Carriage Lighting - reducing brilliance.

[Bulleidboy]

I have a rake of three Hornby Mk1 coaches (R4714, 5 & ^), which came factory fitted with lighting. The lighting is too bright - would fitting a resistor into the wiring circuit reduce the brightness? The power pick-up is from one of the bogies - so track power and as DCC = 15v (ish!).

[Bulleidboy]

Thanks for that very comprehensive reply Chris. I'll try the "variable" voltage method first, and see what happens. The defuser suggestion may also be an easier option. BB

[96RAF]

The speed controlled output from an HM2000 analogue controller dims LEDs nicely. I used it for street lighting.

I have also used the paint over technigue to ‘convert’ electric LEDs to gas lighting levels.

[Bulleidboy]

Thanks Rob -I think the paint-over method will probably be the easiest. I have some Tamiya Clear Yellow Lacquer - which worked well in another situation. BB

[Bulleidboy]

I tried the "variable voltage" test tonight, using my H&M Clipper. The lights came on at a certain point on the dial, and then went up a stage to full brilliance as the power was increased - not a gradual increase. However, they were not as bright as the green set of coaches - Hornby did three in green and three in maroon. There is a slight problem though, in that there is a noticeable degree of "light bleed" through the sides of the carriage. I did not see this on the green carriages. I may have to paint the inside of the coaches?

[96RAF]

Probably to do with the way each of our controllers puts the power out Barry.

Light bleed can be a real problem on some bodies. Coaches can have a lot of clear plastic that prisms light all over the place.

[Bulleidboy]

Thanks for the updated information Rob and Chirs. I onlu used the Clipper as it was to hand - it's sole use is powering the two turntables. Time to get the paint brush  out!

[ColinB]

From my experience with fitting leds into my diesels I have found that modern high intensity leds, don't seem to change their brightness much with a change in current, unless it is really small. Normally I would use a 1 kilo ohm resistor, which should be about 10 ma at 12 volts (12 volts - 2.2 volts across led), but I find I have to go up to values of 20 kilo ohms on some leds to get them to dim. So perhaps your power supply is working ok. Next time you do your testing just put a 10 uF capacitor or above (make sure you get the voltage and polarity right) across the terminals, then you will get true DC. I would be surprised if they use constant current you would need one per led, more probably if you were going to do it, regulate down to 5 volts then run your leds from that, but I may be wrong.

[Chrissaf]

I would be surprised if they use constant current you would need one per LED.

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You certainly do not need one driver per LED. There are numerous commercial examples of LED products that use a single driver for multiple LEDs. Granted, that would give the most accurate control, but it is not a mandatory requirement.

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Here is an example of a commercially available 'after market' coach lighting product that employs a simple Zenor diode 'constant current' driver circuit. I have installed a few of these as after market coach upgrades.

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[96RAF]

My station lights were just all strung together in parallel with a 1K ohm resistor at each light. The HM2000 could wind them down from max to off via incremental dimming with no trouble. I shall adopt the same laymans methodology when I wire up my new layout lighting.

[ColinB]

You are not serious with that circuit, all the leds are paralleled together. It is a well known fact that leds don't all have the same forward voltage drop, you never parallel them together. Plus you cannot guarantee they all get the same current. If one of the leds fails the current through the others increases. My coach lighting definitely has separate resistors. 

[Chrissaf]

I'm sorry we will just have to differ. I've built about 6 of those circuits and installed them from commercial kits and each one has been working absolutely flawlessly for more than 3 years. Although I understand how the circuit functions, and I am totally aware of its technical limitations and compromises ... as a commercial kit it is not one of my designs, but It just works.

[ColinB]

Ok, but if you want to dim the brightness then just increase the value of the resistor on the emitter of the transistor.

[96RAF]

Going back to the OP, Barry’s coaches came with factory fitted lights so have a closer look here at what is likely fitted. 

 

Its a poor image but I can easily get a better one if necessary as I use them as indicator lamps for testing accessory modules (Second photo).

 

If necessary Barry I can link a 1k or 10k ohm resistor in circuit to gauge dimming effect.

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[ColinB]

@RAF96 looking at that picure I am wondering if that chip to the right of the capacitor is a 5 volt regulator, so that the voltage fed to the diodes is constant. It is more of the circuit I would expect. Trouble is to change the lighting you would need to change all those smd resistors. I suspect that chip to the right of the capacitor feeds all the leds, so you could lift the relevant leg and put a resistor in. I won't recommend it though too many things to break. SMD resistors are cheap on EBay so I suppose changing each one is the best way, although it is a pain.

