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Lamp Filament Resistance (Q030E)

 

 

“Can I know if a bulb is 6 or 12 V by measuring the resistance of its filament? With this procedure can I also determine your consumption?”

 

Unfortunately, measuring the resistance of the filament is not a sure way to determine the characteristics of a lamp. What happens is that in cold, when the lamp is off the resistance of its filament is often less than the resistance to warm when it is lit. This difference can be very large and therefore we do not have a reliable indication of its characteristics. What happens is that when being fed the filament dilates and with that its resistance increases. Thus, a small 50 mA x 6 V lamp has a resistance of 120 ohm when lit, but is on the order of only 30 ohm when off. This is also true for incandescent bulbs connected in the power grid and also for other appliances which have heating elements. The measure of the resistance is much more useful to know if the lamp is good or not when the glass is milky and we can not see its filament.