So I have nine 30mA, 1.5V bulbs that are all wired in parallel to two different buildings. I wanted to get a single resistor hooked up before a switch so that I could connect this to the DC power pack. I calculated that I would need a 47 ohm, 3.375 Watt resistor. I found a 45 ohm 5 watt resistor, but the problem is that little *** gets hot as hell, and will actually melt plastic. I needed this resistor because I needed to lower the voltage from the power pack voltage of 14 volts to the voltage that the lights see, which is 1.5 volts. As everyone knows, the voltage drop across a parallel circuit is the same, so I needed to add a resistor before the parallel "ladder system" circuit.
Is there a way that I can drop the voltage from the power pack's 14 volts to 5, 3, or even 1.5 volt without a resistor? I don't want that thing to get hot and burn down my layout!
I would go out and pick up a string of Christmas LED lights and use them with your resistors, the LED's last a longer time and generate no heat and you can get a string of 50 for around $5.00 at the discount stores, just cut the string apart and you have the lights.
By dropping the voltage across a resistor, you are putting 90% of the power into the resistor. Not a great plan!
If you really want to use that supply you could put groups or ten in series, of course then if one in the group goes out, they all go out.
A voltage regulator circuit would be one choice. Do some Googling if you want to go that way. An even better choice might be to look around the house for an old "wall wart" that puts out 4.5V or so, and put the lights in series in groups of three. You might even be able to find a 1.5V unit if you look, perhaps at Radio Shack.
Jeff But it's a dry heat!
I think that putting them in series is your easiest answer, just add one more to the string. 14 volts / 1.5 volts per light = 9.3 lights. 10 lights gives you 1.4 volts per light, they will still be bright enough and will last a little longer. However, as Jeff pointed out before, if one light burns out, all will go out. But if you run them in parallel with a resistor, when one burns out, the voltage will increase on the remaining lights, causing them to burn out faster, which can avalanche and burn them all out if you don't keep them replaced.
As an alternative to building a voltage regulator circuit, consider buying another DC power pack at a swap meet and just adjust it to 1.5 volts. It will be easier than building a circuit, and probably cheaper if you can find a bargain on an old transistor throttle. Jeff's wall wart suggestion is a good solution, too.
- James
With this option, if I cut the Christmas lights, can I hook them directly up to my DC power pack with the resistors? That is good for the future, but that does not help the situation where I already have the lights hooked up on my layout, and I need something to drop the voltage (and heat) of the resistors. I could go the option of hooking up resistors to each light, but I have already soldered the lights together, and that would be a big pain in the rear-end to take them apart and then solder in resistors again.
Better to use a resistor for each lamp. As you have it wired when a lamp burns out the voltage will increase on the other lamps and reduce their life.,
Vail and Southwestern RRBy dropping the voltage across a resistor, you are putting 90% of the power into the resistor. Not a great plan! If you really want to use that supply you could put groups or ten in series, of course then if one in the group goes out, they all go out.A voltage regulator circuit would be one choice. Do some Googling if you want to go that way. An even better choice might be to look around the house for an old "wall wart" that puts out 4.5V or so, and put the lights in series in groups of three. You might even be able to find a 1.5V unit if you look, perhaps at Radio Shack.
Jeff and I think alike. The first thing that came to my mind was a regulator circuit, since they're cheap and easy to build.
But then it would just be the regulator dissipating all that power instead of the resistor, and depending on what package and heatsink you chose, you might just burn that up.
The best option is either higher voltage bulbs, or a lower voltage supply. You have too much of a mis-match for it to be practical.
Steve
The light bulbs are not an option for this particular setup right now. I already have the lights installed. I'd like to consolidate the power supplies too, as I already have two powerpacks for separate trains. I'd like to avoid a third power pack.
I might trade in the two powerpacks for just one with dual controls, but that is in the future some time. It looks like the only option I have is wiring a resistor in with each light. Ughhh. Looks like I have to soldering to do tonight.
I might split the buildings into two equivalent loops in parallel with each other. I would need one resistor with an R value of 33.3 ohms, 0.75W, and another with an R value of 41.67 ohms, 0.6W. I'm going to stop by radioshack in about 15 minutes to see if they have resistors like that. How hot would they get? If I can't find those resistors, I will have to use the 470 ohm, 1/2 watt resistors that I already have.