[96RAF]

I can hook up a DC controller Colin and if it is a regulator then it will click on line at the right time and any extra throttle will make no difference.

[Chrissaf]

Modern coaches with factory lighting are designed to work on both DC and DCC tracks. So they are designed to work on a very wide range of track voltages and yet give a similar level of brightness in the bulk of anticipated track conditions. DC could be as low as 3 volts (bearing in mind that white LEDs usually won't even switch on until they have 3 volts across them) whilst as you already appreciate, DCC track voltage is at an equivalent 14/15 volts once rectified. Thus the coach lighting LEDs need to be as effective as possible over this wide 3 volt to 15 volt range.

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Of the coach factory lighting circuits I have looked at, the majority seem to be based on using a simple semiconductor circuit to create what I call a 'constant brightness' driver. The principle of the operation of this type of circuit is as the name suggests to generate a brightness that is constant regardless of the applied track voltage to it. Thus, it makes it very easy for the coach lighting to remain a reasonably constant brightness over this 3 to 15 volt range.

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This type of circuit applies very elegantly to a LED application, because LEDs are 'current' devices and not voltage devices. It is the current through the LED that defines how bright it is. In very simple LED circuits you can use resistors to control the current and thus the brightness. Notice that I am saying that the resistor is actually controlling the current, any voltage change that might also be effected by adding the resistor is purely coincidental.

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So yes, you could add a resistor to change LED brightness in simple LED circuits, but this is not so clear cut and straight forward if the coach is indeed based on using 'constant brightness' technology in it.

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There are different technical techniques that could be used to create a 'constant brightness' driver. I have seen some commercial 'after market' lighting products that use 'constant current' driver technology. But these type of circuits are not usually used by Hornby in their factory installed lighting coaches. If you add a resistor to the LED circuit that is being fed by a 'constant current' driver circuit, then the 'constant current' driver circuit does its designed for task ... which is to maintain the designed current value. Thus the circuit will compensate for the detected additional resistance and change its output voltage control, to maintain its set current output value. The result of this is that as you add more resistance, the current and thus the LED brightness stays the same, until you reach a point where the added resistance is so high that the LEDs just suddenly go out

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If you feel that your coach LEDs are too bright, then assuming that the LEDs are roof fitted and not table lamps, it would be easier and more effective to fit a diffuser over the bright LEDs. Say for example small pieces of opaque or smoked plastic film or thin sheet. Maybe even a very thinned wash of yellow paint. Something that will dim the LEDs physically rather than electrically. To do it electrically, one would need to have detailed knowledge of the factory installed circuit to decide where and how to make the necessary component changes.

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Experiment

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In order to find out whether your coaches have 'constant brightness' drivers in them, you could try this simple experiment. But ideally it needs a variable DC voltage power supply like those used in test bench environments.

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You might be able to represent something similar with a slightly more modern DC Analogue controller that outputs pure DC that is measurable with a multimeter set to measure DC voltage, but not a PWM controller......see caveat at end of this reply

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Put the coach on a short piece of track and connect your variable supply with the voltage set at zero.

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Slowly turn up the voltage until the LEDs start to light up, this might be anywhere between 3 and 5 volts.

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Now turn up the voltage a little more until you perceive the LEDs to be at their normal brightness, this might be at 6 or 7 volts.

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Now if you slowly turn up the voltage from that measured value to say 12 (or 15 volts if you can) and the LED brightness appears to remain consistent, then the coach employs 'constant brightness' LED driver technology in it.

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Caveat: The results of this experiment may not be conclusive if the variable supply used does not output a pure DC voltage. A PWM analogue controller for example is not really suitable as it pulses the full 12 volt track voltage on its output. It only reads a lower voltage on a multimeter because the meter is measuring the average and not peak voltage. The LEDs react so quickly, they don't see the average voltage, they see the pulses of 12 volts.

[Chrissaf]

H&M Clipper, is not a pure DC output controller. My understanding of the Clipper internals is that they use a wire wound rheostat to crudely control the current to the track. Thus the results of your test might not be able to be totally relied upon.

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To be honest, I can't think of any DC Analogue controller brand that gives out pure DC. Hence the recommendation that the test would only be meaningful if using a lab bench grade variable power supply.

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However that said, you wrote:

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"The lights came on at a certain point on the dial, and then went up a stage to full brilliance as the power was increased - not a gradual increase."

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The fact that the brightness increase was not gradual infers to me that the coach lighting control board does indeed include a 'constant brightness' driver circuit in it.

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Tamiya Clear Yellow Lacquer sounds ideal.