voltage divider with the photocell. When it is light, the cell resistance is so low that the voltage at the gate is too low to allow enhancement (conduction), and the LED is off. When it is dark, the cell resistance is so high that the gate voltage is high enough to turn on the FET.

    It looks like the LED has current limiting built-in, but you cannot know this for sure unless you know the manufacturer and part number for the LED.

  3. Audioguru


    Sep 24, 2016
    You and we need to see the datasheet for the 20W LED you have to see how they show to cool it and its actual range of forward voltage. If it already has current-limiting built-in then what is its range of current when the supply is 10V, 12V and 14V?
  4. mrel


    Jul 2, 2008

    Purchase from Amazon ,amazon website said by colorfulpeal .
    Type : high power led chip,rated power : 20w.35mil emit colors: cool white.
    wide viewing angle 135-140.
    forward voltage (fv):dc12-13v,forward current (if) 1800ma
    color temperature 6500 kelvin.
    That is all information amazon website list on this Led.
    Does not mention anymore built in resistor to the led
  5. Audioguru


    Sep 24, 2016
    Amazon does not know anything about electronic parts and probably does not even know which company made them.
    Most high power LEDs have a forward voltage range much more than you say, like from 10V to 14V. If you get one with a forward voltage of 10V and you power it with 12V then it explodes. If you get one with a forward voltage of 14V and you power it with 12V then it does not light up. Instead you feed it with a constant current source which is powered from about 16V.
    Amazon did not tell you how to cool the LED, did they?

    You should buy parts from a western electronic parts distributor who sell modern name-brand western parts that have a very detailed datasheet.
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