Richmond surface mount LED
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The decoder only has a limit of 100 mA? What kind of decoder is it and for what scale? Is the output actually 1000 mA? Most decoders are at least one amp (1000mA). A 100 mA decoder would be dangerously small, even for Z scale. Even if you believe the LED's are 3.5 volts, you still need to know the output from the lighting teminals of the decoder, You can measure the voltage output with a multimeter when the engine has power. The power output on some decoders can be adjustable using a jumper or can be fixed at anywhere from 1.5 volts to 12 volts. If it's 1.5 volts and your are using 3.5 volt LED's, the LED's won't light. Very few LED's have a voltage rating of 3.5 volts. Given the fact they are rated at 20 mA, I suspect the 1.5 volts is closer to accurate.
ncmrdispatcher
A&O Historical Society
Short answer:
A 1.5K ohm 1/4 or 1/8 watt resistor per LED produces an intense light with a lot of safety margin in virtually every situation. With larger LEDS (3mm diameter and above) a 1K 1/4 watt can be considered a universal solution.
Long answer:
An LED wants to be driven by a current, not a fixed voltage. This is opposite of a normal light bulb that wants to be driven by a fixed voltage.
Nominal voltage output for a DCC decoder, with about 14 volts at the track, would be a steady 12 to 13 volts. The stated 100 mA would be the maximum rated current load for the decoder function. It will work properly if it provides less current.
Most white LEDs have a voltage drop of about 2.8 to 3.4 volts. Most small LEDs, when attached to PC board copper traces for a heat sink, are rated at a maximum of 20 mA current, but a few are rated at only 10 mA. In a ditch light, you don't want to go close to that as the LED won't be properly cooled.
Calculation:
Voltage drop across resistor / resistor in ohms = current through resistor and LED.
(13 volts decoder output - 3 volts LED drop) / 1.5 K ohms = 6.7 mA.
Unlike a regular light bulb, the resistor value for an LED is not at all critical to getting good brightness. We just need to stay well below the LED's maximum current rating. There is no danger using a larger value resistor other than somewhat reduced light output.
Hope this proves helpful. Al the best.
A 1.5K ohm 1/4 or 1/8 watt resistor per LED produces an intense light with a lot of safety margin in virtually every situation. With larger LEDS (3mm diameter and above) a 1K 1/4 watt can be considered a universal solution.
Long answer:
An LED wants to be driven by a current, not a fixed voltage. This is opposite of a normal light bulb that wants to be driven by a fixed voltage.
Nominal voltage output for a DCC decoder, with about 14 volts at the track, would be a steady 12 to 13 volts. The stated 100 mA would be the maximum rated current load for the decoder function. It will work properly if it provides less current.
Most white LEDs have a voltage drop of about 2.8 to 3.4 volts. Most small LEDs, when attached to PC board copper traces for a heat sink, are rated at a maximum of 20 mA current, but a few are rated at only 10 mA. In a ditch light, you don't want to go close to that as the LED won't be properly cooled.
Calculation:
Voltage drop across resistor / resistor in ohms = current through resistor and LED.
(13 volts decoder output - 3 volts LED drop) / 1.5 K ohms = 6.7 mA.
Unlike a regular light bulb, the resistor value for an LED is not at all critical to getting good brightness. We just need to stay well below the LED's maximum current rating. There is no danger using a larger value resistor other than somewhat reduced light output.
Hope this proves helpful. Al the best.
Bob, that's good information but it still doesn't answer the question of what the output voltage is at the light terminals of the decoder. If it's 12 volts and the LED's are 1.5 volts, then your recommendation of the 1k ohm, 1/4 watt resistor is probably good enough. If the output is 1.5 volts, either a much smaller or no resistor would be needed, depending on the current output.
I'm also a little unclear on your statement about the 100mA output for the decoder. If you're talking about the rated output just for the lights, that makes sense, but the decoder itself should be rated for at least one amp to handle all the motor control funtions and the lighting.
I'm also a little unclear on your statement about the 100mA output for the decoder. If you're talking about the rated output just for the lights, that makes sense, but the decoder itself should be rated for at least one amp to handle all the motor control funtions and the lighting.
ncmrdispatcher
A&O Historical Society
Jim -
I'm sorry my answer did not seem entirely clear.
Short answers:
The Tsunami provides between 12 and 13 volts at a function output, which can vary a bit depending on the particular DCC system you use.
The usual (99% of the time) output voltage of any decoder lighting function is 12-13 volts. I have heard of only one or two decoders that had any provision to regulate an output at 1.5 volts, and the Tsunami is definitely not one of them.
