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You are watching: Is a light bulb a resistor

Hi all,Have a "noob" question, and nearly too embarrased come ask...
But...do incandescence lights need a resistor in-line with each one come drop the voltage? I understand LED"s do, but wasn"t sure around regular lamps.The non-LED lights space operating in ~ 1.5V\40mA and would attach to a 9V battery.ThanksRob
Incandescent bulbs behave favor non-linear resistors (their resistance is a duty of their filament temperature). At strength on castle can draw a rise of approximately 8 times their secure state current, i m sorry is why it’s usual to watch a light bulb go negative when you switch the strength on. The voltages room additive, so attach six 1.5 volt incandescents in series and you have 9 volts, no resistor necessary. If you want to simply run a single 1.5 volt pear rated for 40 mA turn off of a 9 volt supply you would require a resistor to drop the voltage come a level the won’t death the bulb, in this situation 180 ohms
1/2 watt power rating. 9V – 1.5V / .04A = 187.5 ohms, use nearest typical value = 180 ohms. In the situation of a solitary bulb v a resistor the filament will warmth up progressively (a 2nd or two) because the resistor borders the magnitude of the initial revolve on current. The lack of a far-reaching turn top top surge will certainly also aid to extend bulb life a tiny bit.
Thanks for the info, I check out what you space saying if the lamps are in series about the resistance.However, ns am assuming the if the lamps to be wired in parallel, ns would need a resistor because that each lamp?
Even through all the lamps in parallel you can technically still usage a single resistor (of a various value than 180 ohms). The trouble with doing that is all the load present flows with that one resistor creating a local hot spot that could reason styrene come sag or melt. By utilizing a separate resistor per pear you can spread the heat around and also hopefully prevent a warm spot.
Gotcha...Speaking that resistors...what does everyone usually use 1/4 or 1/2 watt? Or does the really count on the architecture of the circuit?
The basic answer is the 1/4 and also 1/2 watt are frequently available. The thing to remember is this: the existing flowing v the resistor reasons heat. If the resistor has actually too short of a wattage rating the resistor will certainly burn the end ... With emphasis on "burn". Not so good for the within of a model.

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Calculating the wattage the the resistor have the right to be excellent by utilizing Ohm"s law. I cheat and use one of these calculators.Using the p = I^2 x R formula calculator wherein P is strength in watts, ns is present in amps = 0.040 (40mA) and also R is resistance in ohms = 180 it speak you that it produce 0.288 watts or slightly end 1/4 watt. That"s why you require a greater wattage resistor (1/2) watt to prevent burning it out.For resistors on LEDs you"ll view that 20mA (0.020 amps) and also 360 ohms to produce 0.144 watts. 1/4 (0.25) watt resistors room plenty for this use.If girlfriend don"t want to think around it just buy a chaos of 1/2 watt resistors in helpful resistances (the assorted resistor package of 100 typically have around 90 that you"ll never use, so buy certain resistances).For her incandescent bulb application, take into consideration using a 1.5 volt battery rather of 9V or 9V bulbs and also skip the headache.