I normally use 14 volt lamps or LEDs to light my structures but I have a few places where I want to model outside building wall lamps similar to but much smaller than Walthers wall mounted lights part # 933-1094 (sorry I haven't learned how to post a link yet - a topic for another thread). My problem is the Walthers lamps are way too big for what I want. I want to use Miniatronics 1.2mm 1.5v 15mA bulbs with very small shades on them. My question is, what are my options for powering the bulbs. I had one structure working with power from a train set power pack with a 1/2 watt 10 ohm resistor feeding 4 bulbs in parallel. It worked fine during testing but when I left it on for a while, the resistor turned brown and the bulbs fried. Guess it was the wrong set-up (duh!). Please understand that the detailed formulas for volts/amps/watts/whatever draw a blank when I try to understand them. Your suggestions are very much appreciated.
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
The simplest solution is to use a Miniatronics 1.5 volt power pack.
If you want to build yoru own power pack.
http://home.cogeco.ca/~rpaisley4/PSupply.html
A voltage regulator you can hook to a 12 volt power supply. This way you do not have to work with the 120 vac that powers the supply circuit above.
http://www.bakatronics.com/shop/item.aspx?itemid=407
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
Rich: That was way too easy! I didn't even know they existed. Also, the bulbs have a life expectancy of only 500 hours. Could I use a resistor to lower the voltage a bit to extend their life?
Thanks for the suggestion
You could use their lamps w/shades rated at 12V.
http://www.miniatronics.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=M&Product_Code=72-105-05&Category_Code=9_3&Product_Count=5
I have yet to look at Rich's link, but some voltage regulators are variable voltage output. Their output is controlled by the resistor value.
the 1.5V bulbs have about 50 Ohms each resistance.
the 12V bulbs have about 400 Ohms each resistance
For four 1.5V lamps, you should have hooked them in series with a 200 ohm resistor with plenty of headroom for a 30ma needed to light the bulbs. The only downside is if one burns out, they all burn out.
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
Just lowering the voltage slightly, to 1.2 volts, will GREATLY extend the life of the bulbs. And they'll look better too. About 1.2 volts is the absolute minimum you cna get out of most adjustable voltage regulators, too, so it's quite convenient. You cna hook quite a fewe 15ma bulbs to a regualtor, especially if you mount the regulator to a heat sink. The large 'hat' shaped ones, with heat sink, are generally good to 1.5 amps - and will shut themselves off if overheated, cheap circuit protection. 1.5 amps is 100 15ma bulbs - stick to no more than 75 bulbs and it should be good forever.
WIth multiple lamps in parallel, each should have its own resistor. The current through a resistor in series with a group of lamps in parallel is the sum of the lamp currents. The drop in the resistor is based on the current flowing through it. If one of those bulbs in parallel burns out,t he current goes down as does the voltage drop - resulting in the remaining bulbs getting too many volts and burning out quickly in succession.
4x 15ma bulbs = 60ma. V=current x resistence, so .060a x 100 ohms = 6 volt drop in the resistor. With a 14V power supply you were putting 8 volts into the lamps - no wonder they burned out.
If you want to use resistors, and run the bulbs at 1.2 volts from a 14V power supply, the resistor will have to drop 14-1.2 volts, ot 12.8 volts. Current will be approximately 15ma. R=V/I or 12.8/.015 = 853 ohms. For standard 10% tolerance resistors, you'll have to try an 820 ohm with a 100 ohm in series, or just try a 1K and see if they are bright enough - with a 1K the voltage will be less than 1.2V and the life will be even greater.
Power (watt) rating for the resistor is current x volts. .015 x 12.8, .192 watt, so a 1/4 watt or 1/2 watt resistor will work fine. Use a 1/2 watt for best practices. In your first try, the resistor power should have been .36 watt, so a 1/2 watt resistor should have been fine if a bit warm - sure it wasn't a 1/4 watt resistor? That would have burned up pretty quick.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
You might try a light dimmer that you can plug the power pack into.
