Led lights


Yes, if the input voltage is higher than the LED requires.
Where are you going to use them?
LEDS will not work on DCC track voltage I don't think.
 
Do all led's need resistors whether dc or dcc?
Yes always. An LED above all things is still a diode. As soon as it starts conducting electricity it becomes like a short circuit and will pass as much as is available. So if there are 8 amps it will try to let 8 amps get through and burn itself out in the process. Therefore a resistor is needed to limit the amount of current it can pass. Diodes are creatures of current.
 
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Many decoders are setup so they don't need resistors. Just follow the instructions.
If using a power supply that pushes a higher voltage and than the led can handle, you need a resistor.
 
LEDs & DCC

LEDS will not work on DCC track voltage I don't think.

Leds will work on the DCC tracks if you build a full wave bridge with a filter cap and the correct load resistor for the current limiting requirements of the led. I have a circuit that I found on this forum that will let you use leds for rolling stock and other applications like the red lamp on the back of a car. I will try to find the circuit and add it to my next post.
Doc'
 
LEDs work fine on DCC track power with the appropriate resistor. I've had a red and green LED wired across the rail gaps of a reversing section to indicate whether the "polarity" matches or not. It's still doing its job after more than fifteen years, even if they DO say running an LED on AC will shorten its life. Even if its shortened by half, it should still be good for what ? - 50,000 hours !

Mark.
 
Many decoders are setup so they don't need resistors. Just follow the instructions..
That is true, but it could be misleading. They don't need "additional" resistors because there is already a resistor built into the decoder.
 
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From what I know, LED's only require a resistor if you intend to run them on a higher voltage than they were meant for. It used to be that almost all LED's operated on 5 volts, which is used for most electronics circuits, and layout accessories voltage was 12 volts or higher, and thus you need a resistor to keep from frying the LED. Nowadays there are LED's in other voltages, 12 v being more and more common. The other case where a resistor would be needed is if you wanted to dim the LED while running it on it's intended voltage.

I've not seen an actual 12V LED. Not saying they don't exist, but I think what you're probably seeing is these 12V "ready" LEDs that actually have a resistor built into them. I have some myself and looking at them physically, they don't actually look any different from any other LED. However, they do have the resistor built into them so that you can hook them up to 12V without having to add an external resistor. Probably nit-picking a bit to make that qualification, but there it is for what it's worth :)
 
LED's only care about current, not voltage. They will work on 5v DC or 120v AC no problem, you just need the proper current limiting resistor.
LED's will work just fine on DCC track voltage with the proper current limiting resistor, You don't need to rectify the AC to DC they will do that on their own, it is a Light Emitting DIODE

For the OP, YES ALL LED's require a resistor.
 
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12 volt LED's do in fact exist, click here to see some.

A lot of the indoor lighting LED's are 12 volt, too.

I've been told that the 12V do all actually have integrated resistance. That was my point. I know that "12V" LEDs exist, but from what I'm told they've been manufactured with that built-in resistance. Probably shouldn't have even brought it up. In the end it's a minor point of interest rather than anything that has practical applications. However it's accomplished, "12V" LEDs do indeed work on 12V without any additional resistance applied to the circuit. That's all anybody really cares about & I should have just left it at that instead of being all confusing about it :)

Also, I may have been told wrong, but the source is somebody I tend to think knows what's up. Doesn't mean he knew about the very latest thing, though, and maybe was a bit behind on what the latest & greatest was.

BTW, if you didn't know? All the 12V LEDs came about because auto & truck mfrs. started making LED fixtures for their vehicles. Until then there was very little point or market in making LEDs for specifically 12V. I'd be willing to bet that LEDs with integrated resistors could have been made for decades now if there'd just been a market for it.
 
12 volt LED's do in fact exist, click here to see some.

A lot of the indoor lighting LED's are 12 volt, too.
That is cool but that is also marketing. The "12Volt LED" is an LED encased with a resistor who's value will limit the current properly at 12V. The resistor is in series with the LED. You can even see it in some of those photos on that web site. That tiny black dot inside the plastic is a resistor. Electronically speaking LEDs don't care about forward voltage - just current. On the other hand diodes do care about backward voltage. That is, there is a limit to how much voltage they can block when the polarity is backwards. That would be a rated voltage not an operating voltage.

