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funkjosh 01-02-2013 02:19 PM

high pressure sodium bulb
can someone please tell me why a high pressure sodium bulb will not work in fixture (designed for high pressure bulbs) but a compact fluorescent will?

joed 01-02-2013 02:45 PM

HPS requires a ballast similar to fluorescent tubes. It can not be simply installed into 120 volt light socket. The CFL has a ballast built into it. That is the big bulb at the base.

AllanJ 01-02-2013 02:58 PM

A high pressure sodium bulb (or a mercury vapor bulb) may be installed only in a fixture intended for it, including matching bulb wattage rating. Such a fixture will have the matching ballast in it.

Incorrect bulb usage can result in damage to the fixture or the bulb's exploding. Without the correct ballast, the bulb might draw more and more current as it warms up, until something fails or a breaker trips. Either that, or it doesn't light up at all, not enough volts to jump between the electrodes inside and get it started.

dmxtothemax 01-02-2013 04:19 PM

All discharge lamps have what is known as a
"positive temperature co-efficent".
Which means that as it gets warmer it's
impedance decreases so it draws more current,
as it draws more current, it gets warmer,
so again draws more current. etc etc etc.
This is called "thermal runaway"

So discharge lamps use the ballast to do 2 things
it plays a part in the intial "striking" of the lamp.
And then it limits the current to the lamp.

So all discharge lamps require a ballast.

micromind 01-02-2013 06:38 PM

A typical high pressure sodium fixture works like this;

1) When the ballast is first energized, a voltage is applied to the bulb. It can be anywhere from 100 or so for a small bulb to 350 or so for a big bulb.

Next, a short pulse of about 3000 volts (it varies a lot, but it's always pretty high) is applied to the bulb. This pulse fires an arc across the tube inside the bulb. Once the arc is established, the tube becomes fairly close to a short circuit. The ballast now becomes a current limiter, and supplies the bulb with whatever amount of current it's designed for. The voltage at the bulb falls wherever it will, usually somewhere between 25 and 100.

If the ballast and capacitor are both ok, and the bulb is fairly small, there'll be somewhere around 120 volts present at the socket. This is why the CFL works.

If the ignitor is bad, it won't send the high voltage pulse to the bulb, and it won't light.


funkjosh 01-02-2013 08:33 PM

thanx for the info
this all a lot of real good info, thanx. ill have to look at the wiring of the fixture again but i was told it is new, with new bulb and was wired correctly.

im just looking for things to troubleshoot, why a CF will fire up and not a HFS?

the possibilities may be a bad bulb or bad ignitor. anyone with ideas on how to troubleshoot fixture and bulb?


btw, i do get 120v at socket. is this correct?

is it possible im not being patient enough. does it take time for these bulbs to light? the fixture has a light sensor and is on a timer...

frenchelectrican 01-02-2013 08:35 PM


Originally Posted by funkjosh (Post 1084642)
can someone please tell me why a high pressure sodium bulb will not work in fixture (designed for high pressure bulbs) but a compact fluorescent will?

Most common failure with HPS if not replace the lamp fast engouh is the starter or ingitor which they will send high voltage pulse to get the arc started then the ballast will regulate the voltage/current to proper spec.

There one more important part make sure if you going to replace the HPS lamp make sure it will match the ballast this is true for 150 watt verison due there is two verison it used one is 55 volt arc tube and second verison have 100 volt arc tube both are not interchangeable.

The typical open circuit voltage is 120 volt range for 55 volt arc tube verison ( under 150 watts ) while the larger one it will be about 250 to 400 volts depending on the wattage so that the differnce on that.

You may find the ingitor in big box store but kinda hard to find them but electrical supply centre even lighting centre may have it on hand otherwise they can order ya one. But make sure you mention the correct bulb wattage or bring the old one in to match it up.

But one warning with ingitors when you replace them make sure keep the power off and also watch the connection very carefull it don't take much to get it screw up.


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