A decoder has one maximum current rating for the motor (may be 1 amp or more, sometimes less for N and Z scales) and a different rating for an lighting function. The motor is driven by four large transistors that can provide a lot of current because it is the motor that needs a lot of current.
The lighting functions are driven by tiny transistors, normally rated at maximum of 100 mA. Most useful model locomotive light bulbs and all practical LEDs require far less than this so a tiny transistor can serve.
Long answers:
A decoder starts with a full-wave bridge rectifier to change the alternating-polarity DCC track power to a steady DC voltage. A small capacitor on the output of the bridge helps keep the voltage steady through dirty track. The NMRA DCC standard for HO and O scale calls for 14.5 volts nominally at the track. This cannot be measured accurately with an ordinary voltmeter as they are not designed to measure square waves (just smoothly-changing sinusoidal power line AC.)
The diode bridge drops about 1.2 volts off the 14.5, so the internal power supply runs at about 14.5 - 1.2 = 13.3 volts. The lighting function common (blue wire) connects to the +13.3 volts of the decoder power supply. The decoder lighting functions wires connect to individual NPN transistors which can, when turned on, pull the function output to within about 0.5 volts of ground. Therefore, the function output provides about 13.3 - 0.5 = 12.8 volts.
The function wire is negative relative to the blue wire, so we connect the cathode (either marked on the package or longer lead) to the function output.
If the concepts of voltage and current are confusing, perhaps a plumbing analogy may be helpful. Voltage is like water pressure, current like gallons per minute, and resistance like the diameter of a pipe. Small diameter pipes restrict, or resist, the flow of water just as a resistor restricts current. A small diameter pipe is like a large value resistor. More water pressure can force water to flow more quickly through a small diameter pipe just as more voltage forces greater current through a given value of resistor.
I've installed a lot of decoders in HO and O, often with a dozen or more LEDs (because you can in O-scale) including Soundtraxx, and have been a practicing electrical engineer for over 30 years, so I'm not just making this up.
Bottom line:
Just pick up some 1.5K resistors for these tiny LEDs and have fun. Be sure to insulate all wires before putting the shell on the locomotive, so that there will be no unfortunate surprise short circuits and consequential damage to the decoder.
If you are not highly skilled in soldering, and do not have a very tiny tip soldering iron that is temperature controlled, get the Richmond Controls LEDs with wires already attached. They are easy to destroy by heat, and by static electricity so subtle you can't feel it. Sneeze and you will never find a bare 0603 package LED again!
I sincerely hope that this clears up any confusion. If not, let's continue the discussion!
I'm sorry my answer did not seem entirely clear.
Short answers:
The Tsunami provides between 12 and 13 volts at a function output, which can vary a bit depending on the particular DCC system you use.
The usual (99% of the time) output voltage of any decoder lighting function is 12-13 volts. I have heard of only one or two decoders that had any provision to regulate an output at 1.5 volts, and the Tsunami is definitely not one of them.
A decoder has one maximum current rating for the motor (may be 1 amp or more, sometimes less for N and Z scales) and a different rating for an lighting function. The motor is driven by four large transistors that can provide a lot of current because it is the motor that needs a lot of current.
The lighting functions are driven by tiny transistors, normally rated at maximum of 100 mA. Most useful model locomotive light bulbs and all practical LEDs require far less than this so a tiny transistor can serve.
Long answers:
A decoder starts with a full-wave bridge rectifier to change the alternating-polarity DCC track power to a steady DC voltage. A small capacitor on the output of the bridge helps keep the voltage steady through dirty track. The NMRA DCC standard for HO and O scale calls for 14.5 volts nominally at the track. This cannot be measured accurately with an ordinary voltmeter as they are not designed to measure square waves (just smoothly-changing sinusoidal power line AC.)
The diode bridge drops about 1.2 volts off the 14.5, so the internal power supply runs at about 14.5 - 1.2 = 13.3 volts. The lighting function common (blue wire) connects to the +13.3 volts of the decoder power supply. The decoder lighting functions wires connect to individual NPN transistors which can, when turned on, pull the function output to within about 0.5 volts of ground. Therefore, the function output provides about 13.3 - 0.5 = 12.8 volts.
The function wire is negative relative to the blue wire, so we connect the cathode (either marked on the package or longer lead) to the function output.
