LED and resistor?

[Keusink]

Hello all

I put some digitrax DH1650AO decoders into some locos I bought. They run fine, but the lights don't. The digitrax instructions say that the board output for lights is 15mA, with an option for 30 mA which you get when you scratch out a circuit.

I chose 30, with the intent of putting in icandescents. Then I changed my mind and bought T1 LEDS. The max capacity of the LEDs is 20 mA. I assume that is why I can't get the lights to work. What resistor should I use to lower the mA's to less than 20?
I recall that the LED's are 3 V, 20 mA capacity

Digitrax support is not getting back to me.

BTW, after 3 years advice from Jim Banner, Rich, Old Timer, and many others, our layout is finally finished! Couldn't have done it without you guys. With no LHS, this site has been the source of most of my knowledge and present ability. Thank you all.

Chris Keusink

[Brewman]

Okay, after reading the instructions myself I think you have a few options. If you are good at soldering, you could solder a piece of wire across the two pads where you cut the trace. You could also CAREFULLY scrape the solder mask off were you cut the trace and put a bit of solder to reconnect the two halves. If you are not comfortable with either of those options, adding a resistor will be the only way to go for the LEDs. You could start with a 680ohm resistor 1/4 watt and see how the brightness is, put the resistor on the negative leg of the LED. Make sure the LED polarity is correct to the connection on the board, the negative side looks like the "flag" in the LED. If you already ran these LEDs at too high of a current, they may be burned up. Hope that helps and sorry if I told you things you already know.

[jward]

measure the voltage output of the decoder connections for the lights. let us know what they are.

keep in mind the current rating of the led is a max rating. you can and should operate the led on less than 20 ma.

[utdave]

 ive done some LEDS  like xmas lights  on a string  i cut off like 3 for a building  they run in series    i put a 10k pot to them  on my dc train transformer   from tyco our bachmann    i dont use the the ac voltage .    i have the pot set high 10k ohms   and lower it until the lights light up (lower the resistance and max voltage on transformer  i find where they start getting bright and stop.    i check the ohms of my pot with a harbor frieght volt-ohm meter  and find proper resisters to match close  .                 why i like the the dc transformers is that i can change the brightness on my buildings .  and can run a whole bunch  with little cost .   i want to share this if someone had a hard time to come up with the right resisters for there LEDs.     Dave

[Doneldon]

ive done some LEDS  like xmas lights  on a string  i cut off like 3 for a building  they run in series    i put a 10k pot to them  on my dc train transformer   from tyco our bachmann    i dont use the the ac voltage .    i have the pot set high 10k ohms   and lower it until the lights light up (lower the resistance and max voltage on transformer  i find where they start getting bright and stop.    i check the ohms of my pot with a harbor frieght volt-ohm meter  and find proper resisters to match close  .                 why i like the the dc transformers is that i can change the brightness on my buildings .  and can run a whole bunch  with little cost .   i want to share this if someone had a hard time to come up with the right resisters for there LEDs.     Dave

dave-

I hadn't thought of doing this. Thanks for a great idea.

                                                                        -- D

[richg]

Most LED's operate quite well at 10 ma. A 1k, 1/4 watt resistor is used by many modelers.
Some of the newer super bright LED's use a higher resistance. I don't use the super bright LED's so a Google search is appropriate if you need more info.
Below are measurements I made with a decoder and the NCE Power Caba few years ago.

Measured with 12.2 VDC supply using 20 ma max current LED's.

Resistor      Current
1000 ohm    9.0ma

750 ohm     12.45ma

680 ohm     13.12ma

510 ohm     16.25ma

I have seen this range of resistors used in many DCC forums. Most seem to use 1k.

Rich

[jward]

one thing that should be pointed out is that the actual resistance of the led varies widely with the current.

using rich's numbers to illustrate, given a typical led voltage of 2v.

with 1k ohm resistor: led resistance is 222 ohms.
with 750 ohm----- 160 ohms
with 680 ohm----- 152 ohms
with 510 ohm----- 123 ohms

[Keusink]

Thank you gents. That should get me where I need to go.

Chris

[richg]

Calculating is not important. Get a 5 k pot, clip leads, 12 vdc power, multimeter  and check the current.
I use a 2.5 k pot. And set it at the max resistance, first. This way you can compare brightness of the LED which in certain circumstances you might like.

I have been using LED's since 1972 and have never, ever had to do any calculations.

You can but LED testers right off of ebay for a few dollars. They come right from China. A number of DCC modelers in some Yahoo DCC groups have bought those testers.
Anyone working at what I call the component level should have all these tools.

Rich

[rondon47]

also be aware that the LEDs might be wired backward.  They will only work one way, unlike incandescents.

[richg]

More than more than you will ever need about LED's.

http://tinyurl.com/lkffnus

Store the links you need.

Rich