# What resistor to use?



## rrgrassi

I have some Tomar marker lights for my Cab Forward, running on DC power. The set came as a pair, and have 1.5 volt bulbs in them. Should I run them in series and use a 470 ohm like with a 3 volt LED?

Thanks!


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## Patrick1544

I use this formula for bulbs and LED's. 

Supply Voltage - Bulb or LED Voltage / ma (milliamperes) = resistor value. 

I usually wire 2 lighting sources in parallel, this way they both get equal volts and the same luminance. Haven't destroyed any bulbs or LED's with this formula. It's on the money. Hope this helps.


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## rrgrassi

Thanks, man! How do I figure out the MA of the bulbs? The formula makes sense though.

I was thinking in a series as I have a about 4500 SMT 470 ohm resistors. Does the formula give me the resistor value in ohms or watts?


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## Patrick1544

The MA is usually on the package. Most small e.g. 1.5 volt bulbs are usually 20ma. Same goes for most LED"s


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## gunrunnerjohn

You need to verify the current draw for the bulbs, that's probably the first order of business. Do you happen to have a multimeter?


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## rrgrassi

Gunrunner, I do have a mulitmeter and AA batteries.

The Tomar paperwork does not list MA.


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## gunrunnerjohn

Set the meter to measure DC current, and connect it in series with one AA battery and the lamp in question. You should read the current the bulb requires to light if it's a 1.5 volt bulb.


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## rrgrassi

I'll give that a shot!

I used the formula using an LED MA rating of 20 MA. The result is .45 on the calculator. Is that in ohms, volts amps or watts? If it's ohms it makes sense to get a .45, which I translate to a 470 ohm resistor.

Thanks!


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## rzw0wr

R=Volts/Amps

R=(Supply voltage - 1.5)/.02 (ma = amp/1000) 20 ma / 1000


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## gunrunnerjohn

The cold resistance of a lamp has little bearing on it's hot resistance.


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## rrgrassi

My meter does read anything. Has the settings for like 200 MA, but reads nothing. I may have fried that part when I was trying to get an amp reading on my car's a/c blower motor. Meter maxes at 10 amps. Found it to be drawing 26. 

So, I still need an answer to what ohm resistor to use for a 12 volt power supply to light up a 1.5 volt bulb. 

Thanks!


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## Hutch

We need to know the current that the bulb draws. Do you know exactly what kind of bulb it is so you could look it up? If you know the wattage it can be calculated also.


If you replace the fuse in you meter you could easily get this information.


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## rzw0wr

Assuming that it is a 1.5v bulb.
R= 12-1.5/ .025
The bulbs I checked on the internet state .025 amps.

R= 10.5/.025


R= 420 ohm for .025 amps


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## Mark R.

In all honesty, if you are using a variable DC supply, just using a resistor on a 1.5 volt bulb is not the best approach. Bulbs are voltage devices, requiring a specific voltage and don't work well on a variable voltage supply no matter what value resistor you use.

A MUCH better approach is to use a bridge rectifier in series with the motor lead and wire the 1.5 volt bulbs across the rectifier. The rectifier will provide a constant 1.4 volt source to the bulb regardless of the track voltage - as long as it is above 1.5 volts obviously. I use this set-up for all my clients who still prefer bulbs in their DC engines. The current rating of the bulb is a moot point in this installation - we are only concerned with maintaining a steady voltage, the bulb will only draw the current it requires.

Here's the diagram for installation ....

http://www.awrr.com/ConstantLighting.html

Mark.


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## rrgrassi

Wow. Both fuses are blown. I'll have to go get new ones when I head into town again.


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## rrgrassi

I found some spare fuses. I set the meter at 200 MA DC, and got a reading of 58.5 MA when I reversed the battery, I got a negative 58.5 MA.

This is only a temporary thing until I get a Soundtraxx Tsunami installed. 

Mark, Why is it not advisable to use a resistor to limit voltage?

Thanks for the diagram!


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## waltr

Another way to measure current is a Voltmeter, a know value resistor and Ohm's Law.
The Voltage drop across the resistor is related to current with Ohm's Law, I = E/R.
since I have also blown the fuzes in my DVM I now use this method.
Put a resistor (10 Ohm for 10's of mA) is series with the load to be measured. Put the Voltmeter across the resistor. Apply power and read the Voltage. Divide the measured Voltage by the resistance and you have the current.


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## Mark R.

rrgrassi said:


> I found some spare fuses. I set the meter at 200 MA DC, and got a reading of 58.5 MA when I reversed the battery, I got a negative 58.5 MA.
> 
> This is only a temporary thing until I get a Soundtraxx Tsunami installed.
> 
> Mark, Why is it not advisable to use a resistor to limit voltage?
> 
> Thanks for the diagram!


Bulbs are voltage driven devices and LEDs are current driven devices. Diodes drop voltage and resistors drop current (ohms law dictates the voltage difference based on the resistance). You are mixing two types of technologies using resistors with bulbs.

I'm not saying it doesn't work - it works just fine with a fixed voltage like the light output on a decoder. The problem with using DC is that there is no fixed voltage point to reference from. The voltage swing could be anywhere from 0 to 12 volts, and in many cases, higher than that. 

