Thanks to all of you who answered my previous model rr questions, now I have another one: I am in the process of wiring street lights for my towns and the lamp bulbs are of various wattage (6v, 14v, 12v etc).
My transformer puts out about 19v AC and while I could wire the bulbs in series to drop the volts, I would rather put a resistor on line for each set of bulbs..i.e. dropping one string from 19v to 6,or 19 to 14 etc. Is there an on line conversion chart that could tell me what resistor to use or is there a SIMPLE (and I stress simple because i'm an idiot) formula that I can use.
Any help you can give would be greatly appreciated by the small people in the town who so far have to carry lanterns to make their way around.
The formula to calculate a resistor for an AC circuit is Z = E/I, or Impedance in Ohms is the Voltage (E) divided by the current in Amperes (I). For safety, use a higher wattage resistor than you would with a DC circuit.
tam...(and I stress simple because i'm an idiot)
Instead of insulting yourself, why not just say something like: "I'm not that good at math". That would be a more respectable thing to say and it won't make you look bad compared to your other word choice.
Anyway, here is this current limiting resistor calculator for LED's. I know it's for LED's but the math that determines the resistor value is the same for LED's and lamps. I hope you know your lamps' mA (millampere) rating. This tool (and others like it) are awesome for folks that aren't too good at math.
TONY
"If we never take the time, how can we ever have the time." - Merovingian (Matrix Reloaded)
Do you have a ohmmeter (typically found on multi function voltmeters/ammeters)?
If you don't, you'll need to know the wattage of the bulb, or the resistance of the bulb (sometimes printed on the packaging) (The info on the package would be a better way to go.)
I'll give you directions based on your answer.
BTW: There is an electronics and DCC section of the forum for questions like these. People like me (electrical geeks) have a tendency to hang around there.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
Hi, yes I do have an ommeter. I can't find any info on the packaging of the lights that might help.
I actually do have an ohmmeter, but don't know the wattage of the bulb because nothing is printed on the packaging...i actually had to go to a Walthers catalogue, look up the item, and then figure out the voltage from there.
I was originally going to post the question on the electronics/dcc board but when I looked at it it seemed to concern itself with only DCC so I was afraid my question would be overlooked.
Thanks
TAM
Here's how I do it:
Resistors are cheap, so I went to radio shack and bought an assortment of 1/4 watt resistors ranging from 22 ohms to 470 ohms. Then I just try different ones, or combinations until I get the look I want. I wire them in series to add the resistance together and in paralel to reduce resistance. A combination of series/parallel will increase wattage.
Once you find a combination you use a lot, you can just buy those and use them instead of keeping an assortment.
While we're talking about lights, I'd like to offer a helpful hint: If you find a "deal" on colored GOW bulbs but you really want clear bulbs, go ahead and buy the colored ones. I've found that acetone (nail polish remover) cleans the color off in a heartbeat.
Phil, I'm not a rocket scientist; they are my students.
Measure the resistance across the lightbulb.
Bulb voltage/Resistance = Current.
The current is what you need for your circuit eq.
Since you are putting out 19 Volts AC, 19V - bulb voltage = voltage drop across resistor.
Voltage drop across resister / Current = Resister value.
Example:
19 Volt AC
12 Volt Bulb
100 Ohm resistance across bulb
12V/100Ohm = .12 amps
19-12 = 7 Volts across resistor.
7/.12 = 58.3Ohms (~roughly 60 Ohm should do it)
Confirmation:
.12 amps * (58.3ohms + 100ohms) = 19 Volts AC (V = IR)
Thanks everyone for the directions and formulas. I really appreciate it.