A few questions regards LED's from a simpleton

[big jim]

So I want to add LED lighting to a number of my coaches that I'm running on DCC

 

I plan to add pickups to the coaches and take power direct from the track to various LEDs

 

First up maplin LED strips, the type you can cut down to suit, I've used these in the past but they are too bright inside the coach so I need to dramatically reduce the output by adding a resistor of some sort, any idea of what sort of value resistor I would try?

 

Next up I have somewhere a number of tiny surface mount LEDs, pre wired with resistors that I want to use as spotlights in the likes of mentor's roof, each one comes with an individual 1000 ohm resistor in series (i.e. Soldered In the anode side cable) so they run direct off 12-18v the issue with these are the resistors are quite sizeable so will be difficult to hide the 4 resistors within the coach (6 if I also add 2 solebar lights) my question is can I replace the 4 individual 1000ohm resistors with a single one on one cable feeding all 4 LEDs anodes at once, if so do I need to be looking at a different ohm-age resistor as the power is being split

 

As I say I'm a bit of a simpleton when it comes to electrical stuff so nice simple answers would be appreciated so I can just go into maplin and ask for such and such a resistor without being bomboozled by numbers!!

[APOLLO]

This site may be of help Jim

 

http://www.hobby-hour.com/electronics/ledcalc.php

 

If not I suggest trial & error, use a few resistance values.

 

Brit15

[Penrhos1920]

Unfortunately that calculator doesn't tell you what current you need for dim leds. My guess is you need a 1/20th of the normal bright current. Are the maplin leds in threes? If so don't forget to triple the voltage across the led, but leave the current as is. There will already be a resistor with them but you can probably ignore it as it will be small compared to the new resistor.

[big jim]

Now you see that post is all gobbledygook to me, voltage, current etc

 

Yes the LED's are those strips of 3, probably going to have 3 of them per coach so 9 LED's in Total, as you say each led has its own resistor already

 

I've got a multimeter, can I measure the current at the led strip with that?

[Tricky Dicky]

Can you link to the specific LEDs you intend to use?

 

Richard

[big jim]

Can you link to the specific LEDs you intend to use?

 

Richard

https://www.maplin.co.uk/p/white-12v-5cm-led-strip-n87cz

[Tricky Dicky]

From what I can see each 5cm strip contains the usual 3 SMD LEDs and a single current limiting resistor The strip of LEDs will arrive as one continuous length and you will be able to cut required lengths in multiples of 5cm (5cm of 3 LEDs and one resistor being the minimum length). However long the strips you require will only require a 12v DC supply hooking up to illuminate the LEDs. If the brightness needs toning down then an additional resistance in either of the supply leads will do. The value of the additional resistor is really a matter of trial and error, with LEDs doubling up the resistance does not necessarily halve the brightness, so I would start with a low value in the order of a few hundred ohms and increase the resistance until happy. If you are DCC and pulling the supply from the rails then you probably talking of a 16V rail voltage that means it is not safe to hook up your 12v LEDs as they are but a bridge rectifier or 4 Diodes plus some additional resistance which can be easily calculated will be required. I am going to be out to work in the garden but will look in later this evening if you require any further help. Maybe someone else might contribute in the meantime.

 

Richard

[DCB]

The resistance problem will be the rolling resistance of the pickups not anything electrical if you are not careful.  The old split axle Mainline and early Bachmann wheelsets let you use pin point bearing pick up which will run away down a 1 in 75 or wipers on the axle journals which will run on a 1 in 50 whereas the recent wheel back pickups I have seen won't run down a 1 in 30 which will really compromise train length.

 

My DC LEDs are in parallel, project through a false ceiling under the roof, 1 per compartment, this is important for compartment coaches, and the compartment ends need to go to the ceiling.  The LEDs are fed by a bridge rectifier with the biggest electrolytic capacitor I could hide in parallel with the LEDs and a resistor to keep the LEDs from frying if the power is left full on.  This gives a nice steam era glow at line speed.

My pickup is pinpoint through Lima wheels which are insulated one side only in Hornby Dublo bogies, one bogie positive the other negative.

 

DCC is easier, in some ways, the line voltage is constant and as long as you measure it and don't guess or rely on the sticker on the back of the controller you can set the resistor to give a good light.  I would daisy chain the LEDs in series in this instance as you should get at least 10 to work in series on 16 volts AC through a standard 1 amp diode, maybe more, resistors can be an issue as you may find you need a negative value resistor to increase the brightness!   I would reverse the connections on half the coaches to balance the load across both waves of the aC as each coach would be using only half wave.