It is sort of like the marketing for "batteries". All those "D", "C", "AA", and "AAA" things we purchase under the marketing name of battery aren't. They are "cells". A battery is technically composed of 2 or more cells, like an automobile battery.

A light bulb gets bright and dim by changing the voltage (which in turn changes the current).
An LED gets bright and dim by changing the current (which can be a consequence of changes in voltage).
These two things seem to be the same thing but understanding the difference is essential to understanding electronics.

I never thought of MikeOwnby's comment that this was for the automotive industry, but it makes sense. Why else would that 12V "value" suddenly come on the market. Certainly not for model railroaders. If they made them for model railroaders they would be in 17.5V, 16V, 15V, 14.5V, and 13.5V umm umm encasements? packages? varieties? to match the possible output voltages of the DCC, TCS, and other systems.

Must be late, I am rambling about a topic that has been covered 82 zillion times......
 
Do all led's need resistors whether dc or dcc?

The answer is yes and no. It would depend on the LED you are buying.

LED are diodes, basically a 2 lead semi conductor with a cathode (- negative) and anode (= positive) LEDs or diodes provide no resistance in one polarity once the anode is more positive then the cathode (the amount of voltage depends on the construction of the LED) and theoretically infinite resistance to the opposite polarity. Placing a stand alone LED or diode in the correct polarity across a sufficient power supply will create a short circuit and a theoretically open circuit if polarity is reversed.

Stand alone LED are still available at electronics stores and must be used with a resistor. These are used for building your own circuits.

LEDs designed for specific applications and rated for current flow at a specific voltage could have internal resistance built into them may not require a resistor. The cathode and anode can have resistance built into them, in that case the resistor is not visible, but will show up on a schematic. Read the specifications and instructions for the LED you are interested in.

If you need an LED for a specific application I would start another thread with the specific application you need it for. I hope you are still with us.
 
It might be noted that the milliamp draw of the led tends to be the most important. The resistor in the circuit is for limiting the ma current to keep the led from burning out. As long as you know the working voltage and the manufactures spec for the led's max current draw you can calculate the resistor R=E/I, in addition one should figure the wattage for the resistor so as not to burn it out, P=I*E where E is the voltage drop across the resistor. Again the working voltage must be know to figure the values using ohms law for DC. You can also change the light emitted by the led by increasing the value of the resistor to get a lower current thru the led which will reduce the emitted light. I have attached a circuit that I found on this forum that is used for stand alone LED applications using DCC. Such as lighting rolling stock or a caboose. As DCC is pulse width it appears as AC to the led. This circuit will rectify the voltage for use by the LED. The components can be found at Jameco.com.
 

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Those parts can also be acquired at Radio Shack. I'd also recommend using a voltage regulator to keep the LED brightness consistent and provide more protection.
 
Those parts can also be acquired at Radio Shack. I'd also recommend using a voltage regulator to keep the LED brightness consistent and provide more protection.
As designed this circuit can be used for DC or DCC. If it is DCC the current going to the LEDs should be constant. The capacitor would be to eliminate flicker. I do find it interesting how they drew it. As Stoker pointed out they have one set in parallel and the other all alone. Not that it matters but it is strange. Personally I always use one resistor per LED. I guess that doing it this way does ensure a common brightness for the interior LEDs. Also having the interior lighting "wires" cross the rooftop marker "wires" twice could be very confusing.
 
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A basic knowledge of math, electronics, ohms law, and manufactures spec of the leds you want to use plus the ability to use a DVM will resolve all the issues with the art work. The circuit is for concept and shows the options for both parallel and series uses of leds. I was happy to find it on this forum and have used it for both DCC and DC applications with good results. It even works with my older AC controlled S gauge American Flyer although a voltage regulator can be of help for stable voltage to the leds. Sorry for the imperfections.
 



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