12v transfomer mod

Discussion in 'The Projects Forum' started by alasdairc, Feb 12, 2014.

  1. alasdairc

    Thread Starter New Member

    Feb 12, 2014
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    Hi,

    I've come across a bunch of radio controlled dimmer halogen transformers (240 -> 12V). My intention is to use these to power little installations of doll's house lights around my home (at the request of my better half).

    However there's an issue in that the current draw from 4 or 5 doll's house lights on each installation is no where near enough to cause the circuit to fire up. I'm guessing this is the over voltage protection as the load is lower than designed for.

    Each bulb draws roughly 60mA.

    The transformer is designed to drive between 30W - 105W.

    I'm wondering what the experts here think is the feasibility of swapping the components to provide safe powering of circuits of no more than 15W.

    I suppose the first step would be to survey a board, pull and list all the components and create a schematic, which I'm willing to do and post up here.

    I just wanted to see if there's any immediate "this will never work" thoughts before I get into this.

    I have skills in electronics building and am safe worker with mains electricity, I'm just not an expert with circuit design.

    thanks for reading
     
  2. alasdairc

    Thread Starter New Member

    Feb 12, 2014
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    here are photos of the board, the trackside has been mirrored so that it has the same aspect as the component side.

    240V enters via the lower terminal blocks on the left, leaves at 11.5V on the top blocks.
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    why not simply put one halogen bulb in the circuit to draw curenet and use the supply for the doll houses? or a 5 ohm (or so) resistor? might be simpler than modifying the circuit.
     
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  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    I can not see any big filter caps. So this must must be an AC regulator using a triac (most probably) or thyristor setup. What is markings on the components mounted on the cooling fins and IC 302
    As a note a Triac requires a minimum current draw to work properly. This could a part of your problems
     
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  5. alasdairc

    Thread Starter New Member

    Feb 12, 2014
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    IC302 is a HA17358 dual opamp.

    The two transistors on the cooling fins are BUL38DA, high power NPN transistors.
     
  6. alasdairc

    Thread Starter New Member

    Feb 12, 2014
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    Yeah that is my backup strategy! I quite like the idea of learning how to mod this circuit, but if its too much I'll definitely go with the resistor approach.
     
    Last edited: Feb 12, 2014
  7. alasdairc

    Thread Starter New Member

    Feb 12, 2014
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    A downside about using a dummy load is that this means a lot of wasted energy.

    I just tried with a 25W 4R7 wirewound resistor and it got VERY hot. I don't know if I'm happy with the safety implications of this.
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    Then I must have been wrong. It must use some other control method than using a triac. Which kind of give it self. As I now see that the transformer on the board is quite small. Is it any markings on the IC mounted on the bottom side?
     
  9. alasdairc

    Thread Starter New Member

    Feb 12, 2014
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    The IC on the bottom is an ARC M3D. This is the microcontroller for the RF receiver, control circuits.
     
  10. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Why not just use a off the shelf cheapo wall wart power pack?

    At 60 ma a bulb a common 1 amp unit would be able to power around 16 bulbs without problems.
     
  11. alasdairc

    Thread Starter New Member

    Feb 12, 2014
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    Hi that's not really an option as some of these lamps are going to be installed out of the reach of plugs and I don't want to run new cables in the walls. They will be installed in the lighting circuit - the benefit of the RF remote control is I don't need to install new switches.

    .. yes this is a fairly eccentric proposal, however it will bring lots of happiness.
     
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