Converting twin headlamps to LED, what size resistor?

[IDRick]

I watched a neat youtube video tody that discussed converting bulb lighting to LED.  The locomotive had twin headlights so he mounted a separate LED + 1000 ohm resistor in each slot then connected them in parallel to the decoder.  My memory from college physics (70's vintage) is that two 1000 ohm resistors in parallel will have a series equivalent of 500 ohms (1000/2).  Isn't that too low for LEDs?  Seems like most recommend a 1000 ohm resister for a single LED.  To achieve this with twin headlights, one would need to wire two 2000 ohm resisters in parallel, correct?

[wvg_ca]

nope .. that's is right

one 1000ohm resistor per led

[IDRick]

Okay, thanks.  The 70's were a long time ago...  

After cheating by reading my friend Google, the series equivalent current = Voltage/500 ohms.  The sum of current through two parallel resistors equals Voltage/500 but individual resistor would receive receive a current = Voltage/1000 ohms.  

[Mark R.]

The two resistors themselves aren't in parallel. The two circuits (resistor and LED) are in parallel with each other from the power source .... different math. 

Mark.

[richg1998]

The OP might be interested in the below link. I hope it does not cause any confusion.

http://www.members.optusnet.com.au/nswmn1/Lights_in_DCC.htm

Rich

[IDRick]

richg1998

The OP might be interested in the below link. I hope it does not cause any confusion.

http://www.members.optusnet.com.au/nswmn1/Lights_in_DCC.htm

Rich

 

Makes sense that a 1000 ohm resistor in series will yield same circuit current flow as two 1000 ohm resistors in parallel.  Thanks for the link Rich!

I am a DCC newbie and need be quiet before making myself more of a fool...

 

[IDRick]

Mark R.

The two resistors themselves aren't in parallel. The two circuits (resistor and LED) are in parallel with each other from the power source .... different math. 

Mark.

 

What is your answer then?  Been a long time since I had physics...

I'm a dairy guy, not a electronics whiz.

[richg1998]

The way I see it, each LED, resistor circuit would have 12 vdc across it whch would have about 9 ma current flowing through each circuit which is normal. I have done a few of those, single and dual. No interaction between circuits. The decoder can handle it.

I did resistor LED measurements a number of years ago when I started working with LED's in 1972.

Rich

[Mark R.]

I've done installs in three methods, all with long term success ....

• Two LEDs with a single 1000 ohm resistor on each LED
• Two LEDs soldered together in parallel with a single 680 ohm resistor
• Two LEDs soldered together in series with a single 470 ohm resistor

All three methods work just fine and have lasted years in my own installs. Which method I use depends on the application more than anything.

Mark.

[NeO6874]

and the LED.  Some of those cheap-as-chips chineseium ones on ebay need a bit more in the resistance department. 

 

But hey, when you get a pack of 50 or 100 for $5 ... a little trial-and-error time is fine.

[rrinker]

The physics isn;t wrong - 2x 1K resistors in parallel will yield 500 ohms. However, when you have an LED and a 1K resistoor in parallel with another LED and a 1K resistor - the resistors are not in parallel with each other. The entire circuit of a resistor in series with an LED is in parallel with the entire other circuit of an LED in series with a resistor. You have to account for the LEDs as well. Then the math actually does work out.

For a single LED in series with a single resistor, the current is calculated by (Source voltage - LED voltage drop)/resistance. For typical DCC decooders and white LEDs, it works out to be around 9ma with a 1K resistor. A pair of 9ma circuits in parallel with each other, will draw 18ma from the source. The voltage drop across each resistor will be the same because parallel circuits each get the same voltage - it's not split. So you have the same voltage at twice the current = half the resistance! But that's measured across the entire circuit - each LED still has 1K, not 500 ohms.

 It's actually not quite that simple, but for fairly trivial circuits it happens to work out.

                                          --Randy

[gregc]

IDRick

The locomotive had twin headlights so he mounted a separate LED + 1000 ohm resistor in each slot then connected them in parallel to the decoder. 

seems like this is what he's saying

decoder -----|----R--------LED---------|
             |----R--------LED---------|--- Blue

so why not do this?

decoder ----------R----LED-----LED---- Blue

Mark R.
Two LEDs soldered together in series with a single 470 ohm resistor

                            

[IDRick]

Thanks for the discussion, most appreciated.

