High Pressure Sodium security lamps

The familiar yellow-orange glow of outdoor high pressure sodium lighting have been around for about a half a century now.  HPS street lamps replaced the harsh blue-white Mercury Vapor HID bulbs that had previously replaced high power incandescent lamps before them.  Each advancement in lighting technology was more efficient than the one before, as well as offering longer bulb life, and better illumination.

Some people have thought that the poor color rendition of the Sodium lamps was a step backwards from the Mercury vapor lamps, and indeed Mercury lamps are still in use in industrial settings.  However, the Sodium bulbs have an often overlooked benefit, their warmer color does not havoc our circadian rhythm as the blueish Mercury light does, which can induce sleepiness during prolonged exposure during long nighttime road trips.

Today, LEDs have been replacing the HPS HID lamps when cities install new streetlights or upgrade older equipment.  Replacement parts are still available for the older HPS lamps, in many cases bulb replacement, and fixture repair are still less expensive than out right replacement of the equipment with newer LED lamps.  While the LED equipment is more efficient than the HPS, the difference is actually small, so the savings in cost to operate would take years to recoup.

When my wife and I moved into our house nearly twenty years ago, I installed a HPS HID barn lamp over the entrance to our garage where it lights up our driveway.  During that time I’ve replaced the bulb only three times, even though it burns all night, 365 days a year.  That’s an average life of 5-6 years per bulb.  I’ve also had to replace the igniter twice.  That’s a small circuit that performs the same function as the old fluorescent lamp starter, sending a pulse of high voltage to spark the bulb into conduction when power is applied.  The original igniter lasted over 15 years, the first replacement, barely 2.  Cheap, replacement parts from China.

When the lamp failed to start again a few days ago, I broke down and ordered a new LED fixture.  I did a post mortem on the HPS fixture, and it seems that this time the photo cell has failed, not the igniter, as I had assumed.  If I’d known that, I might have ordered a new sensor as the repair could be done without removing the fixture from the house.  Still, the new LED lamp should last me 20 years, if the 50000 hour life expectancy is correct, even if I have to replace its photo sensor some time.

Maintenance of HPS lamps is easy, so if you have one of these fixtures you might chose to keep them going.  The bulbs will give off less light as they age, but the process is so slow that your eyes won’t notice it.  The dead give away for a bulb that needs replacement, is it blinking on and off every few minutes.  The bulb starts off with a blueish color as the Mercury vapor starting gas fires on lamp start.  As the bulb heats up the Sodium starts to conduct, adding its yellow-orange light to the mix.  However, when the bulb ages, the vapor pressure inside the lamp falls off, and the Sodium stops conducting as the temperature inside the lamp rises.  When the bulb cools enough the igniter fires again, and the lamp restarts, for the cycle to begin again.  If the bulb isn’t replaced, eventually the igniter will self destruct from over work.

The ballast is a large transformer, or inductor inside the fixture that limits the current flow into the bulb.  It must be rated for the correct wattage to match the bulb.  This current limiter is necessary because a HID bulb appears as a negative resistance.  Ballasts rarely burn out, to test one you can screw an incandescent lamp into the socket in place of the HID bulb, and cover the photo sensor with some black electrical tape.  If the bulb lights, the ballast is good.  If not, you’ll have to check the sensor to be sure that that ballast is gone.

The photo sensor operates backwards from what you’d think.  The photo cell controls the current to a relay.  When light hits the cell its resistance is low and the relay pulls in.  The relay is a Normally CLOSED type, meaning that when the relay pulls in, the circuit is OPEN.  When the photo cell is in the dark, the relay drops out, CLOSING the circuit.  You can hear the relay operate, it will make a very faint click as it opens and closes.  My failed photo sensor would pull in the relay when I powered up the fixture in daylight, but the relay would not drop out when the photo cell was covered.  If I had powered up the lamp in darkness the light would come on, and it would go out when the photocell saw light, but it would not come back on again the next time it was dark.  The relay got stuck whenever it pulled it.

If the ballast, bulb, and photo sensor check out, but the fixture won’t start, you have a bad igniter.  Replacement units are housed in a sealed module with three wires.  Note how the original was wired in before removing it and re-wire the new one the same way.  A wiring diagram should be printed on the module.  Be sure to obtain an igniter sized for your bulb wattage.

I’ll probably get a replacement photo sensor and keep my original HPS HID lamp as a spare.  Somehow the old American made hardware seems more trustworthy than its modern tech, made in China replacement.  We’ll see.


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