General > General Discussion
Minatronics 1.5V Incandescent Bulbs
Alan:
Speaking of testing, you may want to wire up a bulb with one of your 1/2 watt resistors to see how hot the resistor gets. Technically, you are within its operating range but being so close to its upper limit I suspect it will get very warm. Try it and see. If it is not hot enough to melt plastic then you can use them. You could also use a higher value resistor so it passes less current. All depends on how bright you want the bulbs to be.
It is important to remember a resistor is a heater. Your toaster toasts bread using resistor wire. Little carbon resistors are the same. They are so small and create so little heat we tend not to think of them as heaters but they are.
Case in point. Switch mode power supplies are notorious for having poor voltage regulation when there is no or very little electrical load on them. I ran into this problem head on with my railroad. There are some electronic gizmos that run on track power yet they draw only a very trivial amount of current when in stand by. I noticed the power supply voltage drifting when there are no locomotives on the track to create an electrical load. This gave me concern about my gizmos possibly being damaged by over voltage. To remedy the situation I placed a dummy load on the rails so the power supply is always loaded.
I did the math, installed the resistors, and checked the system voltage. Nice and steady. Job well done. Until I touched the resistors. Ouch, they were very hot. Not hot enough to damage them, they were operating within their power rating, but hot enough to concern me. There is 2" insulation foam scenery base mere inches above them.
Long story short, today I have forced air cooled heat sink mounted resistance loading on my train track and a lesson well learned - never underestimate the heat a resistor can generate.
TwinStar:
Amazon and me get along very well! Here is what I typically purchase when I need these guys since Radio Shack in gone.
https://www.amazon.com/MonkeyJack-Pieces-Resistors-Assortment-Active/dp/B077JW144B/ref=sr_1_8?ie=UTF8&qid=1535719888&sr=8-8&keywords=470+ohm+1+watt+resistor
A pack of 50 for less than $4 may not be the best deal but I don't know that I'd ever need more. I'll look for the 870 ohm as well.
I will most certainly test them as I've learned that lesson before the hard way. Burns hurt.
Alan:
I'm not anti-Amazon. I too buy a lot from Amazon. Just making you are aware in case you didn't know there are some things that Amazon is not a good deal, with electrical components often being one of those things.
A 1 watt resistor typically sells for 4 to 5 cents so your Amazon find at 7 cents is not grossly out of line. Although, a $4 buy isn't going to break the bank regardless of what you purchase. I tend to be more sensitive to the pricing because for commonly used components like resistors I usually buy in quantities of 1000. That's the industry sweet spot where volume pricing starts kicking in and it means I never have to buy that particular component again. But then I tend to use a lot more than the typical model railroader.
Good luck with the lights.
Alan:
Jacob, here is a thought for you.
The problem with using a series resistor to control the brightness of a bulb, other than needing hefty resistors, is the bulb brightness will vary with voltage. If the voltage on the rails varies for whatever reason so too will your headlight brightness. There is a simple solution that will provide uniform bulb brightness regardless of voltage and uses small 1/4 watt components.
LM317 is a basic, no frills, adjustable voltage regulator in a tiny package. Using the resistor values shown your bulb will always see 1.5v so long as the track voltage is 3.7v or higher. LM modules shut off below 10v so it is safe to say the bulb will always be the same brightness under all conditions. If you were to construct this using surface mount style components it could be made into a very small package.
Just a thought.
TwinStar:
Alan:
I assume I'll need 3 of these? One for the mars, one for the emergency stop, and one for the headlight pair?
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