Hi all – Just trying to sort out the whole resistor thing. I bought some 3 to 6 volt bulbs, 68mA, and I am also using some 3v bulbs in some locos I’m converting. (I have to measure current.)
Output voltage from the white and blue wires on the decoder is 13.5 V. So I’m calculating (13.5V - 3V) / 0.068A, which gives me 154 ohms. At 6 volts, I get 110 ohms. Is my math correct?
And what (watt?) about wattage? I’m assuming voltage times amperage, so 13.5 x 0.68 = 0.918 – a 1 watt resistor (so should I use two watts)? Or should I be calculating based on the voltage of the bulb?
I wish I’d taken electronics in high school, but I didn’t, so I’m just learning all this now…
(Oh, and before you ask, I haven’t found an LED bulb with a color I like… all too blue or too yellow for my taste.)
Your math is correct, but 68ma on a function output is pushing it. Most decoders are 100ma max. Light bulbs also have an inrush current when turning on - the resistor should keep it within decoder limits, but LEDs are FAR more efficient and will never burn out and need to be replaced.
Even though I am a diehard advocate of using LEDs I have to agree. The colours aren’t quite right. Trying to tint them can be an exercise in frustration too. If you apply Tamiya’s Clear Yellow to the lens in front of a bright white LED the lens immediately turns yellow (Duh - what else would you expect) but when you turn the LED on it has an obviously green cast to it. Can’t win! I have had some success with painting bright white LEDs with beige paint to get the colour I wanted but you lose a lot of light when you do that. That may not be a bad thing in some cases.
I have found some LEDs to be much better than others, but it’s pretty much hit and miss when you order the LEDs regardless of how they are described.
You know, Randy, this is a really good point that I hadn’t thought about. The decoder manual talks about limiting current draw. I’m going to see if I can exchange the 3V bulbs for 1.5s that draw less amperage. Same vendor has 40mA bulbs, and I can also buy them locally. Fingers crossed on the former (and if not, anyone want to buy some bulbs, cheap?
I did the math on 1.5V 40 mA bulbs – 800 ohm resistor, if memory serves, and power would be .54W. Can I use a half-watt resistor, or should I go with 1 watt?
At 0.918 watts, I would use a larger than 1 watt resistor, especially in an enclosed space like a locomotive shell; however, 0.918 is incorrect. You use the voltage drop across the resistor - not the total voltage, so if you are dropping the voltage 10.5 volts to run the lamp at 3 volts, the power dissipated by the resistor would be 10.5 x 0.068 = 0.714 watts.
I would find other bulbs if you must use incandescent - you can get 1.5V 15ma bulbs. And probably even lower current.
If the claculated power is .54 watts, you need more than a half watt resistor, a half watt will get very hot and burn up running at over a half watt. And herein is also why I don;t use light bulbs any more - between the resistor and a bulb, they get quite warm. Warm enough to melt the plastic around the bulb. In DC days it wasn;t a huge deal - the locos uses 12 or 14V bulbs, and you didn;t run around the layout at full throttle much anway, so the bulbs were running on maybe 8 volts, which kept the cool. Enter DCC, the function output is full voltage at all times, it doesn;t vary with speed. So whether your loco is idling in the yard or pulling a fast passenger train, the bulb gets the full voltage all the time.
I’ve been very happy witht he Miniatronics Yelo-Glo LEDs for first gen power. The case is clear, so there isn;t a big orange or yellow blob showing through the headlight lens when the lights are off like some ‘golden white’ LEDs. The cheap Chinese ones are fine if you don;t care too much about color - at like $1 for 100 LEDs it’s hard to beat, considering the Miniatronics are many times the cost - but you do tend to get what you pay for. If modeling modern diesels, there are ones listed as “sunny white” that are a less yellow but still not blue. The ones with a bluish tint are useful only for modeling fluorescent lights in buildings and factories, and HID headlights on modern cars.
Thx Randy (and everyone who chimed in). The vendor says he will happily exchange my 3V bulbs for 1.5V/15mA. I am going to try a couple of the Miniatronics LEDs and see if I like 'em.
Now I hope they still make them the same way, it’s been a few years since I last bought some. But the ones I have all have a clear case with a small yellowish-orange square right on the LED die inside. In some locos, like by Bowser Baldwin switches, the headlight lens/light pipe shows the whole LED, but the effect is like there is a small something in the very middle where the bulb would be in the prototype’s headlight assembly. Just don’t look too close because it’s square not round. But it sure beats the stock LEDs they came with, which were rather bright orange when off but a fairly bright white when on. Off, that bright orange filled the lens space and didn’t look right.
When coloring the white LEDs with eg Tamiya clear acrylics, you need to mix colors - white LEDs, especially the ones that look bluish, are usually actually a UV LED with phosphors inside to glow white. So if you do just one color, say yellow, you will end up with unpleasant side effects like the greenish tint. I haven’t tried it myself, but those cheap Chinese LEDs are hard to resist and if I cna make them look like the Miniatronics ones it might be worth it. Most times that directions for tinting LEDs have been proivided, it’s a mix of yellow and orange Tamiya paints.
I remember dipping GOW bulbs in various color paint to make colored lights - after a few years the heat would crack the paint off.
Just a thought, but you could try a RGB LEDs and mix your own hue.
A simple rig with three potentiometers, each wired to the colour pin would let you interactively adjust the colour to your liking. After that, just measure the resistance of each pot, and replace with a (near-by) fixed value resistor.
Only problem with that is unless you have them on “glaring” brightness, you can see the individual red, green, and blue chips inside. It projects a nice shade of light, but looking at the LEDs, it’s very obvious - I have a 3 meter reel of RGB 5050 LEDs and the light it casts can be made all sorts of pleasing (and obnoxious) shades but if you look at the strip, the individual dots in each LED stick out like a sore thumb.
Now for signals - RR CirKits has a really nice LED for modern searchlight signals that has the proper shade of green and yellow. Not that that helps for loco headlights, but these were effectively custom made - order enough and you could get something else custom made as well.
You could add a bit of a thin acrylic panel used in area lighting (I bet most model railroad basements have the lights with those panels anyway) infront of the LED to diffuse the three individual colours.
Oh, by the way many thanks for the heads-up there. I went to RR-CirKits website and the signal looked very impressive, especially that they did a custom LED run and all.
So, afterwards, through serendipity, I went and googled RGY LED and an eBay auction came up among top hits, with one lot left of 100 SMD RGY LEDs (that to me look identical to what RR-CirKits offers) so I grabbed it. [<:o)]
