Resistor for bi color LED ??

[superbe]

Greetings Everyone,

Today I added two resistors but not as planned.

 First I added a 1000 ohm to the lead to the red side of the LED as suggested. My plan as previously stated was to add it before the terminal strip and satisfy all of the  LEDs with the one resistor. So I'll never know how that would have turned out.

Secondly, following the suggestion of adding a 100 ohm resistor I added it to the green LED lead wondering all the while why that was suggested. As it turns out it wasn't suggested for the green at all but for the red.

The final result is both the red and the green are lighting properly !!! All's well that ends well.

Mission accomplished and once again you guys and the forum came through.

Happy Railroading and thanks

Bob

 

[rrinker]

Theoretically this is possible, but it is not recommended to use a single resistor for all LEDs for a couple of reasons. First, unless you cn guarantee they are all from the same manufacturing batch, there will be variations so they won't all have the same brightness. Secod, if you calculate the resistor assuming 10 LEDs in parallel, and fire up the circuit with only 5 of the connected, you will be runnign too much current through them. Or if somethign happens and part of the circuit becomes disconnected, same problem.

 Resistors are cheap - once you know what ones you need, buy a box of them from Mouser, they end up being a penny or two each. Not worth the savings to risk potential problems. Don't buy electronic parts from Radio Shack unless you truly only need 1 to experiment with - they are so overpriced it isn't funny.

 You still only have to run a single wire from the terminal strip for the common lead of each LED.

 WHen it comes down to it, depending on how the two LEDs in the bi-color LED are controlled, safest practice is a resistor for each of the color leads, NOT a single resistor ont he common, becuase with a 3-lead LED it's entirely possible for only one or both LEDs to be active. On a 2-lead bicolor LED it's physically impossible for both LEDs to be active at the same time (not even on AC - they're flipping back and forth at the power frequency, NOT on at the same time!). However if the controlling circuit is something like a DPDT switch which can't physically be in two positions at once, then a single LEDon the common is quite safe.

                                                    --Randy

 

[superbe]

Hi All 

You all have given some good advice and sources to learn more but I have a basic question that may have been answered but I didn't grasp it

By running the power from the transformer to a terminal strip and then running leads to numerous LEDs would one 1000 resistor between the transformer and the terminal strip effectively reduce power to all of the leads leaving the strip? 

Happy Railroading

Bob

 

[bavrail]

Here are a couple of links with some good info on LED's

 http://www.quickar.com/ledbasics.htm

http://www.trainelectronics.com/LED_Articles_2007/LED_101/index.htm

[CSX Robert]

superbe

...I'm using a train transformer with the AC side powering the Peco switch motors and the DC side the LEDs. None of the LEDs have resistors. At first I only had red LEDs and learned from poping them as to where to set the power control (@ 10mph lol)...

This is why your green LEDs are so dim, because the green ones tend to have a higher forward voltage drop. Once you hook them up to a 12 volt supply(or the power pack turned up to about 12 volts) with a ~1k resistor you'll find the red and green LEDs closer in brightness.

As far as wiring: If the red and green are both bright enough and you only have one or the other on at a time, then you can use one resistor per pair on the common leg(it won't hurt anything to have both LEDs on if you do this, they will just be dim). If you want to adjust the relative brightness of the LEDs, then you will need a separate resistor on each leg of the LED.

[daveb]

Bob,

Sorry if I wasn't clear about what I meant. I am suggesting that you wire a 1K  0.25W resistor in series with the common leg (anode lead in your case) of each and every LED.  The use of a separate 12VDC supply as suggested by Randy is also a good idea.

Dave.

[rrinker]

 The 1K resistor will be good for 12-14 volts, so if your power pack is in that range at full throttle then yes, you could just use that. But a dedicated regualted power supply is usually better for electronics because the output is smoother DC than you get from a train set power pack, and the voltage remains faitly constant from no load to near th maximum, instead of being way high with a small load and droppign way below the rating whent he load increases.

                                    --Randy

[superbe]

Dave,

My wiring is as follows; The wires powering the LEDs are from the the DC side of the transformer and go to separate terminal strips. From the strips one wire goes to the anode of a LED and the other goes to the common feed of the accesory switch on the turnout. Then two wires leave the acc switch and are soldered to the cathode leads of the LED.