The refinement of a capacitor or ten is probably a not on with DCC as it may affect the signal so flickering lights will spoil the illusion unless everything is spotlessly clean [my life is too short]

 

Another approach is to use button cells to operate lighting.  If the train spends most of its time in a hidden siding a button cell or two operating the LEDs through a resistor with a change over reed switch mounted low in the battery box or under a bogie can be arranged to switch off the lights when at rest over a magnet or 12.  This is easy to cobble up using chinese super neo magnets at £1 for 10.  That way the batteries last a surprising time.  Poor quality batteries are available for about £3.20 from motor factors for key fobs or better quality ones from Poundland as packs of 24 assorted sizes.   Some say "You get what you pays for."   Then again some believe the earth is round as well.

 

Coach lighting is great, especially at night with the last train of the day heading back down the garden to the terminus at 9pm on a September evening.  That way you can't see the loco is an ancient Airfix Prairie with oval wheels and the coaches 1960s Triang Suburbans...

[tractor_37260]

https://www.maplin.co.uk/p/white-12v-5cm-led-strip-n87cz

 

It may be worth considering these also, they will give a warmer tone of light more like tungsten bulbs, your other choice will have a very cold blue/white Xenon light effect..........

 

https://www.maplin.co.uk/p/warm-white-12v-ip65-5cm-led-strip-n91na

[Crosland]

DCC is easier, in some ways, the line voltage is constant and as long as you measure it and don't guess or rely on the sticker on the back of the controller you can set the resistor to give a good light.  I would daisy chain the LEDs in series in this instance as you should get at least 10 to work in series on 16 volts AC through a standard 1 amp diode, maybe more, resistors can be an issue as you may find you need a negative value resistor to increase the brightness!

 

DCC is square wave so the peak and RMS voltages are the same. If the track voltage is 16V then you will get, at most, 15 V across the LED string which is way too low for 10 LEDs.

 

The refinement of a capacitor or ten is probably a not on with DCC as it may affect the signal so flickering lights will spoil the illusion unless everything is spotlessly clean [my life is too short]

On the contrary, a small capacitor is recommended with DCC to avoid flickering caused by intermittent pickup. Much like a stay alive capacitor used in a loco, but a smaller value will do. You just don't connect it across the track.

[PatB]

Might I suggest an alternative (or, more probably supplement) to trying to dim the LEDs? Instead, art shops sell transparent paints for use on glass but which should also work on plastic LED casings. A coat of a suitable colour would help to mute the very harsh whiteness which LEDs tend to exhibit. With care and varying thickness of coating it should be possible to replicate anything from C19th oil lamps, through gas and dim incandescent installations to modern(ish) fluoro lighting.

[DCB]

Might I suggest an alternative (or, more probably supplement) to trying to dim the LEDs? Instead, art shops sell transparent paints for use on glass but which should also work on plastic LED casings. A coat of a suitable colour would help to mute the very harsh whiteness which LEDs tend to exhibit. With care and varying thickness of coating it should be possible to replicate anything from C19th oil lamps, through gas and dim incandescent installations to modern(ish) fluoro lighting.

White LEDs are a different breed to Coloured as in yellow / orange/ Red/ Green and the Vf is considerably greater, white LEDs need 3 AAA cells to light them while coloured are good on 2 AAA cells.  I use yellow and orange LEDs for steam era coaches and White for HST era stock

The white lights of late 70s HSTs was quite dramatic, we used to watch them from just west of Wotton Bassett on both Bath and South Wales lines.

 

DCC is square wave so the peak and RMS voltages are the same. If the track voltage is 16V then you will get, at most, 15 V across the LED string which is way too low for 10 LEDs.

 

On the contrary, a small capacitor is recommended with DCC to avoid flickering caused by intermittent pickup. Much like a stay alive capacitor used in a loco, but a smaller value will do. You just don't connect it across the track.

From other posts on this forum some DCC controllers deliver over 25 volts which is excess of the NMRA standards and leads to fried chips,

24 volts would light 10 yellow leds very nicely 16 volts would be good for units of 5 in series with suitable resistors.

 

My anti flicker capacitors are as big as possible and probably a higher value than the typical stay alive as in too big to fit in most locomotives. 

[big jim]

my mind is boggled now!

 

im thinking of going down the decoder route instead as ive done with another coach i have on the go, ive got a function only decoder that has 4 functions (2 for head/tail lights) and 2 stand alone functions, i can map the head/tail light finctions to stay on in both directions according to the instructions but i only really need 2 functions per coach, one for the interior lights and another for 'ditch lights' or other various recording equipment lights depending on the coach in question

 

i know this is going off on a bit of a tangent but what sort of voltage's are decoder function outputs?

 

the coach ive already done has a Hornby light bar fitted via the lais dcc decoder and that seems to work alright (if not a bit too bright but ive not adjusted the output yet) so im assuming the maplin strips should just wire straight to the decoder too

[14Steve14]

Once you get to work out the resistor value, could you not use something like a preset potentiometer with a similar value. That would allow further brightness alteration if required.