I have a 4.5V, 500mA power supply I want to use for some limited lighting in N Scale (small bulbs ok). I have five 1.5V 30mA lights from Miniatronics and I want to use them with the power supply. If I hook one light up I think I'm supposed to add a resistor to reduce the voltage and using (Vpwrs - Vlight)/mA light = Ohms resistor, I get (4.5-1.5)/0.03 = 100 ohms.
I tried this and it worked for a while but then the bulb burned out. How am I supposed to do it and what if I want to use 5 lamps, what value of resistors do I put in and where in the circuit? Thanks,
Eric
I'm kinda likin this stuff
Actually, after fiddling with it, the bulb didn't burn out at all. So that circuit might not be a problem (?).
The 100 ohm resistor will be fine to drop the 3V you need to get down to 1.5V. Make sure you use a separate resistor for each lamp. You can get away with an 1/8W resistor but I'd use a 1/4W.
Engineer Jeff NS Nut Visit my layout at: http://www.thebinks.com/trains/
Do I use a 100ohm resistor for each lamp or for 2 lamps do I use (4.5V- (1.5V + 1.5V))/0.030mA ? I think its the former.
Also, since the PS is 500mA do I divide 500mA/30mA bulbs to get the number of lamps (16-17 lamps). What if I want the the voltage at the lamp to be lower, just put in a bigger resistor? If so, how much bigger?
Thanks,
A few things.
1. The 500 mA (0.5 Amp) rating on the power supply merely tells you the maximum current the supply can furnish before it over heats, gets up set, or does a smoke release. The amount of current it does furnish is controled by the number of lamps. Assuming you do it right, each lamp uses 30 mA, so you can power about 16 lamps before the power supply is over loaded.
2. Your 100 ohm resistor computation is correct, far as the math goes. Look at the lamp. If it is glowing an ordinary yellowish incandecent lamp color, you have the resistor right. If it is glowing very white, it's running hot and the lamp life will be shortened. If you run the lamp on something less that rated voltage it will last much longer. The lamp burnout you experienced might be just a tired lamp, or it might be you need a bigger resistor to drop a bit more voltage. If the lamp is burning much whiter than an ordinary light bulb, it's a sign that the lamp is getting more voltage than it should for decent lamp life.
3. You can hook up the 1.5 volt bulbs in series. For instance 3 1.5 volt bulbs wired in series gives you 4.5 volts which is just what your supply furnishes. The down side to series connection is when one bulb burns out, it opens the circuit and all the bulbs go dark, leaving you wondering which bulb needs to be changed.
4. Was it me, I'd get a bunch of 100 ohm resistors, and put one resistor in series with each lamp and go from there. That way, if a bulb fails to light (not uncommon) you just change the dead bulb and get on with it.
David Starr www.newsnorthwoods.blogspot.com
My advice, and it is my advice, is to run your bulbs at well below their rated voltage.
I've got a bunch of 16-volt bulbs that I'm running at 10 volts, and not one has burned out. They give a warm, yellow incandescent glow, which is much nicer than the glaring, bright look of burning them at their rated voltage.
Model railroading is about the image, the impression, after all. By running your bulbs cooler, they last longer, and, IMHO, they look much better.
It takes an iron man to play with a toy iron horse.
Instead of using one resistor for each bulb, why not put a bigger (higher resistance) in series with the power supply output. What you're doing with the resistors is dropping the voltage the bulb sees. It doesn't care if it has its own resistor, or a one larger one dropping the voltage for the whole circuit. And, oh yeah, wire the bulbs in parallel. I use distribution blocks (dbs for this discussion). I connect one wire from the power supply to each db. On one side, I connect a wire to all of the terminals on one side on both dbs. Then, I connect one wire from the bulb to one db, and the other to another db. With 10 terminal dbs, you have enough for 10 lights. I have mine wired so that street lights are on one pair of dbs, upstairs lights on another, and downstairs lights on yet another. This way, I can turn them off and on separately.
And, oh yes, like Mr. Beasley wrote, by all means, run them at below their rated voltage; they will last much longer.
Marlon
See pictures of the Clinton-Golden Valley RR
Medina1128 wrote: Instead of using one resistor for each bulb, why not put a bigger (higher resistance) in series with the power supply output. What you're doing with the resistors is dropping the voltage the bulb sees. It doesn't care if it has its own resistor, or a one larger one dropping the voltage for the whole circuit. And, oh yeah, wire the bulbs in parallel. I use distribution blocks (dbs for this discussion). I connect one wire from the power supply to each db. On one side, I connect a wire to all of the terminals on one side on both dbs. Then, I connect one wire from the bulb to one db, and the other to another db. With 10 terminal dbs, you have enough for 10 lights. I have mine wired so that street lights are on one pair of dbs, upstairs lights on another, and downstairs lights on yet another. This way, I can turn them off and on separately. And, oh yes, like Mr. Beasley wrote, by all means, run them at below their rated voltage; they will last much longer.