Here is the youtube video that I watched: https://www.youtube.com/watch?v=j46QbNDG4I0

Note each LED has a black and red wire.  Near the end (4:45) he describes soldering the wires in parallel (ie the two blacks are soldered together then the two red ones).  Earlier he shows the resistors but not show them being attached.  Oops, I missed that the first time... (probably because I had be looking at LEDs with attached resistors prior to watching the this video).  With his wiring approach, where will he install the resistors?  Thanks

[gregc]

i'd need to see how he connects the LED so the decoder

[rrinker]

 As one of the comments rudely states, to which he rudely replied - he doesn;t show that kind of important bit of information. Makes the electrical portion of the video pretty much useless.

 Unless thoose LEDs already have resistors in line somewhere, it would appear he is hooking 2 LEDs in parallel and then using one resistor. This is probably the least favorable way to do it, but it will work. The issue can be that the two LEDs are not well matched, especially if you buy the cheap Chinesium ones. They may not have equal forward voltages, so one may end up brighter than the other. It would be better to put two LEDs in series and use a smaller than 1K resistor if you want to reduce the number of resistors used.

 As for the ditch lights, looks like he's doing a BC Rail model, so it's OK what he did, but if you are modeling a different railroad such as Conrail, NS, or CSX, you don't want to combine the two ditch lights, you want each light on its own function output so they can alternately flash. Canadian railroads don't do that, so they can be wired together.

                                             --Randy

 

[IDRick]

Randy, thanks for your discussion and always great advice!  

I came across a good deal for a newbie like me on ebay.  I purchased this: https://www.ebay.com/itm/PARTS-Athearn-Blue-Box-locomotive-warm-white-LED-head-lights-DC-DCC-EASY-INSTALL-/202466058665?oid=202421016879

This will outfit front and rear lights of both of my athearns on Front and Rear lights.  Provides directions for DC connection and then how to change when upgrading to DCC.  Has nice set of directions.  May be a bit overpriced but for a newbie, it's perfect given the well described directions and easy way of converting between DC and DCC!  Do you agree, did I read it right?

[rrinker]

 Those will certainly work. But they are only 1 LED per end, not double, Which is another way to do it - several companies make clear lenses, and you can put one LED on the inside and have both holes filled and illuminated.

                                      --Randy

 

[IDRick]

Shouldn't have pulled the trigger so fast.  After further review, I have canceled the order as I don't care for all the light in the cab and there's very little room to add a light block.  Plus the LED leads are rather short which makes it difficult to attach/take off a shell with  attached LEDs.

[IDRick]

I found another video that does a great job on how make your own LED + leads and install in the headlamp openings.  Lighting looks awesome in the locos!  Unfortunately, he didn't show how he completed the final wiring from shell to the locomotive...  Grrr...

https://youtu.be/huH4jN-Wk1E

[wvg_ca]

he actually didn't go straight to the loco [DC] in the video ..

he went to the pads on a TCS decoder [DCC] with keep alive

[IDRick]

wvg_ca

he actually didn't go straight to the loco [DC] in the video ..

he went to the pads on a TCS decoder [DCC] with keep alive

 

 

As a newbie, I would love for the videographer to show how and where he made the connections with multiple LEDs on his locomotive.  I can guess how he did it but I don't know for sure...  It would provide an excellent teaching moment for newbies like me.  

[wvg_ca]

as a guess, he went to pads that had 12v or so to them, he mentions using a 470ohm resistor ...

you could alwaya leave a comment on that video ??

[IDRick]

wvg_ca

as a guess, he went to pads that had 12v or so to them, he mentions using a 470ohm resistor ...

you could alwaya leave a comment on that video ??

 

 

Turns out he actually had an additional video that discussed his new kits that include ditch lights and gave a quick discussion on how he connected the lighting wires to the decoder.  I have exchanged an email with him and I'm hoping to use his product in my athearns.