Really, dropping the voltage for light bulbs through resistors is not a great plan. There is going to be heat. I think I already said it, but you are dissipating almost 10 times the power in the resistor as in all of the lights put together! And that power turns to heat. That's why it gets hot. A lower voltage supply is the best solution, a cheap wall wart, or one you already have laying around the house doing nothing would be a much better solution.
I vote for finding a low-voltage wall-wart, too. Even if the best you've got is a 4.5 volt, that's still a lot less than the 14-volt system you're using, so it will produce a lot less heat.
Next time you do an order from an electronics place, see what they've got in low-voltage supplies. If you're lucky enough to have one nearby, pay them a visit. I'm not talking about Radio $hack, by the way. We have a place called You-Do-It Electronics here outside of Boston - lots of fun to just browse through the aisles. You can get these things online, too, of course, but actually finding a place with bins of #22 wire spools and LEDs is kind of fun. Rarer than an LHS, actually, but a great asset if you've got one not to far away.
It takes an iron man to play with a toy iron horse.
Vail and Southwestern RRA lower voltage supply is the best solution
I agree.
Actually I would suggest you run your lights and accessories from a separate supply and leave power packs for trains only.
Jeff Wimberly had a tutorial on converting an old computer supply for hobby use. I've always liked that idea since old computer power supplies are cheap and most can deliver 3.3V, 5V and 12V right out of the box. All that's needed is a resistor or lamp load to keep the supply turned on. There are a number of sites that detail how to do it too. Google is your friend.
When you are sizing power resistors, use one that is at least twice the wattage needed. Fully loaded power resistors are supposed to run hot.
MisterBeasleyI vote for finding a low-voltage wall-wart, too. Even if the best you've got is a 4.5 volt, that's still a lot less than the 14-volt system you're using, so it will produce a lot less heat. Next time you do an order from an electronics place, see what they've got in low-voltage supplies. ... If you're lucky enough to have one nearby, pay them a visit. I'm not talking about Radio $hack, by the way. We have a place called You-Do-It Electronics here outside of Boston - lots of fun to just browse through the aisles.
Next time you do an order from an electronics place, see what they've got in low-voltage supplies. ... If you're lucky enough to have one nearby, pay them a visit. I'm not talking about Radio $hack, by the way. We have a place called You-Do-It Electronics here outside of Boston - lots of fun to just browse through the aisles.
In the Chicago area, there is store chain called "Frye's Electronics." Much more stuff than Radio Rot. They have a warehouse store almost as big as a "Home Depot" and about 1/6 is parts. The rest is computer and home theater stuff.
Miniatronics has a 1.5 volt supply.
http://www.miniatronics.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=8
Here is a circuit using a LM317 voltage regulator. Many modelers use this IC for different voltages.
A few components. Feed the regulator with 12 to 14 volts DC. I think the LM313 can handle up to about 35 volts DC at the input. For different voltages out, vary the resistor between the ADJ terminal and ground. The regulator is self controlling. To much current being drawn and it simply shuts down. You can get the parts at Radio Shack.
http://home.comcast.net/~mjmx/lights.htm
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.
Thanks for the tips.
I had a linear voltage regulator on the lights, plugged them in, and the lights blew. I was mad. %^*&^%^&% Radio Shack.
I put in some small 12-16V lights under the gas station canopy, and just plugged those into the switch and the power pack. It worked well. Now, when I get cheaper 1.5 lights for N scale cars and buildings, I will also put in resistors in series with these lights. I want to avoid another power pack because this is a small traveling layout (4' X 2.5') that I bring to and from work sometimes. I guess I forgot to mention that.
gatrhumpyThanks for the tips. I had a linear voltage regulator on the lights, plugged them in, and the lights blew. I was mad. %^*&^%^&% Radio Shack.
What was the voltage out of the regulator before connecting the bulbs? I always check a regulator with a voltmeter before connecting anything.
The package said +5V.
Yeah, that will blow out 1.5 volt bulbs. When I referenced a regulator at Radio Shack, I meant buying the components and building your own 1.5 volt regulator. I got the impression you could work at the component level. Sorry for misleading you.
