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Watt You Need To Know: The Lowdown on HPS Grow Lights

Get the low down on one of the most common types of grow lighting. Get the low down on one of the most common types of grow lighting.


Greg Richter is an aviation and hydroponics engineer who created the PurGro GroBot Evolution. He contributed this article to Rosebud Magazine to help our readers better make sense of one of the most common types of grow lighting out there. Get the lowdown on HPS grow lights - check it out:

High-pressure sodium (HPS) bulbs are the standard against which all other grow lights are measured. They’re efficient, cheap and perfect for most grows. Here’s how they work, what makes them stop working, and a few things to save you trouble with your HPS grow lights.

HPS bulbs consist of an arc tube filled with xenon gas, and a little sodium mixed with mercury mounted inside a glass bulb with a screw base. The xenon provides an easy way to initiate an arc and get the sodium amalgam hot enough to vaporize. Once the arc is stabilized, it’s the incandescent sodium vapor that produces the characteristic yellow light of the HPS lamp.

Keep the bulb wiring short, keep the bulbs themselves cool and change them often, and your HPS lighting will yield you a bounty of produce.

To light a bulb, 2,000–4,000 volts are needed to strike the arc. A hot bulb requires as many as 20,000 volts, which is why it often won’t strike until cooled. Once the arc is struck, the initial phase of warming up the sodium amalgam requires current-limiting to keep the arc stable. Once the bulb is up to temperature, the arc can be maintained with a constant voltage source. All this staging is handled by your ballast, which strikes the arc, maintains constant current during startup, and regulates voltage and sometimes frequency to maintain constant bulb output power.

Aging bulbs get harder to strike as the sodium chemically combines with the arc tube materials, and the arc becomes harder and harder to maintain with less sodium available. If your bulb strikes, warms up and goes out, it’s time to replace it. Bulbs also color shift as they age, which is why growers tend to replace them long before they get hard to strike.

HPS bulbs for growing come in 400-watt, 600-watt and 1,000-watt sizes. The 400- and 600-watt bulbs run at around 130 volts whereas the 1,000-watt bulbs run closer to 230 volts, which is why you don’t want to try to dim a 1,000-watt ballast to drive a 600-watt bulb. Some ballasts have an overdrive feature to run a 1,000-watt bulb at more than 1,000 watts, which makes the light brighter and bluer, but shortens its life. For some growers, that’s a good trade.

There is some discussion about digital bulbs, which are intended for high-frequency electronic ballasts as opposed to the 50 or 60 hertz that mag core ballasts put out and most bulbs are designed for. The bottom line is that most bulbs will work with just about any ballast, but if you’re blowing bulbs all the time this may be worth looking into. Keep the bulb wiring short, keep the bulbs themselves cool and change them often, and your HPS grow lighting will yield you a bounty of produce.

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Have a look at the system Greg Richter invented.
Last modified on Tuesday, 16 April 2013 06:32

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