Which size resistor will I need to ballast a fluorescent.

Topics relating to lighting control gear, including magnetic & high frequency ballast and ancilliary devices such as starters, ignitors and switchgear.
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MetalHalideHater
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Which size resistor will I need to ballast a fluorescent.

Postby MetalHalideHater » Sun Apr 29, 2012 12:59 pm

I have a load of high wattage resistors and wonder what Ohomic value they will need to be to ballast a 20w T12.

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Tom
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Re: Which size resistor will I need to ballast a fluorescent

Postby Tom » Sun Apr 29, 2012 2:15 pm

Very good question there MHH! I've often wondered the same! Just make sure your lamp is on something earthed so it starts reliably! 100w GLS would be worth a try too!
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SuperSix
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Re: Which size resistor will I need to ballast a fluorescent

Postby SuperSix » Sun Apr 29, 2012 2:22 pm

You'd need a resistor with a value around 480Ω and it would need to dissipate at least 70W, would be best to choose one capable of dissipating a fair bit more power than that so it doesn't run quite so hot.

Tom's idea is best, a 100W GLS lamp is a fairly close match! :woohoo

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Re: Which size resistor will I need to ballast a fluorescent

Postby MetalHalideHater » Sun Apr 29, 2012 2:26 pm

I tried a 70 Ohmer on a 20w modern sylvie. Needless to say, it worked for a while, until one electrode went pop. It still works with that electrode shorted out.

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Re: Which size resistor will I need to ballast a fluorescent

Postby Tom » Sun Apr 29, 2012 2:31 pm

Yep one of those big ally bodied jobs would be good. You may want to heatsink it too! What would be good for 4-6-8w T5s?
I once fitted a 4w inside an amp to backlight the VU meters. Worked well but the 8w ballast caused magnetic interference! Would have probably been ok if it was resistor ballasted!
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Re: Which size resistor will I need to ballast a fluorescent

Postby SuperSix » Sun Apr 29, 2012 2:44 pm

You're looking at around 1500Ω for a 4/6/8W circuit and the resistor would need to dissipate almost 30W for a 4W circuit! Terribly inefficient! Alternatively a 25W GLS lamp is close enough match.

In theory a 2mfd capacitor could be used as a ballast for the miniature T5s if you don't have a choke handy although I'm not sure how stable the arc would be and whether the lamps would last very long.

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Re: Which size resistor will I need to ballast a fluorescent

Postby Globe Collector » Tue Jun 12, 2012 8:11 am

In Viet Nam they sell little night lights. They are tiny phosphor coated tubes with three seals at each end. I presume there is an oxide coated, pre-heatable type of electrode between the two outermost seals and a "probe" type electrode, positioned underneath the transverse, oxide coated affair. The two probe electrodes at each end are connected by a 100K resistor and the outer two simply joined. On end goes to the "neutral" and the other, via a 100nF 275V A.C. film capacitor and 1 ohm "smoke" resistor to the "active".
I have tried capacitive current regulation of small discharge lamps, it works, but its hard on the electrodes every time the arc re-strikes with each new half cycle. Since Xc = 1/2pi f C, one does not want the frequency to go up. Imagine a lightning strike, one of those "jab five times, then jab fade" types of bolts which collects charges from a new area of the cloud as the previous jab alters the electric field in the cloud. These can induce a string of high dv/dt pulses onto the mains similar to an ignitor. These go straight through a capacitor as its reactance is low to rapid changes, and pof goes your lamp. Chokes and inductors are used because they are single pole low pass filters which knock off all this rapidly changing stuff.
If you know the inductance of the ballast inductor or choke, then finding Xl, its inductive reactance, is simple, by using the equation Xl = 2 pi f L. This reactance comes out of the equation in "ohms", and if you substitute a resistor with the same omic resistance value as the calculated reactance of the correct choke it will work, however, since the voltage and current are no longer at quadrature, you must calculate the power dissipation of the resistor. Some types of Philips Iron-Cobalt halide suntanning lamps and Ediswan Pointoloite lamps use resistive ballasting, not to mention MLL, MBTF and other self ballasted mercury lamps. When I was young, before I had any chokes, I used banks of G.L.S. lamps as ballasts for high pressure mercury lamps.


One of the really odd things about inductors is how they are marked. Capacitors are marked in capacitance and working voltage, (and occasionally ripple current), resistors are marked in resistance and power dissipation, but, rarely are chokes and wound single inductance ballasts marked in inductance and current, and possibly peak flash over voltage and onset of core saturation current. Even low pressure sodium leak transformers should be marked with the basics, primary inductance and mutual inductance as well as turns ratio and any other useful guf.
I have exhaustively measured the properties of any inductive control gear I can get my hands on and I will list these values under a separate subject heading.


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