The resistor trick can work if you understand the voltage/resistance/current/power formula. Some get into trouble if they use a wrong resistor. Bulbs do not light or burn out, sometimes burning out the resistor.
When you add or subtract bulbs or a bulb burns out, stuff can happen to the rest of the bulbs.
I would use the below voltage regulator circuit and set the resistor to give 1.4 volts. The voltage will stay the same if you add or remove bu;lbs or bulbs burn out.
In the below link is a voltage regulator diagram and calculator. Put 33 ohms in the resistor box and calculate. You will see 1.42 volts if you make the circuit and use the 33 ohm, 1/4 watt resistor. The parts are at Radio Shack. Attach the LM317 to a piece of aluminum for a heat sink. I do this for different voltages at out local MRR club.
The LM317 can handle up to about 36 volts DC input I believe.
http://www.electronics-lab.com/articles/LM317/
Store the link for future reference or to give to others.
Dave,
I have my exterior lights wired "in parallel" (vs. "in series") and hooked to the DC terminal of a MRC Railpower 1370 power pack:
I like this arrangement because this allows me the advantage of "dialing down" the voltage via the knob.
I typically operate my lighting at 50-55% power, which makes them last a lot longer. AC, on the other hand, is on full - all the time - so it is not affected by the rotation of the knob. Therefore, you must use a resistor to reduce the voltage going to bulbs.
Another advantage of wiring "in parallel" is that you then have the capability to control (e.g. turning off & on) lights individually with switches. Lights wired "in series" in a string can only be controlled as a string of lights.
The downside to this is if someone mindlessly turns up the power on the power pack, you run the risk of blowing out all your lights - i.e. if your light bulbs are rated lower than the total DC output of the power pack. However, you could always create some sort of physical stop for the knob to keep this from happening. A sign works, too, but...you know how some folks respond when they see a sign that says "Wet paint".
For lighting needs where size is important for realism (generally exterior lighting), I use 1.2mm - 1.5V Miniatonic incandescent bulbs. For interior lighting, I use the slightly larger 1.7mm bulbs because they are 12V. Actually, if you're willing to compromise a little, Dave, the 1.7mm incandescents look pretty decent as exterior lights...AND...they're rated a 1,000 hrs instead of 500 hours.
The one thing that I've found with the Miniatronics 1.2mm incandescents that I don't like is that the lighting from bulb to bulb is inconsistent. Some bulbs diffuse the light beam evenly; others concentrate the light in a single beam - e.g. like an LED. If I use 1.2mm incandescents for lighting, I test them out first to make sure they diffuse the light evenly.
Hope that helps, Dave...
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.
Rich, Randy, DigitalGriffin, Tom
Thanks for your very helpful suggestions. The light is actually starting to come on (sorry - I couldn't resist the pun)! I also noticed in Rob Paisley's web site that I shouldn't be relying on an old train set power supply to test these circuits so I will have to invest in something more reliable. If anyone has a suggestion as to where I can find a reliable power supply or what kind to use I would apprciate the info. I have used Ron's circuits in an attempt to build power supplies for solenoid switch motors and I couldn't get them to work properly. I know now that it was likely the power pack that was at fault. I'm off to try again! Hopefully I won't let the smoke out of any more bulbs.
DigitalGriffin
I have used Miniatronics 12 volt bulbs and their brass lamp shades on other structures and if the building is large enough they look fine. However in this case I want to put outside lights on a very small engine shed (just big enough for one small industrial switcher - the one you see in my avatar) and on a few small buildings in a very small mining camp. The first photo shows the engine house that the lamps will be used in (complete with the first set of burned out bulbs!). The second photo shows the relative sizes of the wall lamps. The one on the left is a Walthers built up. The middle is a Miniatronics 12 volt bulb with their brass lamp shade and the one on the right is a Miniatronics 1.5 v, 1.2mm bulb with a plastic lamp shade from a fake lamp set from (I believe) Grandt Line or possibly Tichy. The one on the right is what I am trying to use.
Again, thanks for your help.