# Resistor value for a smoke unit?



## San Juan (Jan 3, 2008)

I'm not sure if this is the correct section for this question, but since I'm using a battery/RC Airwire G2 board hopefully this is.

I'd like to install an LGB 5 volt smoke unit into a steam loco I'm working on. I already have a spare 5 volt smoke unit, and these are expensive and a little hard to find (either in 5, 18, or 24 volt varieties) so this is what I'd like to use.

The wiring hook up is as follows:

Battery and Airwire G2 board in the tender. 4 wires going to the loco. 2 for the motor, and 2 for the headlight. I've spliced into the headlight wires and use it as the power supply for lights and hopefully for the smoke unit, so lets call it the "headlight wire". This headlight wire is controlled via function 0. So if function 0 is on, the headlight is on. The headlight is an LED on a resistor (680 ohm 1 Watt). There are also two class lights that use the same headlight wires but are interrupted by an on/off switch, so they don't have to be on if the headlight is on (they too use 680 ohm, 1 Watt resistors). There is another on/off switch available for the smoke unit, so it too will not always have to be on if the headlight is on. I have not installed a resistor on this on/off switch yet.


The output voltage, measured with a volt-meter, (battery-Airwire G2 Board-headlight wires) that I will hook the smoke unit up to is roughly 16 volts. So I need to bring this down obviously for a 5 volt smoke unit.



What is bothering me is what resistor to use for the smoke unit. Due to what I assume will be a much higher amp/milliamp draw then the LEDs, I'm wondering how high I have to go on the resistor watts? And then what value I have to use to bring the 16 volts down to 5 volts. 




Does anyone know what voltage the LGB 5 volt smoke unit should run at? 5 volts? Higher? Lower? And at the desired voltage, how many amps/milliamps does it draw?

Is there a good online resistor calculator you might know of that could help me figure out what resistor to use for the smoke unit?


By the way, I installed the LEDs this weekend and they look great. 3mm Golden Whites. I also found a perfect flashlight reflector for the Trackside Details headlight I'm using. I'll have to post some pictures.


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## K27_463 (Jan 2, 2008)

In general, this is not a good idea. The lighting outputs of the g2 are not designed for smoke, and any smoke gen will easily exceed the current design limit of the lighting outputs of the g2. The g2 has 2 outputs specifically designed for higher current loads, they are labeled A and B function outputs, and they are there for a reason. In addition, the 5 volt smokers from the older Lgb stuff actually draw more current beacause good old Ohms law says voltage times current equals power which is what you want to make heat. So, if low voltage, then more current. The LGB 5 v smokers actually work best at right around 8-9 volts. At design 5v they produce poor results. Overdriving them works, but will shorten life somewhat. Best solution is to use a smoke gen that is actually rated for your working battery voltage, which is what the Airwire function outputs will produce. Smoke output will be better, and higher voltage will draw less current. The old Aristo SD 45 unit is ok, the MTH ones and others, all will run at 15-18v and produce reasonable to good output. A resistor in line to the smoke unit will not produce the voltage limit you are looking for, you need a regulator. A resistor will simply burn energy . 
A smoke unit driven by a 9 volt three leg regulator will load the regulator enough that the reg (Lm 7809 or =) will go into shutdown with out a heat sink of its own. Things get complicated from here. 
jonathan/EMW


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## San Juan (Jan 3, 2008)

Thanks for the input Jonathan.

I wasn't sure if this was a good idea or not. I'm not really all that much of a fan of smoke units outside anyway (never looks right). Thanks for the heads up. This makes my hook up a lot easier now...I'm done


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## Dan Pierce (Jan 2, 2008)

Seuthe has smoke generators in a variety of voltages. 

You could find one matched to the battery voltage and just turn it on via a relay as a relay has a low current draw but large contact current rating for a smoke unit. 

Here is a reference to the voltages and current ratings of many seuthe generators. 

http://www.modelrailcraft.com/Articles.asp?ID=133


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