With a resistor, the intensity of your bulb will still vary throughout the entire throttle range - all the resistor is doing is protecting it at the highest throttle setting. It will also require more than 1.5 volts on the track before the bulb will even glow. 

With the diodes, as soon as the throttle reaches 1.5 volts, the bulb is at full brightness and the motor is only seeing 0.1 volts - not enough to even begin to move. So, your lights are at full brightness while your engine is still sitting still. The bulbs will remain at this same brightness throughout the entire throttle range.

So, in a nutshell - it's not a matter of you can't / shouldn't use a resistor .... it's just not the best practice for the best results.

Mark.


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## gunrunnerjohn

For a 1.5V bulb and a 12V input voltage, you need to drop 10.5 volts. You'll need a 180 ohm resistor, and it will dissipate about .6 watts. Given that, you'd probably need a 2 watt resistor to avoid it getting pretty hot, so it'll be of a significant size.

I like the idea of the diode bridge here, it solves the problem neatly and doesn't cause excessive power dissipation. The bridge is probably smaller than the 2W resistor as well.


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## rrgrassi

I guess, the best thing to do is by the LED version of these marker lights.


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## Mark R.

Getting good looking lighting on DC has always been a challenge. Going to LEDs on DC is going to be even more challenging as they require around 3.4 volts DC to operate. More than double the voltage of the 1.5 volt bulbs, so you won't get any illumination until the engine is well under way. Also LEDs are polarity sensitive which means you will automatically get directional lighting, but it takes a bit of work to keep them on in both directions.

You must also install a resistor on each LED which will again raise their on time slightly.

The link I gave you earlier also has a similar set-up for LEDs using the bridge rectifier method, but it really steals a lot of initial voltage from your motor. 

For constant LED lighting, this is a much better approach, which gives you a constant current source - remember, LEDs are current devices ....

http://www.pollensoftware.com/railroad/index.html

Mark.


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## rrgrassi

Mark, This locomotive is my Riv Cab Forward that I re-motored using a Faulhaber 2224 12 volt coreless DC motor. I have converted the head light and added a rear light using Miniatronic Yeloglo LED's using a 470 ohm resistor and a 4001 diode to prevent back fed voltage. I have not tested the amperage draw yet. I can say that the locomotive springs to life on DC when you barely move the dial. For DC power, I use the MRC Tech II 2400 for bench testing and a old copper topped MRC Dual head for running stuff on my portable layout. I do not have my home layout started yet. 

As soon as I save up some money, I'll be adding a Tsunami 1000 decoder made specifically for a Cab Forward. It has the capability to leave the headlight on and turn the rear light on when backing and turning it off when moving forward, like they did in real life.

The links you posted are wonderful reference tools. They are excellent for those staying with DC. I am slowly converting all my DC stuff to DCC. That is why I am reluctant to do all the additional wiring. I got bitten by the DCC and sound bugs.


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## Mark R.

In all honesty, if you are planning to put a decoder in it, it's probably not worth the time and effort to tear it down and modify it for constant lighting on DC at this point, only to tear it down again and install a decoder later as the components you used for DC would be scrapped.

Mark.


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## rrgrassi

Mark, that is my thinking as well.

Thanks for your help!!!


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## tr1

*Interested in purchasing Competant Dcc Control*

DO all or most DCC control stations with a decoder drive an LED to simulate a yellow(LED) as a rotary beacon? 
Regards,
tr1


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## rkenney

*****


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## T-Man

Mark R. said:


> Getting good looking lighting on DC has always been a challenge. Going to LEDs on DC is going to be even more challenging as they require around 3.4 volts DC to operate. More than double the voltage of the 1.5 volt bulbs, so you won't get any illumination until the engine is well under way. Also LEDs are polarity sensitive which means you will automatically get directional lighting, but it takes a bit of work to keep them on in both directions.
> 
> You must also install a resistor on each LED which will again raise their on time slightly.
> 
> The link I gave you earlier also has a similar set-up for LEDs using the bridge rectifier method, but it really steals a lot of initial voltage from your motor.
> 
> For constant LED lighting, this is a much better approach, which gives you a constant current source - remember, LEDs are current devices ....
> 
> http://www.pollensoftware.com/railroad/index.html
> 
> Mark.


Mark I like those links you gave. SO far I have use voltage regulators, they make it work. This is something I have to investigate more. Thanks.


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## gunrunnerjohn

The LM334Z only goes to 10ma maximum, I use the CL-2 constant current regulator, it's a very simple two pin device that just sits in series and regulates to 20 or 25 milliamps, depending on the one you select. You can also simply parallel them for greater current output if needed.


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## T-Man

That explains the difference.


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## tr1

*Red and green signal masts*

I was curious, if anyone here could direct me to a signaling system with two aspects of
red and green LED's. For the most part, I think I'll be able to make my own signal masts. What I think I could use is a schematic, or some type of directions. I plan on using NCE hardware when this project eventually gets of the ground. Thank you for your reply. Regards,tr1
:hah: publications directions,etc.
Thank you!


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