Converting 24VAC Halogen SCR Dimmed Fixture to LED

Discussion in 'The Projects Forum' started by Tim Davis, Jun 10, 2015.

  1. Tim Davis

    Thread Starter New Member

    Jun 10, 2015
    Hello, This forum looks like a great place.. I hope I can get some help. I have been reading alot but hoping some experts can chime in.

    I have an "intelligent lighting fixture".. Ie. A robot with a lamp.. known as a "HES Trackspot" Used in nightclubs. The fixture is from 1992 with a simple macro size PCB. I am attempting to convert this unit to use high current energy efficient LED for less power and brighter colors. The issues I cannot resolve are

    1) Fixture uses SCR for dimming, LED's mostly use PWM
    2) Output voltage for lamp is 24VAC @ 250W, Input voltage for LED's is DC

    So basically im trying to install this LED

    This is the schematic for the old Fixture (Note page 14 shows the dimming circuit) (PIC)

    So im guessing I need a constant voltage for the LED, an LED IC Driver for the dimming/power and some way to marry the information from the SCR circuit to the IC Driver.. all while using the Switching, Dimming of the original fixture. The original bulb was 250 Watt halogen and I believe the dimming for the bulb terminates with a decreased voltage to the bulb.

    Hope this makes any sense!
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
  3. Tim Davis

    Thread Starter New Member

    Jun 10, 2015
    Hello.. Thanks for your reply. Yes we need higher color temperature. The link you provide says 250 Watt equal but the lumens are only 2500 Lumens. The lumens of the halogen lamp are 5600-7000. I have since purchased this LED which is actually only 1 inch across.

    So my question now is I will purchase a seperate power supply for the LED as its the only option for a constant forward current to the LED. The power supply accepts 1 - 10V dc for dimming. So now I only need two things.

    1) A way to trigger the LED to power on when the original lamp was supposed to strike
    2) A way to hook into the SCR circuit (diagram above) to transform to 1-10V DC
  4. Tim Davis

    Thread Starter New Member

    Jun 10, 2015
    I just realized that the dimming is already wired into the final 24V out (to the halogen lamp).. I could take this 24V and convert it into 10-12V somehow. It would need to adjust by voltage so say 24VAC would give 12VDC but 12VAC (dimmed) would give 5VDC. My only DUMB question is the bulb was rated at 250Watts @ 24VAC (10AMPS) but the max input current of the dimmer is 0.5A
  5. ebeowulf17

    Well-Known Member

    Aug 12, 2014
    Specs, model number on dimmer? I would expect your 1-10V dimming signal input to be high impedance and not draw significant current unless driven with way too much voltage, but that's just a guess. Need to see datasheet to really know.

    As for 24V SCR to 1-10V signal conversion, it sounds to me like an ideal application for an op amp integrator circuit... but I'm still new at all this and have only read about them, never built one. You'd definitely want someone more experienced to confirm or deny my theory.
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    The LED in post #3 has this spec:-
    DC Forward Current(IF): 2400mA
    DC Forward Voltage(VF): DC 28.0-36.0v

    That means your existing 24VAC supply won't be directly usable to power the LED; an AC/DC adapter will be needed to give a DC supply > 36V. Do you already have one? If not, a dimmable LED ballast is called for.
    It would be relatively simple to convert the existing dimmer output (phase-controlled 24VAC) to a 0-10VDC (or similar) control signal for the ballast.

    Here's how you could do the conversion.
    R4 is a dummy lamp load to maintain the SCR current above the 50mA minimum holding current for much of the mains cycle. The graphs show the converter output for dimmer settings of 100% (yellow) and 50% (blue).
    Last edited: Jun 13, 2015