Basic electrical question, resistor for wiring building lights

[grayfox1119]

The thing to remember here is that the current is the same in a series circuit, and the voltage is constant in a parallel circuit. So it matters how you hook up the lights.

[Don Gibson]

BRUCE - absolutely correct.

My point was far more basic.

Incandecent bulb's use 'WATT's of power, and use of Resistor's WASTE power. (Both dissapate heat).

Lghting model building's would require multiplicity of bulbs, and w.w. resistor's.

[ukguy]

Thanks for the advice guys, the power source I am thinking of using is an old computer PSU, these have a 5v and 12v output. From what has been said I could wire a dimmer switch to the 12v output, run my 6 distributor blocks from this in parallel and then run each diorama's accessories from these blocks.
Thanks for the time and detail in your response Bruce, although I'm a smart guy all that math and calculation seems to make my innitial idea not viable, maybe viable but alot of work and inflexible.
Thanks also Don, if I understand you correctly I can wire my lights directly and they will be fine, I always thought if you wire a 3.5v bulb to 12v power it would blow, shows what I know, thats why I always pay for electricians and mechanics, know your limitations, esp. where fatality could be involved.

Thanks again.
Karl.

[Blind Bruce]

The use of one resistor will work ONLY if all lights have the exact current draw. If you have a building with, say, two lamps and each draws, say, .01 amps, then the building lights will draw.02 amps. Now, if you add another structure, with, say, three lamps, then the draw for this structure would be .03 amps which would require a different resistor value. The two together would dray .05 Amps.
To calculate the value of the resistor, you need at least two known quantities. Usually the voltage that you have to supply the current and the current that will be required. In our example above, for building no. one we need .02 amps. Lets say that you have a 15 V supply and your bulbs are rated at 12Volts. That means that the resistor must drop three volts at .01 amps each. Using ohms law where resistance = volts divided by current, we see that volts=3 and current = .02 which is 150 ohms. For building two, we still have the three V to drop, but now have .03Amps current. Therefore divide the volts (3) by yhe new current .(.03) and arrive at 100 ohms. The two together would require a resistor value of 60 ohms.
The power required for this resistor is P= E X I or .06, .09 and .15 Watts respectively.
Every time you add or subtract a bulb, the resistance must be changed.
You could also wire all the lamps in a series configuration and the resistor value will not be as critical but then, if one bulb fails, the all will go out making it hard to finf the one bad unit.
BB

[Don Gibson]

Only LED's require resistors. 1 per matches output's.

INCANDESCENT bulbs do not need resistors - they waste power.
All you need is an adequate power supply. = X AMPS.

MY 5 stall roundhouse with 3 5v lamps per stall, requires 5 AMPS'. I use a separate 5v. 10 amp supply for this and all other incandescent lighting.

[cacole]

If you have an old DC power pack from a train set or whatever, connect your wiring to the variable DC terminals and you can use it for your lights. This will let you adjust the brightness as lights are added and draw more current, or dim the building lights if you darken the room.
.

[Ibeamlicker]

Why resistors?If your running DC use a dimmer switch or some other reostat.

[ukguy]

I am almost ready to instal some basic lights into several buildings and locations, my logical mind tells me that instead of putting a resistor on each light I can place one resistor on the feeder wires to a distribution block and then wire 10 or so lights to this block without the need for resistors on each bulb.
I have built my 'town' in a diorama fashion, each section lifts out to be worked on easily.
Overview of layout

Industrial Area

Dioramas


My thinking is to have a distribution block attached to the underside of each diorama with all accessories wired to this. Then have power feeder wires with a resistor to feed each block.

If all accessories per diorama are within the same power range is my thinking correct. The only wiring/electrical experience I have is when I wired my track, and that took me all day to get the operation I wanted.

Many thanks in advance.

Have fun & be safe.
Karl.