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Thread: Why do certain LED strings not dim?

  1. #1
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    Default Why do certain LED strings not dim?

    Some of my LED strings dim, and some (typically newer purchased ones) will not dim, but retain the same brightness until they switch off at a certain minimal on time of the duty cycle.

    I'm guessing there's extra circuitry in these strings, they have a bigger plug. Anyone know what's inside them to cause them to
    behave differently?

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    Default Re: Why do certain LED strings not dim?

    The non dimming ones probably have a capacitor in them, hence the big plug. Could be wrong since this is just a SWAG.

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    Default Re: Why do certain LED strings not dim?

    Quote Originally Posted by macebobo View Post
    The non dimming ones probably have a capacitor in them, hence the big plug. Could be wrong since this is just a SWAG.
    That would be my guess as well. The cap is probably there to reduce 60 or 120 Hz flickering (or 50/100Hz flicker if it's for a country with 50Hz power).
    Phil

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    Default Re: Why do certain LED strings not dim?

    The other possibility might be circuitry to reduce the voltage going to the LEDs if it's a short string.
    Phil

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    Default Re: Why do certain LED strings not dim?

    Interesting. I've also noticed that the plug gets somewhat warm too.

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    Default Re: Why do certain LED strings not dim?

    Quote Originally Posted by jetty View Post
    Some of my LED strings dim, and some (typically newer purchased ones) will not dim, but retain the same brightness until they switch off at a certain minimal on time of the duty cycle.

    I'm guessing there's extra circuitry in these strings, they have a bigger plug. Anyone know what's inside them to cause them to
    behave differently?
    One possibility is if the package that they were in states that they are "Full Wave Rectified" then there is a full wave bridge rectifier with in the plug and possibly some other circuitry to do a constant current. Therefore if you use phase dimming techniques that will not work. You have to use PWM to dim them. To get around flicker with LEDs is to do full wave rectification of the AC line. At 120 Hz the flicker is not recognizable to the human eye. OnSemi has a line of constant current regulators that are designed to work with a bridge rectifier and supply between 20 and 350mA constant regulated current. They will maintain current regulation with as little as 0.5VDc across the device. They are small. Anywhere from a SOD123 package to D2PAK package. neat devices. They will also work on Incandescent lights to.

    Kind of neat to control dimming of incans with essentially a DCSSR using PWM instead of Phase dimming. There are a couple of other constant current sources and sinks out there that could also work. Diodes Inc make a current sink that will work with up to 100 VDC and about 150mA.

    james

  7. #7
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    Default Re: Why do certain LED strings not dim?

    Are they Martha Stewart Led's? If so they have capacitors.
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    Default Re: Why do certain LED strings not dim?

    Quote Originally Posted by buckanear View Post
    One possibility is if the package that they were in states that they are "Full Wave Rectified" then there is a full wave bridge rectifier with in the plug and possibly some other circuitry to do a constant current. Therefore if you use phase dimming techniques that will not work.
    You can do phase angle dimming with a full wave bridge (I use it), so there must be some other components in there to prevent the dimming.

    Quote Originally Posted by buckanear View Post
    They will maintain current regulation with as little as 0.5VDc across the device.
    Could you post the part# for that?

    don
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    Default Re: Why do certain LED strings not dim?

    Quote Originally Posted by djulien View Post
    You can do phase angle dimming with a full wave bridge (I use it), so there must be some other components in there to prevent the dimming.



    Could you post the part# for that?

    don
    NSI45020 will do 20mA constant current. I am planning on using those and the NSI45035J which is adjustable from 35 to 70mA. Also the NSI45090J which is adjustable from 90 to 165mA. All these will handle 45 Volts peak voltage across them. Average is about 7Volts. These are from Onsemi.Also from Onsemi is the NSI2020 which will withstand peak voltages of 120V at a regulated current of 20mA.

    From Diodes Inc consider the AL5801 or AL5802.

    attached is a app note to use a CCR with a bridge rectifier.

    Also a design note how they are used in a LED T5 fluorescent replacement bulb.

    also

    james
    Attached Files Attached Files

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    Default Re: Why do certain LED strings not dim?

    Quote Originally Posted by SteveMaris View Post
    Are they Martha Stewart Led's? If so they have capacitors.
    It would be interesting to see what they looked like inside. i wonder if they are doing something like a drop cap technique?

    james

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