Hi all,

Have a 'noob' question, and almost too embarrased to ask...:rolleyes:

But...do incandescent lights need a resistor in-line with each one to drop the voltage? I know LED's do, but wasn't sure about regular lamps.

The non-LED lights are operating at 1.5V\40mA and would connect to a 9V battery.

Thanks
Rob

Incandescent bulbs behave like non-linear resistors (their resistance is a function of their filament temperature). At power on they can draw a surge of up to 8 times their steady state current, which is why it’s common to see a light bulb go bad when you switch the power on. The voltages are additive, so connect six 1.5 volt incandescents in series and you have 9 volts, no resistor necessary. If you want to just run a single 1.5 volt bulb rated for 40 mA off of a 9 volt supply you would need a resistor to drop the voltage to a level that won’t kill the bulb, in this case 180 ohms @ 1/2 watt power rating. 9V – 1.5V / .04A = 187.5 ohms, use nearest common value = 180 ohms. In the case of a single bulb with a resistor the filament will warm up slowly (a second or two) because the resistor limits the magnitude of the initial turn on current. The lack of a significant turn on surge will also help to extend bulb life a little bit.

Thanks for the info, I see what you are saying if the lamps are in series about the resistance.

However, I am assuming that if the lamps were wired in parallel, I would need a resistor for each lamp?

Even with all the lamps in parallel you could technically still use a single resistor (of a different value than 180 ohms). The problem with doing that is all the load current flows through that one resistor producing a local hot spot that could cause styrene to sag or melt. By using a separate resistor per bulb you can spread the heat around and hopefully avoid a hot spot.

Gotcha...

Speaking of resistors...what does everyone generally use 1/4 or 1/2 watt? Or does it really depend on the design of the circuit?

The simple answer is that 1/4 and 1/2 watt are commonly available. The thing to remember is this: the current flowing through the resistor causes heat. If the resistor has too low of a wattage rating the resistor will burn out ... with emphasis on "burn". Not so good for the inside of a model.

Calculating the wattage of the resistor can be done by using Ohm's law. I cheat and use one of these calculators (http://www.the12volt.com/ohm/page2.asp).

Using the P = I^2 x R formula calculator where P is power in watts, I is current in amps = 0.040 (40mA) and R is resistance in ohms = 180 it tells you that it produces 0.288 watts or slightly over 1/4 watt. That's why you need a higher wattage resistor (1/2) watt to avoid burning it out.

For resistors on LEDs you'll see that 20mA (0.020 amps) and 360 ohms produces 0.144 watts. 1/4 (0.25) watt resistors are plenty for this use.

If you don't want to think about it just buy a mess of 1/2 watt resistors in useful resistances (the assorted resistor packs of 100 usually have about 90 that you'll never use, so buy specific resistances).

For your incandescent bulb application, consider using a 1.5 volt battery instead of 9V or 9V bulbs and skip the headache.

^Steve, thanks for the info..very helpful, and now have a better understanding of it all.