LED amp draw

[hon30critter]

Hi everyone!

This is perhaps a dumb question about how much current various sizes of LEDs draw.

I know that a typical 0603 LED draws somewhere around 20 - 30 milliamps depending on colour (or at least I think that is the case - I could be wrong).

I am installing much larger 5030 warm white LEDs behind the numberboards in a Bachmann Spectrum FM 16- 44 using a Tsunami-AT1000 decoder. I am using the larger LEDs to try to get more even light distribution in the number boards.

Do the 5030 LEDs draw more current? I bought the things on eBay so of course they didn't come with any specifications at all. My concern is that I don't want to overload the decoder's lighting outputs.

As always, your sage advice is greatly appreciated.

Thanks

Dave

[modelmaker51]

Typically, LEDs draw bewteen 10 and 50 miliamps, so unless you're hanging half a dozen or so on each function, you don't have to worry about current overload. If your function output is 12Volts, just put a 1000 ohm resistor on one leg of each LED and you'll be good to go.

[Mark R.]

No danger of overloading the decoder's output rating, but you might want a much higher value resistor than 1000 ohms for your numberboards. Don't think you'd want them as bright as your headlights !

Try some different values between 5000 and 10,000 ohms for your numberboard LEDs. You want them to be visibly on, but not glaringly bright.

 

Mark.

[hon30critter]

Thanks Mark and Jay.

I thought I would be ok but I wanted confirmation in case I let the smoke out of the decoder.

Mark:

The Spectrum number boards are not done very well at all. They are simply a clear molding with a black outline printed on it to outline the letters. When I put a 0603 LED behind them I got a lot of light in the center of the board but little on the outside two numbers. The lettering quality and method made the number boards virtually unreadable with a single 0603 LED behind them.

In order to improve the appearance of the number boards what I have done is to first hollow out the back of the number boards with a dremel to make enough room to install a small piece of .010 white styrene behind the numbers. I then glued 5030 SMD LEDs to the back of each board, and I replaced the original printed numbers with Microscale decals with a white background and black overlay. The numbers stand out far better than the originals, and the larger LEDs with the styrene give much more even light across the numbers, although it is not perfect. The styrene blocks enough of the light that I don't need to tone down the brightness of the 5030 LEDs.

Thanks for your help!

Dave

 

[rrinker]

 Key thing to remember is that when an LED is listed at say 25ma, that is the MAXIMUM it can handle. LEDs work different than light bulbs, there is no set current that they will draw - they will keep drawing increasing current up to the limit of the power supply, until either the LED literally pops (on the 5mm size, you can hear it - and sometimes it blows a chunk of the case off too) or the power supply is supply as much current as it can. That's why you need dropping resistors or a current limiter circuit with LEDs, so that they get no more than a suitable amount of current.

 It's like a light bulb and voltage - you can keep feeding more and more voltage to a 12V lamp until you either hit the max voltage of the power supply, or the filament burns out.

 If the LED is rated around 25-30ma, configuring the circuit to around half of that is usually fine. For white LEDs used for headlights, they are typically rated at no more than 25ma, with a 1K resistor they get just over 9ma and are still plenty bright - plus there's a huge cushion should something go wrong and double the track voltage, it won't be the LED that blow up.

                    --Randy

[BroadwayLion]

LEDs do not draw current. They pass current. Think of them as rectifiers. You will loose about 1.2 volts for each rectifier that you pass through and the same with the LEDs. But LEDs cannot handle much current, thus the resistor.

Below a certain voltage they will not give light. More than that and they will light up. If too much current passes through them they will, as said, explode (or simply die with a flash).

It is a new way of thinking of things. You can put thousands of them on a circuit (Think Christmas Lamps strung together all passing through a 3 amp fuse) and it will work just fine.

[richg1998]

Most of the LED's I have used since the 1970s are 20 ma max. There are other values in catalogs but I have not seen them in model railroad use. I generally run my LED's at 50 percent, 10 ma.

Ten years ago I bought some green, 3mm LED's rated at 2.0MA for battery operated test equipment.

Today there are some super bright LED's showing up and you need to increase the resistance until you are happy with the results.

Rich

[gandydancer19]

BroadwayLion

LEDs do not draw current. They pass current. Think of them as rectifiers. You will loose about 1.2 volts for each rectifier that you pass through and the same with the LEDs. But LEDs cannot handle much current, thus the resistor.

Below a certain voltage they will not give light. More than that and they will light up. If too much current passes through them they will, as said, explode (or simply die with a flash).

It is a new way of thinking of things. You can put thousands of them on a circuit (Think Christmas Lamps strung together all passing through a 3 amp fuse) and it will work just fine.

Now that is an interesting way to look at an LED.  I hadn't thought of them in that way, and you are absolutely correct.  Basically they are creating a short circuit, so you have to use a resistor to keep them from shorting out the power.  Thanks Lion.

[Texas Zepher]

gandydancer19

Now that is an interesting way to look at an LED.

???  That is the only way to look at an LED.   It is a diode that just happens to produce light as a side effect.