Question about bulbs and resistors.

Model Railroader General Discussion (Model Railroader)
1

I am lighting the buildings for my town. I have a 5.6 volt DC power source and 3.5 volt bulbs. How do I determine what resistor to use and where do I wire it in?

I also have some 1.5 volt bulbs I would like to wire in. What resistors do I need there?

How does one figure that out, there must be a formula.

Thanks in advance

2

Art,

You really need to know how much current the 3.5 v bulb is rated for. For example, a 1.5 v bulb that draws 30 ma(.030 amps) is about 50 ohms of resistance. With that 5.6 v power supply, one would have to add a 137 ohm resistor in series with that bulb to keep the current flow at or under .030 amps. The key here is to keep the current flow at or under the rating of the bulb.

Here is a good website to calculate this stuff:

http://www.csgnetwork.com/ohmslaw.html

Jim Bernier

3

Art, all the bulbs should be wired in parallel. In a parallel circuit, the voltage is constant, and current is the sum of the parallel branches. Lets say for instance that you have 6 houses with one bulb in each house. All these bulbs will be wired in parallel. If your 5.6Volt power supply is not variable ( voltage can be adjusted ) then you will need to adjust the voltage for the bulbs so that you don’t either burn them out, or shorten their lives by applying overvoltage. The bulbs will have a current draw spec. and for the sake of math, lets say that each 3.5V bulb draws 1 milliamp ( 1 ma ) If you have 6 bulbs, they will draw 6 ma total from the power source. So if you place a 350 ohm resistor in series with the power supply feed, you will drop the 2.1volts and the bulbs will receive 3.5 volts and be very happy.

Voltage = Resistance in ohms multiplied by the current in amps. 1 ma = .001 amps.

4

I think the simplest thing is just to buy 6 volt bulbs and prevent burnout.

(Save 1.5 bulbs for headlights using 2 diodes).

3.5V bulbs can be paired (series) to make 7v - and mixed with 6 v. lamps on a ‘ladder’ - albeit somewhat dimmer .

5

Do NOT do it this way!! Each bulb should have it own resistor. With the above method, if one bulb burns out, the voltage to the other bulbs will increase, and if the initial design was close to the voltage limit of the bulbs, all of the others will get brighter and quickly burnout.

Do as Jim B. suggests above.

6

Nigel, using a zener diode will clamp the voltage at 3.5V, thus protecting the other bulbs from burning out from higher voltage and ultimate burn out. One zener instead of a quantity of resistors.

7

Dick;

Agreed, but you did not mention using a zener diode in your previous post…

8

formula: (supply voltage - bulb voltage) / bulb milliamps = resistance in k ohms.

Always round up, not down, to next standard resistance. Safer and dimmer to go even higher.

Safe Resistor wattage = (supply voltage - bulb voltage) * bulb current * 2

Good luck

Karl

9

This is how I used to do it, when I didn’t have an exact power supply for the lamps. The lower voltage will also make the bulbs last a lot longer.