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#18814 - 12/14/02 05:11 PM Questions on Hid lighting
ant540 Offline
Junior Member

Registered: 08/14/02
Posts: 5
Loc: Antioch, IL
I recently installed a 150watt HPS flood. The light draws 4 amps on startup and 3 there after at 120 volts. I was wondering why so high, dosent conform to Ohm's law? Could the ballast be going?Also does anyone know what a 400 watt HPS light draws at 120v? If I wanted to change the fixture to a 400 watt MH would I have to gut the whole thing or could I just change the ballast?

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#18815 - 12/14/02 07:46 PM Re: Questions on Hid lighting
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Three amps at 120V is 360 voltamperes, but these days the fixture should have fairly high power factor, so probably under 200 voltamperes [under 2 amps] would be more reasonable.

Contact the manufacturer for a specific rating...they may have a web site with electrical ratings of their products.

The fixture should be hotter than blazes of it's actually using that much power. Make darn you have an accurate meter, that the right tap is used on the ballast and that the line voltage is in range.

#18816 - 12/14/02 10:31 PM Re: Questions on Hid lighting
ThinkGood Offline

Registered: 08/07/02
Posts: 1084
Loc: Milwaukee, WI

#18817 - 12/18/02 12:12 AM Re: Questions on Hid lighting
Scott35 Offline

Broom Pusher and

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA

Regarding the 150 Watt HPS, does this lamp have a Medium base by chance?
Sounds like a Straight Reactor type Ballast, which will draw more Starting Line Current, then taper down to a steady running level when the lamp is at operating intensity.

I can see this Ballast drawing >300 VA, since the Power Factor of a Straight Reactor coil is,,, err,,, crappy! Connecting a 55 MFD 170 VAC Capacitor in Parallel with the input (Line + Common) should bring the Line Amperes down to around 2.5 +/- for starting, then maybe around 1.5 to 2.0 Amps at normal operation.
If not, check for a shorted winding segment (if the PF corrected line amperes is high with known good Capacitor installed)

Be sure the correct Voltage Lamp is used with this Ballast!!! 150 Watt HPS Lamps come in two flavors: 55 Volt (Medium Base) ANSI S55 and 100 Volt (Mogul Base) ANSI S56.

The 55 Volt S55 uses the Straight Linear Reactor for Line Voltage at 120 VAC.
The 100 Volt S56 uses the Constant Wattage Autotransformer (CWA) for all input line Voltages - 120, 208, 240, 277 and 480.
For Multi Voltage input (120, 208, 240, 277/480) to an S55 lamp, the High Reactance (HX) Autotransformer is used.

These components (Ballast + Arc Tube Lamp, plus possible PF capacitor) are REACTIVE ELEMENTS - they draw Volt-Amperes (Apparent Power) from the AC supply.

Within the Apparent Power (VA) is the true Power (Watts) and Reactive Power (VAR).

FYI, the Ballast could draw 4 amps with absolutely no TRUE POWER (Watts) being "Consumed". This would be a "0"% Power Factor, or all the Volt-Amperes are Reactive (VARs - Volt-Amperes Reactive).

Anywhere heat is produced in the circuitry, is a transfer of True Power; meaning that some level of Wattage is being drawn from the supply. Where the heat is, so is Ohms Law.

For the Lamp, Power is transfered in the form of Light and heat [actually both are light, but let's not go there now!].
The True Power (Wattage) drawn from the supply is a total from all points in the circuit where true power can "do work".
Most points are dissipations through pure resistance [R], while the others are through an Impedance [Z].

Impedance is the total opposition to an AC.
Simple formula: Z= ^(square root) of R^ + X^
X = total Reactance (XL-XC).


If a Fixed Resistor with 3 Ohms Resistance was connected in series with an Inductor having an Inductance of 4 Ohms, the following would result:

    [*]Connected to Direct Current, total resistance will be 7 Ohms,

    [*]Connected to Alternating Current, total Impedance will be 5 Ohms.

    [*]If the DC voltage is 7 VDC, a current of 1 Amp will flow through this circuit.

    [*]If the AC voltage is 7 VAC, a current of 1.4 Amps will flow through this circuit.

    [*]True Power drawn by the circuit driven by DC = 7 Watts.

    [*]True Power drawn by the circuit driven by AC = 9.8 Watts.

Per a 400 Watt HPS fixture:
If the Ballast is a typical CWA, the total Volt-Amps drawn will be around 480 VA at full operation. This works out to be 4 Amps @ 120 VAC. The same will apply for a 400 Watt Metal Halide Lamp and Ballast kit.

The Luminare should be made to accept a common Ballast mounting kit - which should be similar for CWA Ballasts that drive either an HPS or MH Lamp.
The coils are completely different as far as their Electrical make-up goes, but physically they are very much the same.

Don't try driving the HPS Lamp from the MH's Ballast - or vice versa. One has too low of operating voltage that can be applied across the arctube, the other can create excessive voltage across the lamp's arctube.

QUIZ: Guess Which One Does What???
P.S. Pulse Ignitor can be used on either lamp without causing the effects listed above.

If you have more Q's, feel free to post 'em!

Scott s.e.t.
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!


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