If the concepts of voltage and current are confusing, perhaps a plumbing analogy may be helpful. Voltage is like water pressure, current like gallons per minute, and resistance like the diameter of a pipe. Small diameter pipes restrict, or resist, the flow of water just as a resistor restricts current. A small diameter pipe is like a large value resistor. More water pressure can force water to flow more quickly through a small diameter pipe just as more voltage forces greater current through a given value of resistor.
I've installed a lot of decoders in HO and O, often with a dozen or more LEDs (because you can in O-scale) including Soundtraxx, and have been a practicing electrical engineer for over 30 years, so I'm not just making this up.
Bottom line:
Just pick up some 1.5K resistors for these tiny LEDs and have fun. Be sure to insulate all wires before putting the shell on the locomotive, so that there will be no unfortunate surprise short circuits and consequential damage to the decoder.
If you are not highly skilled in soldering, and do not have a very tiny tip soldering iron that is temperature controlled, get the Richmond Controls LEDs with wires already attached. They are easy to destroy by heat, and by static electricity so subtle you can't feel it. Sneeze and you will never find a bare 0603 package LED again!
I sincerely hope that this clears up any confusion. If not, let's continue the discussion!
Last edited by a moderator:
tanks for the infos Bob, Il will try the 1.5K and 1000 ohms and pick the one wich will look the best, ill post pics soon.
I thougt I made clear it was for a locomotive decoder function output ( a tsunami sound decoder). Obviously it wasnt clear enought.
An other power related question. If i hook up an 12V 6 amp power supply to a digitrax Super empire builer kit, will the world ends or will it be fine ?eh ? ( i run in HO)
I thougt I made clear it was for a locomotive decoder function output ( a tsunami sound decoder). Obviously it wasnt clear enought.
An other power related question. If i hook up an 12V 6 amp power supply to a digitrax Super empire builer kit, will the world ends or will it be fine ?eh ? ( i run in HO)
Thanks for the clarification, Bob, I understand what you were saying now. For some reason, I didn't click on the fact this was a Tsunami board, so I was the one confusing the situation. As you say, the light output from the Tsunami is a nominal 12 volts so it''s just a matter of knowing if you're dealing with 12 volt micro bulbs or 1.5 volt micro bulbs or LED's. I usually use a 1.5K, 1/4 watt resistor and bench test the LED or bulb to make sure it's still bright enough. Having a larger resistor cuts down a lot on the heat problems caused by resistors that more closely match the formula you gave.
Julien, any power supply that delivers the maximum amps your system requires will work fine. The Super Empire Builder is rated at 5 amps so a six amp power supply will work without any magic smoke.
.
Julien, any power supply that delivers the maximum amps your system requires will work fine. The Super Empire Builder is rated at 5 amps so a six amp power supply will work without any magic smoke.
.
ncmrdispatcher
A&O Historical Society
Most DCC systems I've seen require about a 14 to 16 volt AC transformer as does my NCE Powerhouse Pro. The requirements will be spelled-out in the owner's manual. A 12 volt transformer will lower the track voltage a bit; whether that is a problem is for you to decide. A 12 volt DC supply will certainly drop the track voltage too much.
For safety it is quite important to use a power source that provides adequate current to the DCC system, so that the built-in track short circuit detection can function correctly. NCE mentions this in the Powerhouse Pro manual.
I have witnessed smoke rising from a derailed locomotive on two different layouts when the DCC system failed to detect a short circuit. These were both large layouts that have been featured in Model Railroader magazine. In both cases an operator responded quickly to the situation, but you can imagine what could happen if the layout was left unattended.
For safety it is quite important to use a power source that provides adequate current to the DCC system, so that the built-in track short circuit detection can function correctly. NCE mentions this in the Powerhouse Pro manual.
I have witnessed smoke rising from a derailed locomotive on two different layouts when the DCC system failed to detect a short circuit. These were both large layouts that have been featured in Model Railroader magazine. In both cases an operator responded quickly to the situation, but you can imagine what could happen if the layout was left unattended.
Whoops, bad reading on my part again. 
Yes you do need a 14-16 volt AC power source for your DCC controller. A 12 volt DC source is not a good idea. If, for some reason, you don't have the DB150 power supply that should ahve come with the Super Empire Builder, you can get the appropriate power supply at http://www.modeltrainstuff.com/ProductDetails.asp?ProductCode=DIG-PS515.
Yes you do need a 14-16 volt AC power source for your DCC controller. A 12 volt DC source is not a good idea. If, for some reason, you don't have the DB150 power supply that should ahve come with the Super Empire Builder, you can get the appropriate power supply at http://www.modeltrainstuff.com/ProductDetails.asp?ProductCode=DIG-PS515.
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