Following your suggestion I would install the 1000 ohm resistor between the transformer and the positive terminal strip feeding the common feeds of the accessory switch. In addition I'd install an additional resistor to the Red leads to brighten the Green LED. If this is what you mean would I then be able to use the full power of the transformer or are you also suggesting using a separate 12 V power supply?

Happy Railroading

Bob

[daveb]

Bob,

I would suggest that before doing anything else that you wire a 1K resistor in series with the common lead of all your LEDs to avoid damaging them by turning up the power control.

Once the 1K resistors are in place you can then operate the LEDs from a fixed 12VDC supply and then experiment with other values in the red lead to get the same intensity.

Dave

[superbe]

rrinker

 Before you go buying anything - how are you powering the LEDs now? If from a 12V power supply, I assume you have resistors there already? If so, ADD the 100 ohm to the red side lead, don't REPLACE.

 Green LEDs tend to be the dimmest at a given current, sometimes you might have to decrease the reistor on the green side AND increase the resistor ont he red side to make the intensity match. Or one resistor in the common lead that runs both LEDs a somethign less than the max current rating, plus an extra resistor in the red lead to reduce the current on the red side even more. If you get LEDs from a major supply house like Mouser you can look at the specs before you buy and check the intensity in mcd and find ones that are close. Same with buying discrete LEDs where you want ones of differnt colors to have the same intensity. Green usually do not go up that high, but you can get red ones that are lower and thus will match. Different bicolor LEDs are matched differently as well.

                                      --Randy

I'm using a train transformer with the AC side powering the Peco switch motors and the DC side the LEDs. None of the LEDs have resistors. At first I only had red LEDs and learned from poping them as to where to set the power control (@ 10mph lol)

This is my first experience wiring any kind of lighting so on future projects I'll check out sites like Mouser. My current LEDs came from eBay at a good price and with super fast mailing. I also have some grain of what bulbs but haven't done anything with them.

For now I'll try the 100 ohm resistor and see what happens. My favorite learning tool is trial and error.

Thanks and Happy Railroading

Bob

[rrinker]

 Before you go buying anything - how are you powering the LEDs now? If from a 12V power supply, I assume you have resistors there already? If so, ADD the 100 ohm to the red side lead, don't REPLACE.

 Green LEDs tend to be the dimmest at a given current, sometimes you might have to decrease the reistor on the green side AND increase the resistor ont he red side to make the intensity match. Or one resistor in the common lead that runs both LEDs a somethign less than the max current rating, plus an extra resistor in the red lead to reduce the current on the red side even more. If you get LEDs from a major supply house like Mouser you can look at the specs before you buy and check the intensity in mcd and find ones that are close. Same with buying discrete LEDs where you want ones of differnt colors to have the same intensity. Green usually do not go up that high, but you can get red ones that are lower and thus will match. Different bicolor LEDs are matched differently as well.

                                      --Randy

[superbe]

DMNJRR

 Correct.

Mine are 3-lead and the resistor is on the Red lead. 

 

Well it's off to R S this weekend to pick up some 100 OHM resistors. Hopefully my specs will be the same as yours.

Thanks everyone.

Happy Railroading

Bob 

[richg1998]

 Most people are not aware that bi-polar green/red LED's have different intensities. Check the specs.

Rich

[DMNJRR]

 Correct.

Mine are 3-lead and the resistor is on the Red lead. 

 

[superbe]

Hello DMJRR & Randy,

I should have stated that the LEDs have 3 leads. So as I understand it I could get by with using the resistor on the red lead. Correct?

Thanks and

Happy Railroading ,

Bob

 

[rrinker]

 The two different resistors trick only works if you are using 3-lead bicolor LEDs. For the 2-lead types, you also have to add a pair of regular diodes so as to bypass the larger resistor when the polarity is such to light the green LED, and bypass the smaller resistor when the current flow is for the red led.

                            --Randy

 

[DMNJRR]

I am also using bi-colored LEDs on my control panel, They're powered by either Atlas Dwarf Signal boards or Atlas Non-Derail boards. When I chain two together the RED is brighter than the green. I found that a 100 OHM resistor on the RED lead evens it out. 

Hope it works for you!

[superbe]

I'm using a bi color LED (red / green) as a turnout directional indicator on the control panel.

These are powered by the DC side of a transformer. When I turn up the power to a safe level the red shines brightly but the green is dim.

What size resistor would you suggest for the red lead to make the green brighter??

Your help will be appreciated.

Happy Railroading

Bob