resistors

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dk-espee

Member
What size resistor should I use for 1.5volt, 1.2mm incandescant bulbs? These are going in digitrax equipped locos for ditch lights. Also, if I will be running two lights from the same power source, do I need to use two resistors or will one work?
 
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grumpybob

Lake Shore Lines
Normally I use a 1k 1/4watt resistor for each bulb. I have not done one for digitrax in quite awhile and didn't know whether their boards already had resistors built in. Would suggest you review the digitrax manuals for more info on them.

Bob
 

B_Kosanda

Member
I start out with 1 K Ohm and work down to a lower value using a voltmeter across the bulb. I usually try to shoot for just under 1.5 volts across the bulb when the loco is moving. Seems like different decoders require slightly different resistor values.

Bill
 

B_Kosanda

Member
The voltmeter is the only real way to make sure you don't over-voltage those bulbs and cause yourself some early bulb replacement work.
 

RexHea

RAIL BENDER
Try this link and click on "ohms for light bulbs.";)
http://www.walkersquawker.net/
I meant to add that some of the sizes in the chart may be hard to find if at all. Always go to the next larger size that you can buy. Also, if you're not sure of the current draw for the bulb, you can usually look it up at one of the online electronic supply stores where they list the specs.;) And as Bill said, double check the lamp supply voltage out of the decoder with a meter before you do anything.
 
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Maxitrains

Member
Hi, I hope I'm not missing anything from the above, but I would like to know if a Resistor is enough to reduce voltage from 14.5 to 6 volts. My main concern is that the resistor might heat up after a long time working and might blow and maybe cause some fire on the scenery ( I know very few about electronics ).

So if a simple resistor is ok to reduce voltage, what type and size do I need?
1/4 Watt or 1/2 watt, and is 10K alot?

Thanks
 

GAPPLEG

SP modeler
I would have concerns about dropping that much voltage inside a plastic shell.I personally would move to LED's on any thing like this, current draw is so minimal and heat also.
 

Maxitrains

Member
My problem is that I am using 12 - 14 V bulbs inside buildings, but there are certain buildings which I have to add lampposts and hanging street lamps, and since they will be attached to the same building that already has lights in, I thought to connect the 6.5 Volts lamps to the wiring of the 12 - 14 Volts bulbs, but again, since the small lamps are almost half the voltage I need some reduction in voltage, and I cannot use other kind of bulbs or LEDs. The lamps I'm using have a buld approx 1.5mm dia. with 4 mm lenght, these are the smallest bulbs (lights ) I came across, so I have to stick with them.

My only chance is either find a solution through electronics, which has to be something simple that fits in small houses, or I have to start double wiring every building I have for 12 - 14V and 6.5V, which willt ake me double the time and material.
 

GAPPLEG

SP modeler
Ok , gotcha, didn't know your application. I built small voltage regulators for my lighting circuits. Ic an feed voltage in and by using a trimmer pot on the regulator I can get an output to match the particular bulb I'm feeding. I too ran into lighting situations where I have different types of bulbs. Kits like these are available:
http://www.rainbowkits.com/kits/ebps-5p.html
I more or less designed my own.
I used LM317 voltage regulators from radio shack , I think they even showed a circuit on the package. I input 16v and then adjust down to what I need.
 
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Maxitrains

Member
But such circuits does not always fit into buildings, that is why I wanted to know if a resistor would have done the job, to reduce the voltage from 14.5V to light up a single 6.5V lightbulb. The query was for a single lightbulb, when I will light up a whole street of 6.5V lightbulbs I will make a circuit or else use a 6.5V PSU directly. I could have taken a strand from teh 6.5V PSU directly to these houses with that single bulb, but then there will be too much wiring under the table.

Can you get my point, or am I confusing you with the way I'm explaining?

Thanks :)
 

GAPPLEG

SP modeler
No problem , I understand completely , if you built the circuit i'm talking about, you could tap off the other line 14.5 ? and place the circuit under that table and it would even be available for future use. My is built on a 1.5 x 1.5 inch piece of phenolic board. Maybe too much trouble for you, but I've built 5 of these that I use to power odd ball stuff.
 

Maxitrains

Member
Well if I can have the diagram of how it can be built on a veroboard ( stripboard ), it will be no problem for me to make similar ones, I can manage with the soldering iron, the only think I cannnot manage is to convert a diagram onto a board or invent something in electronics :S.

I am already building speed pots for differnt layout sections, even though I have to check them all again, because some of them work well and other no :S

Anyway, this would be another step forward in electronic building :)

Thanks
 

RexHea

RAIL BENDER
Maxitrains:

Yes you can drop any voltage you want with a resistor in series, but the size in resistance is dependant on the amount of current in the circuit (lamp current) and the amount of voltage. The wattage is also dependant on these factors.

Example: 6 volt Lamp@ 50ma, amount of voltage drop@resistor_8.5 volts.
R is RESISTANCE
E is Voltage
I is Current
P is Power in Watts

R= E/I
R= 8.5/0.050
R=170 ohms

Pr= (I^2)R
Pr= 0.050x0.050x170
Pr=0.425 or 1/2 watt minimum, (Engineering "rule of thumb"=double minimum for safety)=1 watt

The key is knowing the amount of current needed by the lamp. Then just substitute the values.
 
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NickB

Wannabe Engineer
Ahh Rex ya beat me to it :D Yea I think your way is the least component complicated, I just have an old habit of going to a voltage divider don't ask me why because I honestly couldn't tell ya :)
 

Maxitrains

Member
So if I understood your calculation, with every 6.5V bulb I have to put a 1Watt 170 Ohms resistor to drop the voltage from 14.5 to 6.5

is that right :)?
 

RexHea

RAIL BENDER
Nick: Danged ole College Boy!:D :D

Maxi: That resistor size, both resistance and wattage, is completely dependant on the current that the lamp requires. You need to look this up. You can put that value back on this thread and we can figure for you if you want. Yes, each lamp has to have a resistor which is not a big expense or work.
If you tried to hook up more bulbs in the circuit your total current would require a huge wattage for the resistor or a more elaborate design. If you want to keep it simple, just buy a 6 volt power supply at Radio Shack and parallel the bulbs. I supply my layout with one of their 12 volt models.
 
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Maxitrains

Member
yeah I understand, but the problem now is how to know the wattage consumption of these bulbs, I bought these ready made and only know the voltage, then had no package or anything that indicates teh wattage, the only thing I can tell you is that they are very small bulbs.

As for hooking more lamps to one fuse, I'm not going to do that, I only need this resistor for only one of these lamps at a time. For more of these lamps that will be connected together I will have a 6 volts PSU that will have wiring directly to multiple lamps ( eventually street pole lamps )
 




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