Series/parallel LED Resistors

[WIMorrison]

I plan to install LEDs into some old locos and will use bicolour LEDs for the lower  lights (red/white) and a single white micro LED for the third upper forward light.

 

i have created this circuit and calculated what I think the traitor value needs to be and I wonder if someone who knows more than me can confirm that the value is correct?

 

 

If I have it wrong - which is highly probable - can you also tell me how to work out the correct value?

 

thanks

 

iain

[Hamburger]

Hello,

 

it appears to me as if this setup eventually could cause some problems.

But certainly it depends on how you want to switch the lighting (red/white separately, and/or direction dependent etc.).

 

First, it seems that you intend to use LEDs with common cathode which is possible but would make wiring more complicated.

 

It is always recommended to use such with common anode. So you can connect all anodes commonly to the blue wire, the cathodes have to be connected to the respective outputs.

Resistors should be between output and cathode. Without having calculated it, 300 Ohms seem to be not sufficient. Just by experience, between 1 and 2 kOhms should suit better.

 

Second, white and coloured LEDs most often need different resistor values because they need different voltages (roughly up to 3 V for white, up to 2 V for coloured).

Always recommended to use one resistor per LED.

 

Third, you should use the same bicolour LED also for the top light, if mechanically possible, in order to get the same appearance.

Different LEDs will often give a different 'white' colour.

Just connect only the 'white part' of the LED on the top light.

 

Take into account that the decoder would not always supply 12 V.

Usually you have track voltage minus 1.4 V on the blue wire.

 

Wolf

 

[WIMorrison]

Quote

 

Wolf

 

Thank for the views, I thought that I had drawn them as using the blue anode wire and the yellow or white cathode?

If I could get a bicolour into the top light I would do that however I cant which is why I am planning to use a micro-led for the top light. I also thought that by placing LEDs in series that like resistors they would drop the appropriate voltage depending on the LED resistance as they will be in a constant current circuit?

 

Taking the different reverse voltages for red/white I think it would work out as a 470 ohm resistor?

[Phil S]

Have you considered buying pre-wired SMD LEDs?? - they come, with or without attached resistor according to supplier - there are a range of sizes  down to a pinhead .... it makes it easy to get the light in the right place without 'light pipes' ... and the wires come back to a more convenient position.   [ Plenty available via Ebay, or Digikeijs also sell packs of them, as do DCC Concepts... ]

 

Other observations:  If the same make and batch, parallel LEDs will usually work and appear as bright as each other  - but otherwise a separate Resistor provides the option to adjust them individually - eg for Day/night switching.    Resistors do not have to be specifically on the cathode or the anode side - as long as they are there in series** - whichever s most convenient for your circuit and physical arrangement - that said, it may be PREFERABLE to work to a common standard so that, once legs are trimmed, and joints sealed with heat shrink, you can still identify which way things are !

 

**With 3 legged LEDs,  a common resistor can be used on the central pin, but with no control on the relative brightess of the 2 outputs -

with resistors on both the outer legs, they can be adjusted indiviually.  With Loco-based decoders, it is EASIER to use Common ANODE (+ve) because the common (Blue wire) is positive, and the standard outputs pull down to grouns ie 0V when 'On'  [ But some decoders now support connections to high and to low - see their instructions for details ! ]    For (Stationary) Accessory Decoders - ihese may vary with different makes.  CHECK with a meter.

 

I would recommend buying a small 'LED Tester' - the size of a PP3 (v battery (which is what is inside the box) - and with a series of contacts labelled from 1mA to 50mA allowing you to judge the required resistor value by the light output obtained .... because modern LEDs are SO much more efficient than older types, you could be using values in the 2-5k region for a single LED across the onboard power .... and I would not assume only 12V if considering MINIMUM resistor values - because the dcc specification allows for, and many systems will have, a higher voltage - typically 14V [ the extra 2 volts, with a '12V LED lighting strip is enough to DOUBLE the current ] ... and can go as high as 20V within spec (after the diode bridge drops) ....  but I think you would find it a very inefficient old LED that required the resisitor to be such a low value that it damaged the LED.

[Suzie]

The chances of the two whites having the same forward voltage are slim so you should put them in series.

 

How are your bi-colour LEDs wired? they normally only have two pins.

[WIMorrison]

36 minutes ago, Suzie said:

The chances of the two whites having the same forward voltage are slim so you should put them in series.

 

How are your bi-colour LEDs wired? they normally only have two pins.

 

Suzie

 

Thanks, I think that you confirm my thinking on having them in series?

 

The bicolour LEDs are 3 pin with a common anode - I specifically buy these to enable the switching using the decoder outputs.

 

Any views on an effective resistor size?

 

Thanks

 

Iain

[Suzie]

It depends on how bright you want them. I would normally use 680R for the red and 1K for the two whites in series as a starting point. If your decoder has brightness adjustment use that to dim them down a bit if required.

 

Your diagram will not work because of how the LEDs are wired. Just connect the common anode pin to the blue wire and the resistors to the other legs. Connect the extra white LED in series with the white resistor.

[John ks]

If I understand the first post correctly the one of these 2 circuits may be what you want

The lower circuit separates the red lights from the headlight circuit so that the Red lights can be switched independently from the white lights. CVs can be set so that the red lights are directional & switched by one function button

 

 

Modern hi intensity LEDs can be very bright. a resistor of 1K would be my starting point, change the value up and down until you get the desired brightness.

On older locos you may need to go to 5K or more to get the lights to look right

 

Led calculators will give you the rated current through the LED for maximum brightness(& shortest life)

This is great if you are trying to light a room but for models it can be way to bright

 

Hope this helps 

John

 

[Pandora]

On 08/05/2019 at 09:46, WIMorrison said:

I plan to install LEDs into some old locos and will use bicolour LEDs for the lower  lights (red/white) and a single white micro LED for the third upper forward light.

 

i have created this circuit and calculated what I think the traitor value needs to be and I wonder if someone who knows more than me can confirm that the value is correct?

 

 

If I have it wrong - which is highly probable - can you also tell me how to work out the correct value?

 

thanks

 

iain

I can see the 12V feed from the decoder,  but I cannot see the Ground for the diodes in the circuit

Edited July 24, 2019 by Pandora

[RobjUK]

Look at the last diagram, posted by John ks

 

The decoder typically switches the outputs to ground.

The common to all the LEDs is a feed from the decoders internal positive supply; the blue connection in that diagram.

 

[Izzy]

In my experience most bi-colour SMD LEDs seem to be anode common, certainly those I have been using (red/green) in some 2mm searchlight signals. Although I have used 2-2K5 resistance with them, and some smaller single white ones, (theatre indicator) all on a regulated 12v supply, I would suggest that, if you are using a decoder with dimming capabilities I.e. Zimo, that this is also used (cv60 for Zimo) to knock the output down further. Just adding ever higher resistance is a faff and not half as easy or adjustable.

 

Izzy

[Phil S]

As a side issue.... whether to use the resistor value or the decoder's pulse width controlled dImming feature .... it may depend on whether you intend photographing your layout  ( or using video monitoring )  as to how well the pulsed v constant LED output appears in either stills or video ( which may also be affected by ambient lighting levels affecting overall exposure )