Converting dual filament bulb to single filament

Discussion in 'General Electronics Chat' started by Bubblehead1985, Sep 11, 2008.

  1. Bubblehead1985

    Bubblehead1985 Thread Starter New Member

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    I am converting my turn signal/parking lights from dual filament bulbs to single filament bulbs. This is a common conversion on motorcycles, hotrods, etc. I am using a 3 wire to 2 wire converter(see attached pic). The turn signal comes into the circuit(red wire) through a 1N4001 diode. The park light signal comes in the circuit(grey wire) through a 27 Ω resistor and a 1N4001 diode in parallel to the turn signal. The two wires are joined at a common output(black wire) the goes to the positive lead of the turn/park signal and the negative lead is attached to a ground wire.
    When I apply 12VDC to the circuit, the parking light does not light and the resistor gets very hot. When I turn on the turn signal, the light flashes as normal and the resistor returns to normal temp. Or, if I start with the turn signal on, the light works as required and the resistor remains cool until I turn off the turn signal. All readings are as expected on the components. I am stumped!!!

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  2. beenthere

    beenthere AAC Fanatic!

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    Is your bulb socket still the kind with two contacts for the dual filament bulb? It sounds like there is a bad path for current in the park circuit (around the bulb filament).
  3. Bubblehead1985

    Bubblehead1985 Thread Starter New Member

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    No, single contact socket.
  4. beenthere

    beenthere AAC Fanatic!

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    Can you verify that the gray wire has 12 volts on it only when the parking lamp is selected, and that the red wire only has an intermittent voltage present when the turn signal is selected?

    That black wire should run to the center contact of the bulb socket, and the outer part of the socket should be in common with the frame and battery negative.
  5. Bubblehead1985

    Bubblehead1985 Thread Starter New Member

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    Voltage on the grey wire is constant and does not go away when turn signal is on. Voltage on red wire is intermittent with flasher. Black wire is to the center contact of the bulb and the other wire connects from the outer shell to the negative/ground.
  6. beenthere

    beenthere AAC Fanatic!

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    About the only condition that would cause your problem is for the diode on the red wire to be shorted, and the red wire to connect to ground when not flashing. Unsoldering the red wire from the diode should tell if this is the case (the lamp should be at park brifgtness and the resistor ill not get more than warm).
  7. Bubblehead1985

    Bubblehead1985 Thread Starter New Member

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    I will give that a try this weekend. Unfortunately production decided to work tomorrow and I have to support. Recently we have only had to work Monday thru Thursday with every weekend off. I will post back with results. I appreciate your time and assistance.
  8. SIcam

    SIcam Active Member

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    Maybe I am wrong but the filament in the bulb initially has a resistance of practically 0 at room temperature. The in rush of current causes the filament to get hot and its internal resistance to increase and emit photons. I think your resistor has limited the current to the point that the internal heating resistance of the filament is not reaching photon emitting temperature. I think you need to drop your resistance to about 5 ohms and get a potted ceramic high temp resistor ( Your basically connecting a 27 ohm resistor across a 12.8 - 0.7v or 12 vdc source. That means the you are dumping 5 watts across that small resistor. Your turn signal is working because it allows enough current to the filament to get it cherry red (or to the photon emitting temperature.) You need to rethink your resistor value or series chain about put a zener in the circuit to drop park voltage to about 6-7 volts.

    What bulb you using?
  9. Bubblehead1985

    Bubblehead1985 Thread Starter New Member

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    12V 23W single filament bulb. It is what came in the new lights I bought.
  10. SIcam

    SIcam Active Member

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