The only problem with that approach is that if a couple of bulbs burn out you can lose all ten. The voltage drop is dependent upon the current going through the lit bulbs. If a couple burn out then the voltage drop across the resistor goes down and then goes up across the bulbs. At some point it will be enough to burn out the remaining bulbs. There is no way to tell for sure but trial and error. Even with individual resistors you can still turn as many on/off with a switch. It just depends upon how many bulbs/resistors you wire to the switch output.
As Mr. Beasly said, by running your lamps at lower than rated voltage they will last significantly longer (probably a lifetime for us older modelers). However, the downside is also significantly less light per lamp. For background lighting this may not matter, but if you are lighting building interiors and want to see inside it matters. I run some of my 12 volt lamps at 9volts and the results are less than expected. By going to the rated voltage the appearance of my building interiors was much improved.
What I have done is use a 9 volt and a 12 volt supply controlled by a spdt switch. This way I can use 9 volts for general viewing and switch to 12 volts when I want any visitors to see a nicely lighted city and depot with detailed interiors.
Ron K.
I was using a 100ohm resistor in front of my 1.5V, 30mA bulb and it really did finally burn out, it had a short life. I'm using the 4.5V 500mA PS as above and the resistor was 1/2 watt.
I tried a 220ohm (1/4 watt) resistor with a new 1.5V, 30mA bulb but it was pretty dim. Am now using a 1/4 watt 100ohm resistor with the new bulb. Would the resistor wattage make a difference with how fast it could burnt out?
I have some 1/2 watt 63 and 33 ohm resistors and some 1/4 watt 10 ohm resistors that I was thinking of putting together with my 100 ohm (1/4 watt) resistor to lower the voltage. Will this work well and what, if any, are the effects of mixing resisitors of different wattages?
Thanks for all your replies,
misterconsister wrote: I was using a 100ohm resistor in front of my 1.5V, 30mA bulb and it really did finally burn out, it had a short life. I'm using the 4.5V 500mA PS as above and the resistor was 1/2 watt.I tried a 220ohm (1/4 watt) resistor with a new 1.5V, 30mA bulb but it was pretty dim. Am now using a 1/4 watt 100ohm resistor with the new bulb. Would the resistor wattage make a difference with how fast it could burnt out? I have some 1/2 watt 63 and 33 ohm resistors and some 1/4 watt 10 ohm resistors that I was thinking of putting together with my 100 ohm (1/4 watt) resistor to lower the voltage. Will this work well and what, if any, are the effects of mixing resisitors of different wattages? Thanks for all your replies, Eric
Resistor wattage will make no difference. The only problem you can have with resistor wattage is being too small. In that case the resistor burns up. I am guessing that the problem you have is that the 4.5V supply really isn't 4.5V. It is probably 4.5V under full load and somewhat higher at les than full load. Couple that with the small tolerance you will likely have with the 1.5V bulbs for over voltage and you have this situation. The next standard size resistor is 150 ohm. That may still be too dim but you can try it. You might try putting the 100 ohm and 10 ohm in series with a bulb and see how you like it and the bulb life. If you still have problems try the 100 ohm in series with the 33 ohm.
jbinkley60 wrote: Medina1128 wrote: Instead of using one resistor for each bulb, why not put a bigger (higher resistance) in series with the power supply output. What you're doing with the resistors is dropping the voltage the bulb sees. It doesn't care if it has its own resistor, or a one larger one dropping the voltage for the whole circuit. And, oh yeah, wire the bulbs in parallel. I use distribution blocks (dbs for this discussion). I connect one wire from the power supply to each db. On one side, I connect a wire to all of the terminals on one side on both dbs. Then, I connect one wire from the bulb to one db, and the other to another db. With 10 terminal dbs, you have enough for 10 lights. I have mine wired so that street lights are on one pair of dbs, upstairs lights on another, and downstairs lights on yet another. This way, I can turn them off and on separately. And, oh yes, like Mr. Beasley wrote, by all means, run them at below their rated voltage; they will last much longer.The only problem with that approach is that if a couple of bulbs burn out you can lose all ten. The voltage drop is dependent upon the current going through the lit bulbs. If a couple burn out then the voltage drop across the resistor goes down and then goes up across the bulbs. At some point it will be enough to burn out the remaining bulbs. There is no way to tell for sure but trial and error. Even with individual resistors you can still turn as many on/off with a switch. It just depends upon how many bulbs/resistors you wire to the switch output.
Oops, I forgot to mention the fuse that is wired into the circuit, for just that reason. Calculate the total amperage of the normal circuit, then use a fuse rated slightly above that. IF a bulb goes out, the current will spike, blowing the fuse. Make sure you use a quick trip fuse, not a "slo-blow".