It sounds like you do not have a multimeter. As long as I have been working with model railroad electrical, I have always used a multimeter.
I had a large enough resistor in series with the equivalent resistors in the two buildings. The lights should not have blown. The resistor dropped the voltage another 3.5 V, so I don't know what happened.
And yes I have a multimeter.
I have replaced those four 1.5V lights under the gas station canopy with three 12V lights. They are wired in series to a switch going directly to the power pack.
I have another building with a front canopy light and a back door light, and those lights are 1.5 V with the appropriate resistor in the building. I wired those lights in parallel with the gas station lights, and now the switch turns off the lights in both buildings.
I ordered 20 12V 2.4mm lights from miniatronics last night, along with the arc welder lights. I'm excited about installing those bad boys. I will put five lights into the truck warehouse that I have, with four over the bay doors.
My next step is to get a bunch of sound modules and place them all around the layout from www.ittproducts.com.
Perfectly workable multimeters can be found at Harbor Freight for $2.99.
A linear regulator like a 7805 5 volt version needs some additional components to work properly. They also need a DC power supply, not AC. There are also 'adjustable' regulators that output various voltages based on what resistors are hooked up with it - these can get down to 1.2 volts which is perfect for 1.5V bulbs.
Theother factor is load - the regulator can only provide so much current. It's related to the overall ability of the regulator to dissipate heat, so the higher input voltage you use, the less output lload it can handle since the excess voiltage is given off as heat. If you're setting up a regulator for 1.2 volts, don;t feed it 12 volts, feed it 5.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Yeah I already have a multimeter. Thanks though! :D
I thought the power pack was DC, not AC.
Here are the specs for the LM317 voltage regulator.
http://www.national.com/mpf/LM/LM317.html
I have done the same with a 7805 since I did not have a LM317 at the time. I now have a few LM317s in case I need to slap a regulator together. With a trim pot, I can also make it a variable regulator.
Many people use the LM317, TO220 case now a days. Bolt it to a piece of aluminum for a heat sink. Normally good for 1.5 amps with a heat sink. It shuts itself down if you draw to much current.
When I use to run DC, I made my own controllers using the LM317 regulator as did many in a club I belong to. We now run DCC.
What's cheaper ? a 1.5 V transformer that can handle you 9 lights, or a burned down layout.
What's less work? Plug in the Transformer, or rebuilt part of the layout.
I guess that's one way to put it.
Maybe I have been doing this too long at 50+ yrs in model railroading but this is an old, simple and relatively cheap solution. Look for a center tap filament transfomer rated at 6.3 V. Tie a 50 ohm rheostat of 5W or so across the center tap and one leg of the 6.3 V side. Be sure you are wiring the 115V across the 115 Vac and not the 6.3 vac or otherwise your output will be about 20x the 115vac. With an output now of approximately 3 v your rheostat has a lot less to drop. You don't get into all of the problems of series wiring ... especially in buildings. And, it is a whole lot less complcated and probably cheaper than building a voltage regulator. Be sure you use good wiring practices on the 115 vac connection. Both of these are probably available in surplus shops or somewhere on-line. Beauty of this, is you can dim the voltage below 1.5 v and extend the life of your bulbs. Since it is ac, you won't want to do this with LEDs. If you can find an old selenium bridge rectifier, the rheostat will be dropping even less on this circuit and the output will be dc. Or, you could use a solid state bridge rectifier.
Something that I have not seen anyone mention. When I am playing with 1.5V light bulbs, I calculate so that I really run them at 1.2V supply to them. They last a whole lot longer. Run 12V lamps at 11.5V, 3V lamps at 2.5V etc.
I will often also put two Zenier diodes in series with each other and in parallel with the lamp in order to regulate the voltage (four diodes if the power source is AC). When used with lamps in series, this also keeps one burnt out lamp from turning off the whole series.
I like the little resistors that are in a metal case that can be mounted to a heat sink.
I've got many a caboose with two 1.5V marker lamps, a bridge rectifier (four diodes), and a metal resistor mounted to the steel plate car weight. The heat gets distributed through the whole length of the caboose instead of melting down the plastic roof.
Those are good thoughts. Once I get the buildings wired up and some more scenery done, I will start to concentrate on rolling stock and installing FREDs.