I am going to install a 1.5 V 15 ma (.015) amp incandescent lamp, attaching it to a steam Tsunami board. I am using a Zephyr so am assuming voltage is 12.8 (per the book); my averaging meter reads it as 13.0, following Digitrax's instructions.
Doing the math, I need an 850 ohm resistor of 1/4 watt, using the 12.8 Volts input. BUT, there is a voltage drop across the decoder of, I presume, 1.5 Volts to give me 11.3 Volts at the lighting contacts. The question is, should I be using 11.3 volts and a 750 ohm resistor? I guess there is a second question, now that I think about it: am I correct in assuming a 1.5 Volt drop at the contacts? I'd appreciate some quidance.
You need to calculate the resistor based on how much voltage it has to drop - the sum of the voltages of the loads in a circuit equals the total input voltage - Kirchoff's voltage law. So if the output at the function is 12.8 volts, and you have a 1.5 volt bulb, the resistor must drop the remaining 11.3 volts. The current of loads in series is equal across each element, so the resistor will be passing 15ms, just liek the bulb. So you would use the 11.3V/0.015A ~ 750 Ohms. Check the actual voltage at the function outputs with meter. I don't think there's any standard for the circuit design of function outputs, just the colors of the wires, so it pays to check. It's usually track voltage with perhaps one diode drop from the transistor 'switch'. A slightly (liek next value up) resistor will only slightly dim the light but greatly extend the life, while one value too low will greatly shorten the life of the bulb. This is why I like to use LEDs when possible.
Here is a good site that explains some of the series dropping resistor calculations, in case anyone is interested. http://technology.niagarac.on.ca/courses/etec1120/Files/Lab4.pdf
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I second the LED. If you are going to the extra for a resistor than an LED is just as simple. After melting an RS1 shell from a bulb I did a wholesale round of sunny whites and never looked back. One bit of advice is to ohm the resistors you get. I got a bad batch and popped a couple LEDs before realizing it.
Pete
I pray every day I break even, Cause I can really use the money!
I started with nothing and still have most of it left!
Wow - where did you get a bad resistor from? In I don't know how many years messing with electronics, close to 30 I would guess, I don't think I've EVER had a resistor that was color coded one value and measured something else. And I've gotten them in kits, in boxes of leftover 'junk' people have given me, from Radio Shack, local electronic stores (once upon a time we actually had one), and mailorder places like Mouser.
An averaging meter sees voltage about the same way as your light bulb sees it. No problem there. One way to attack this problem is to compute the resistance of your lamp (V/I = R, 1.5V/0.015 = 100 ohms). The current in the lamp is the same as the current in the dropping resistor. The equation of the circuit then becomes
V = I * Rdroppingresistor + I * Rlamp. (From Kirchoff's voltage law)
You have I (0.015A), you measure V with your meter, You know Rlamp = 100 ohms. Do the algebra and solve for R dropping.
Assuming V = 13 Volts, I get R dropping equal to 766 ohms. 740 ohms is a readily available standard value and close enough.
You don't need to assume a voltage drop, just measure the voltage at the output of the Tsunami board and that's what it is. Be truly crafty, measure the voltage under load. The 0.015A draw of the lamp may load the voltage down, and under load it drops to something less than what it reads open circuit. You can check this by touching the dropping resistor across the Tsunami board output and see how much the voltage drops, if any.
David Starr www.newsnorthwoods.blogspot.com
Randy
Pack of 5 at Radio Shack. I picked up 2 packs of 1K 1/4 watt and only 2 out of 10 tested around 1K. Some tested less than 300 ohm and some were almost infinite. They had no 4th stripe just the brown black and red. Now I look for the gold or silver stripe and still test it. A10 cent resistor popped a 2 dollar LED.
It's not a bad idea to measure resistors with a meter before installing them. It's a double check in case you didn't read the color code correctly. Or read the intsy teensy writing on the more modern resistors. The only resistors I ever saw without the tolerance color band (gold = 5%, silver = 10%) came out of a real antique 1920's vacuum tube radio set.