Auto transformer for hid lights
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  1. #1
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    Default Auto transformer for hid lights

    Can someone please explain how the transformers function in these types of lights. Specifically in a mercury vapor set up with no capacitor. I measured the no load voltage in the bulb socket it was 244 volts. I measured the voltage at start up with the bulb installed it was about 14 volts and rose to 122 volts after it warmed up. How is this all possible with a transformer that has 1 winding. I forgot to mention its 125 volt input and the bulb is 175 watts. Thanks everyone.

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    When cold, a bimetallic strip closes and shorts the discharge gap. This presents a low resistance path for current to flow through a filament, which heats enough to allow mercury to vaporize. Once the filament is hot enough, the discharge gap opens, and current flows through the mercury vapor, making bright light in the process.

    The transformer manages driving this by being a spectularly bad transformer. It's not as simple as a winding/core/winding. There is a chunk of core material bypassing magnetic flux around just the primary. This limits the current the transformer can put out through the secondary. When at 14 volts heating the filament, the transformer is likely producing much more heat in itself than it does in the bulb. But the bulb is small and it doesn't take much heat to vaporize mercury so it's not a big deal. Arcs can't reliably be sustained with a set voltage. The amperage should be constant, and the voltage should fluctuate to maintain it. This requires a certain amount of voltage loss for the current regulation to work.

    Some of this voltage drop is poor power factor, and doesn't become heat, while much of it is resistive/magnetic losses, and the reason ballasts get so damn hot.

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    Is power coming from different parts of the transformer during start up and regular operation? Or is the bulb just putting different loads on the transformer when the bi-metallic strip is in different positions.

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    Quote Originally Posted by camaro_dan67 View Post
    Is power coming from different parts of the transformer during start up and regular operation? Or is the bulb just putting different loads on the transformer when the bi-metallic strip is in different positions.
    The bulb presents different resistance. The transformer supplies a mostly constant current through that resistance, resulting in a variable voltage.

    Not only does the bulb have the on/off discharge bypass, but the resistance of the filament increases a bit as it gets hot, and the resistance of the gas decreases as it gets hot. The latter part is why these can't run off of constant voltage. The gas gets hot, and the resistance drops. With constant voltage, that means the current would go up, making the gas get hotter, making the resistance decrease, increasing the current, making the gas get hotter, decreasing the resistance, increasing the current, making the gas get hotter, and so on until something pops. There are other ways around this but current limiting is by far the easiest.

    That brings up the note that the open circuit voltage is double the operating voltage of the bulb. The current limiting setup needs some head room, but not that much. The voltage has to be high to arc through the mercury vapor when it's only sort of warm by the filament. The gentle opening of the electrode gap also helps "pull" an arc, but the cold electrodes tend to have an arc cooling effect on short gaps, so the extra voltage helps start it reliably. Also the operating voltage may go up as the electrodes within the bulb wear, or mercury gets absorbed by the electrodes or contaminated with metal from them. Lots of design dependent factors here.

    Also worth noting is that the transformer isn't that great at regulating current. The current it can supply is still dependent on the voltage drop across its windings, but this effect is much weaker that the gas temperature/resistance relation and so doesn't have much affect on the stability of the arc.

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    Could someone give me a detailed wiring diagram for one of these lights with the transformer taps included in the diagram. Thanks again Dan.

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    I forgot to ask in the last message about including which taps on the transformer go to which parts of the bulb socket.
    Dan

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    Generally speaking it should be on the ballast itself. One-size-fits-all diagrams are not good when a manufacturer decides to do things a bit differently.

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    Quote Originally Posted by camaro_dan67 View Post
    How is this all possible with a transformer that has 1 winding. I forgot to mention its 125 volt input and the bulb is 175 watts. Thanks everyone.
    The lamp is nonlinear. It's not a simple resistor.


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