# What Resistor do I use?



## jimtyp (Jan 2, 2008)

I just got a harbor models smoke unit. It requires 12v and uses up to 2amps. I thought I would try running it through a bridge rectifier off of track power since I use DCC. I hooked up a bridge rectifier to the rails and with a volt meter see 20.5 volts output from the bridge rectifier. So I assume I need something that can reduce the voltage to around 12v. I used an online calculator and it says use a 4.7 ohm resistor capable of at least 18.8 watts. This seems to be hard to find. I saw Radio Shack has an 8-Ohm Non-Inductive Resistor rated at 20w 
Could I use two of these to get 4-ohm 20w resistor? 

Is there a better way to power the 12v smoke unit?


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## Therios (Sep 28, 2008)

i think you would do better to get a dc to dc voltage regulator.


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## Greg Elmassian (Jan 3, 2008)

Actually what you are thinking is a great way, just mount the resistors since you WILL be dissipating 20 watts of heat continuously. 

I had the same situation in a sweeper car. The problem is that you would have a **** of a time getting a regulator and heat sink combination to shed that heat. 

Now, remember also, resistors are RARELY made to run at full wattage, they will get ridiculously hot... rule of thumb run at 1/2 rated wattage. 

So, 2 resistors in parallel will change from 8 ohms to 4 ohms and 20 watts to 40 watts, and you need around 20. 

Works... 

(For those of you in doubt about the logic, look up the wattage capability of a 3 amp regulator)... 

20.5 - 12 = 8.5 volts times 2 amps = 17 watts... but the voltage is probably a bit higher, put a small electrolytic cap on the rectifier output and measure again. 

Put the motor on 12 volts and measure the actual current draw, don't take the manufacturer's specs for granted, when you are dealing with this much current and heat. 

Regards, Greg


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## krs (Feb 29, 2008)

The problem using a resistor to drop the voltage (besides the power dissipation) is that the voltage drop through the resistor varies with current. 
So if the load current is pretty constant and predictable - a resistor may be an option. 

Otherwise I would look for a 12 volt 2 to 3 amp three-terminal *switching* regulator with an efficiency in the 85 to 90% range. 
They are not that easy to find but they do exist and the price is reasonable as well. 

Knut


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## Greg Elmassian (Jan 3, 2008)

Yep, pretty constant and predictable with the smoke unit... meets that requirement. 

Yep, switching regulator would be nice, lower heat, but bigger, more costly.... ya gets what ya pay for! 

Here's one here (Scroll down to the 3 amp one)... *http://www.endurance-rc....trong>** not sure about efficiency though, looking at the size of that heat sink, something really efficient should have a pretty small heat sink... but it IS 3 amps after all.


I did the quick and dirty on my sweeper... in a loco, maybe it's a lot better idea than finding a way to deal with the heat. 

Depends on space inside.. 

Greg*


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## Dan Pierce (Jan 2, 2008)

LM350 with a heat sink will do the job. 3 amp adjustable regulator. If you need more amps, LM338 for 5 amps. 

USA TRAINS uses the LM317 a 1 amp adjustable regulator in their engines for smoke units and lights.


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## Greg Elmassian (Jan 3, 2008)

You better consider the T0-3 package not the T0-220, and need some ventilation and a LARGE heat sink. 

I think this is impractical due to the heat sink size... or maybe you could fan cool it... 

Get a higher efficiency regulator... 

again, I mentioned that you have to dissipate 20 watts, this is a lot more than you think in terms of heat output, and you are most likely putting it in a steam loco. 

Been there and done that... 

Greg


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## jimtyp (Jan 2, 2008)

Thanks for the help gents. I appreciate it. 

What about this unit, says it can handle 3amps and 36watts:  switching regulator


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## Greg Elmassian (Jan 3, 2008)

It says input voltage range is 17-35 volts... sounds fine... 

Cheap enough... 

Get one and let us know... note it ships from Hong Kong, and note it might not be super high quality. 

I'd say buy 2. 

Greg


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