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F.S.U's and Wattage Ramblings.

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An F.S.U. is a little canister unit that helps start a fluorescent tube in an old magnetic ballasted fluorescent fitting. It is a Fluorescent Starter Unit, thus F.S.U. We don't use them now in modern electronically ballasted fluorescent fittings or with L.E.D. tubes.


In the days when unicorns roamed the Serengeti, we had two popular tubes of F.S.U., one was rated at 4 to 65 Watts, the other up to 125 Watts. This referred to the rating of the tubes. 4 to 65 Watts covers a single tube up to 5 feet long, and the other F.S.U. (up to 125 Watts) covers up to 8 foot long tubes.


I had to change some tubes and starters in an old animal sanctuary yesterday. When I came to buy some new F.S.U.s they were rated 4 to 65 Watts or 70 Watts. The 70 Watt version covers the modern 70 Watt 6 foot tubes. 


Now, a Watt is a very small unit of power. Old tungsten car side lights were just 5 Watts. Dashboard indicator lights might have been just 1.2 Watts. Bright headlights 55 Watts.


So why make two F.S.U.s these days just 5 Watts apart? Why not make just one rated 4 to 70 Watts?


Z.


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    I refer here to the voltage across the lamp under steady state operating conditions. This voltage varies from about 25/30 volts for a 6 inch 4 watt lamp, up to about 160 volts for an 8 foot 125 watt lamp.

    The striking voltage of the starter must be greater than the lamp operating voltage, but less than the mains voltage, and by a generous margin in both cases to allow for manufacturing tolerances and supply voltage variations.


    The starters for 6 foot and 8 foot lamps have to reliably strike from a 240 volt nominal supply mains that is down to 220 volts actual, but also to reliably NOT strike from the normal lamp voltage which might be about 175 volts under adverse conditions.


    The nearly universal starters have a much wider voltage tolerance and are therefore cheaper to make.


    Because 6 foot and 8 foot lamps operate at a higher lamp voltage than other types, they need a nominal 240 volt supply for reliable operation. Such lamps on standard switch start control gear do not work well or reliably from 220 volt circuits, which may in practice be below 200 volts at the point of use.

    6 foot and 8 foot lamps can of course be used from 220 volt circuits if the fitting incorporates a step up transformer, but the extra costs, losses and complications tends to drive the choice to five foot or shorter lamps in 220 volt countries.

    An electronic ballast can of course be designed to work any lamp from any supply voltage, including say an 8 foot lamp from a 220 volt supply.


    Longer lamps a UK thing, and also popular in the USA from 277 volt circuits.

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  • 0
    I refer here to the voltage across the lamp under steady state operating conditions. This voltage varies from about 25/30 volts for a 6 inch 4 watt lamp, up to about 160 volts for an 8 foot 125 watt lamp.

    The striking voltage of the starter must be greater than the lamp operating voltage, but less than the mains voltage, and by a generous margin in both cases to allow for manufacturing tolerances and supply voltage variations.


    The starters for 6 foot and 8 foot lamps have to reliably strike from a 240 volt nominal supply mains that is down to 220 volts actual, but also to reliably NOT strike from the normal lamp voltage which might be about 175 volts under adverse conditions.


    The nearly universal starters have a much wider voltage tolerance and are therefore cheaper to make.


    Because 6 foot and 8 foot lamps operate at a higher lamp voltage than other types, they need a nominal 240 volt supply for reliable operation. Such lamps on standard switch start control gear do not work well or reliably from 220 volt circuits, which may in practice be below 200 volts at the point of use.

    6 foot and 8 foot lamps can of course be used from 220 volt circuits if the fitting incorporates a step up transformer, but the extra costs, losses and complications tends to drive the choice to five foot or shorter lamps in 220 volt countries.

    An electronic ballast can of course be designed to work any lamp from any supply voltage, including say an 8 foot lamp from a 220 volt supply.


    Longer lamps a UK thing, and also popular in the USA from 277 volt circuits.

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