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    Photo Cells for LED

    I recently found out that you have to use a photo cell rated for LED when controlling LED lights. Why would an LED need something different than just a normal photo cell? I thought it was just basically a switch?
    [COLOR=navy]If you aim at nothing, you will hit it every time![/COLOR]

    #2
    190920-2433 EDT

    Little Bill:

    Rating is not the real issue, but design is. You will need to start running experiments, and taking things apart to try to study how they work. I feel that in many cases a rating may not really be a good indication of how something will work.

    For a photo switch to switch LEDs I would suggest that one that requires power from line voltage, and that uses a electromechanical relay would be the best choice. This will be off when it is off, and it will cycle on when it needs to.

    .

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      #3
      Actually it's called a photo control. Some use a solid state device and work much like a dimmer only with two states- on and off. And you know how dimmers work with LED lighting. What you want is the ones that use contacts to turn the switch leg on and off.

      -Hal

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        #4
        Absolutely essential is that the photo control make use of the line neutral as a voltage source instead of depending on conduction through the load or to the EGC. Similar to the difference between a two terminal dimmer (only really satisfactory for incandescent lamps) and a three terminal dimmer.

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          #5
          I only found this out when I was at the SH asking for a photo cell (control). They asked if I was using it for LED. I suppose they found out from their rep to ask customers wanting a PC. Anyway, the brand that I use now says "LED" on the box. I'm not sure if they are also for incandescent or if they still make ones just for incandescent.
          [COLOR=navy]If you aim at nothing, you will hit it every time![/COLOR]

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            #6
            I do not see any reason that a dimmer specifically for LEDs cannot support incandescent as well except maybe that it could not handle the inrush of a cold tungsten filament or the surge of a high wattage incandescent when it burns out.

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              #7
              So what makes it 'led' rated...? ~RJ~

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                #8
                190921-0509 EDT

                romex jockey:

                Most photocell controlled circuits with an incandescent bulb would not use a load less than about 100 W. A 100 W 120 V incandescent is about 10 ohms with no current thru it and at room temperature. When powered with 120 V its resistance rises to about 120*120/100 = 144 ohms.

                With a series powered switch, a two wire switch, the incandescent load is a sufficiently low resistance to provide plenty of power to the series connected electronics without illuminating the incandescent load in the off state. There will be a small current thru the load.

                When the photo switch turns on there is sufficient load current to provide reasonable power to the photo switch to keep the photo switch on without too much voltage drop across the switch.

                An LED light with equivalent light output to an incandescent requires much less current when on, and probably does not look like a resistive load, and does not have the same change of apparent impedance between its on and off states.

                .

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                  #9
                  Originally posted by Little Bill View Post
                  I only found this out when I was at the SH asking for a photo cell (control). They asked if I was using it for LED. I suppose they found out from their rep to ask customers wanting a PC. Anyway, the brand that I use now says "LED" on the box. I'm not sure if they are also for incandescent or if they still make ones just for incandescent.
                  Bill, your suppliers are behind the times. Those led photocells have been around for about 2 years or so. We were having so much trouble with the standard ones that we complain bitterly about it. Our suppliers check it out with the manufacturer and they said oh, we have led photocells....
                  They say I shot a man named Gray and took his wife to Italy
                  She inherited a million bucks and when she died it came to me
                  I can't help it if I'm lucky

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                    #10
                    Originally posted by Little Bill View Post
                    I recently found out that you have to use a photo cell rated for LED when controlling LED lights. Why would an LED need something different than just a normal photo cell? I thought it was just basically a switch?
                    One issue is that the SCR or Triac on the output of a typical photocell unit has a series R-C "snubber" across it to prevent it from false triggering on voltage spikes. This snubber causes a leakage current that could be enough to light up an LED lamp dimly and with possible flickering. The LED compatible photocells might have something like a large resistor to shunt this current to the neutral wire which should be provided as GoldDigger mentioned.

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                      #11
                      Gar’s last paragraph is the answer for most of the cheap photocontrols now.
                      some manufactures recommendation...
                      “Ideal for LED fixtures totaling 1650 W @ 277 VAC”

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                        #12
                        LED lights have high inrush current (up 70 times of nominal), it could be a real problem for contacts in dimmer (for example). if you check Lutron Maestro dimmers for example, you will see that LED permitted load is less than other types of lighting loads for the same dimmer.

                        Sent from my ONEPLUS A6013 using Tapatalk

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                          #13
                          Originally posted by victor.cherkashi View Post
                          LED lights have high inrush current (up 70 times of nominal), it could be a real problem for contacts in dimmer (for example). if you check Lutron Maestro dimmers for example, you will see that LED permitted load is less than other types of lighting loads for the same dimmer.

                          Sent from my ONEPLUS A6013 using Tapatalk
                          We're talking about photo control switches, unless you're just relating that to them.
                          [COLOR=navy]If you aim at nothing, you will hit it every time![/COLOR]

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                            #14
                            and motion detectors too.....?

                            so now our diagnostics would be to try an incandescent , vs. bad pcell, etc....?

                            ~RJ~

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                              #15
                              Years ago I remember a home owner who installed cfl's in the motion sensor flood. The unit stopped working. Took out the cfl's and installed leds and the unit worked again
                              They say I shot a man named Gray and took his wife to Italy
                              She inherited a million bucks and when she died it came to me
                              I can't help it if I'm lucky

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