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Wattage of Resistors? Where does that come in to the formula?

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Wattage of Resistors? Where does that come in to the formula?
Posted by Otis on Monday, March 17, 2008 12:16 PM

Where does the wattage of resistors fit into the use of a resistor ?

I will be trying 100 ohm and 220 ohm resistors to get roughly a .1 amp to .05 amp load on the decoder.  That is safe as I understand.  Any lower ohms might be nearing too much load to the decoder.

I have available resistors rated 1/8 watt, 1/4 watt, and 1 watt in that range of ohm values.

But not being an electronics guy, I don't know what the diff the wattage will make.  It doesn't have a place in ohms law that I am using to determine which resistor to use.

Question [?]

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Posted by jfugate on Monday, March 17, 2008 12:40 PM
You will find a very detailed discussion of the formulas here, with examples ... along with some great alternatives to just using resistors for DCC lighting.

Joe Fugate Modeling the 1980s SP Siskiyou Line in southern Oregon

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Posted by Otis on Monday, March 17, 2008 1:41 PM

Thank you for the link.  Now I seem to need to know what voltage I am dealing with.  What voltage do I need to drop from the 13.8 V that the Zephyr controller puts out across the resistor to the decoder?

Anyway, I think both the 1/4 watt and 1/8 watt resistors would work to simulate a motor load on a decoder.

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Posted by 60YOKID on Monday, March 17, 2008 2:08 PM

Wattage is the power rating. In the case of a resistor, the power is dissapated as heat. You need a rating high enough to avoid burning up the resistor from too much heat.  Two simple formula will get you quite a ways in electricity.

Power formula is Voltage times Current = Power in Watts.  ( E X I = W)

And Ohms law is E = I X R 

The decoder puts out DC to the load. If you assume a DC voltage of 12 volts, then for the 100 ohm resistor: 12V = .12A X 100 ohms. And 12v X .12A = 1.44 Watts

Good practice is to use aapproximately a double rating, or 2.88 Watt resistor. Since the next rating commonly available is 5 or 10 watts, I would use three 330-ohm 1 watt resistors in parallel.

The 220 ohm resistor is like this: 12V = .055A X 220 ohms.  12V X .055A = .66 Watts.  Therefore, I would use at least a 1 Watt resistor. 

These wattage ratings are for continuous operation. If you only intend to operate at full load for a few minutes, such as for testing, then you could reduce the wattage.

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Posted by jktrains on Monday, March 17, 2008 4:49 PM

60YOKID,

Your example might be a bit misleading.  Consider that if the OP uses a 12-14v bulb with the decoder than no resistor is required.  Now, becuase the OP is talking about needing to use a resistor, I'll conclude that he's using something other than 12-14v bulb and more like either 1.5v bulb or a LED.  The misleading part of you example is that it doesn't use realistic values for current.  most 1.5v bulbs are not a 1/10th of an amp or 12 hundredths of an amp but are in the range of 20mA or .020A (if my math serves me correctly).  Using your formulas you get around .24 watts or a 1/4 watt resistor.  This corresponds to what most decoder manufacturers recommend in the manuals.  Using your example needing 3 330ohm/1 watt resistors, you'd be hard pressed to fit them inside a loco, especially considering you need that for each light function.

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Posted by 60YOKID on Monday, March 17, 2008 5:53 PM

jktrains,

You are correct if the OP is talking small 1.5V lamps or LED's. Thanks for pointing that out.

However, he mentions 50 to 100 mA which doesn't usually apply to small lamps or LED's, and would therefore require a higher wattage rating. Maybe he has O or G gauge with more room.

If we had a little more info it would be good.

 

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Posted by BlueHillsCPR on Tuesday, March 18, 2008 7:44 AM
Indeed!  Some background information helps put things in perspective and allows the guys trying to help to understand exactly what it is you are trying to accomplish. Smile [:)]
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Posted by Phoebe Vet on Tuesday, March 18, 2008 8:04 AM

60:

If you use a 5 watt resistor on an HO decoder, it will have to be mounted externally.  Whistling [:-^]

Perhaps the G scale assumption is accurate.

Dave

Lackawanna Route of the Phoebe Snow

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Posted by Otis on Tuesday, March 18, 2008 11:41 AM

 BlueHillsCPR wrote:
Indeed!  Some background information helps put things in perspective and allows the guys trying to help to understand exactly what it is you are trying to accomplish. Smile [:)]

Thanks for all the comments.

Oh, oh. I tried to keep the original post short rather than re-explain the project.

The information about the use of the resistor is actually complete.  I should have stated "motor load" perhaps.  Then readers would not be thinking of my using a light bulb or LED.

The stuff below might give you some zzzz's if you have read about it before.  Zzz [zzz].

The resistor is to put a motor load on some Soundtraxx LC decoders used with some other Soundtraxx DSXs and a Soundbug that I have lying around as my experiment in stationary sound.  Some people believe the LCs won't respond as stationary decoders unless a motor load is simulated.

Those that might be wondering why I am doing this....I have more than a dozen LCs and Tsunamis installed in locos and want to add (6) sound sources as stationary sounds hooked through my stereo and subwoofers (as others have done) as an experimental alternative to adding dozens more sound decoders to all the locos waiting in my cabinet.  Six decoders of a mix of EMD, Alco and steam locos will give me some range of ambient, loud, MU'ed loco noise.

The project is taking so long because I am 250 Km from any LHS or electronics shop so have to find and order everything online.  The first few shipments of transformers, switches and LEDs were all wrong Banged Head [banghead].

Now I have everything except the 3 resistors needed for a motor load on the LCs.  The single decoder prototype sound system worked so I am now almost away to the races!Big Smile [:D]

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Posted by BlueHillsCPR on Tuesday, March 18, 2008 3:02 PM

Thumbs Up [tup] That makes it crystal clear to me. Smile [:)]

BTW, Otis...your avatar pic is great! 

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Posted by Otis on Tuesday, March 18, 2008 7:10 PM
 BlueHillsCPR wrote:

BTW, Otis...your avatar pic is great! 

Thanks.

I grew up alongside the tracks as my father was a grain buyer in Alberta.  This shot could have been taken at one of his first job locations, Poe, Alberta, but it is elsewhere.  Might even be from SK.  Poe is now on the list of ghost towns of Alberta (although it never was a town).

I have the lucky chance of driving by once about twenty years ago and snapped a shot of a F series loco pulling a long grain train past the elevator my old man worked in from 1950 to 1953.  I was born just down the road.

Zzz [zzz].  Ok more than anyone wanted to hear. Sign - Off Topic!! [#offtopic]

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