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24 Volt Dimmer

Discussion in 'Electronic Design' started by Matt Durkin, Jul 6, 2004.

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  1. Matt Durkin

    Matt Durkin Guest

    Hi All,
    I posted this on .basics but have had no response. Can anyone here help me?
  2. Tim Mitchell

    Tim Mitchell Guest

    Use a separate "dimmable electronic transformer" for the lights, then
    use a standard wall dimmer to feed the mains power to it.

    Make sure that the dimmer is compatible with low voltage electronic
    transformers and that the transformer itself is dimmable, some aren't.
  3. I read in that Matt Durkin
    An ordinary light dimmer circuit will work, except that you can't use a
    35 V diac for triggering the triac. Use a low-threshold trigger circuit.
    Obviously, the triac needs a current rating much higher than 4 A to
    allow for the inrush current of the lamps, but 20 A triacs are freely
    available and cheap. Just check that 20 A is enough.
  4. CFoley1064

    CFoley1064 Guest

    Subject: 24 Volt Dimmer
    Hi, Matt. There are two basic problems with using a standard lamp dimmer with
    24VAC. The first is that the triggering devices that are still made have
    trigger voltages starting at 27V, which means tour AC waveform will be at
    around 60 degrees before the diac triggers.

    The second problem is that most of these simple dimmer circuits have turn-on
    hysteresis, which will drive you crazy with 24VAC. You will have to turn the
    dimmer control up to get turn-on, and then turn the dimmer control down to get
    to the lower voltage. If there's a line voltage sag, the dimmer turns off.
    Don't go there.

    On another s.e.d. question, someone mentioned the Velleman K8003 kit, which
    provides opto-isolated voltage input for a dimmer circuit. They're about $25
    USD each from various hobbyist sources. If you look in the manual for the kit
    (assembly required), you see that it has components for 120VAC as well as 24VAC
    operation. All you will then need is a voltage source for the input, which you
    can do like this (view in fixed font or M$ Notepad):

    Zener 10V Power Supply
    .--------. 1K Ohm 1 Watt
    | | ___
    o----o ~ +o---o----|___|----o---o
    | | | | |
    | | +| 1N4740| .-. 1K pot +
    24VAC | | --- Vz = 10V/-/ | |<-------o
    | | --- ^ | | Input to
    | | |470uF 50V | '-' Velleman K8003
    | | | | |
    o----o ~ - o---o-------------o---o---------o
    | | -
    | |

    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    Good luck
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