HPS Grow Light Fundamentals.
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High-pressure sodium (HPS) grow lights are known as gas-discharge lights. At the core of and HPS bulb is a small glass arc containing mercury and sodium. Electrical voltage passes through the arc causing the gasses inside it to glow. The entire reaction is fairly sensitive to fluctuations in voltage, and so to keep the glass arc from exploding the input power for the entire system is cleaned and regulated via a device known as the ballast.
More specifically, the principles behind the lamp is the following:
• An AC voltage source with an inductive ballast is used to provide the lamp with a clean and constant supply of current.
• The current enters the arc and begins to heat it causing the metals found within it to shift from solid to liquid.
• The current continues to raise the temperature in the arc causing the sodium and mercury to mix together into an amalgam.
• As the temperature increases, the amalgam shifts into a gaseous form and begins to emit light.
• As the pressure in the arc increases from the increase in gaseous metal, the intensity of the light increases.
• Eventually an equilibrium point is reached with the ballast regulating the input current to keep the pressure in the arc at a steady state.
HPS grow lights can have a life expectancy of over 20,000 hours. As they near the end of their life, they begin to exhibit a behavior called “cycling.” This is caused by a change in the amount of sodium in the arc, caused by chemical loss over time, causing the amount of voltage required to keep the light running to increase. The ballast is no longer capable of supplying the lamp sufficient voltage and it shuts itself off to prevent it from destroying itself.
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