Adding a lighting circuit back

[hedley_reynolds]

Hi it’s just me again 

I have been looking at Chris’s wiring diagram for a TTS decode lights  anyway I have an n gauge loco and the 6 pin socket was loose and the loco out of warranty so here’s my question will the resistors be the same value for n gauge and 00 gauge also there’s only one of each wire to ei end and they’re white at t top ie roof and yellow in the middle and blue at the bottom so I’m going to make a small circuit board for the resistors and wiring from a 6 pin harness for my decoder.

 I would be very grateful once again for any help with the wiring and the board as in the TTS diagram shows 4 resistors and as far as I can see I only have 2 on the old board as I have to use a magnifying glass for to see them and I had fried the circuit board so I can’t use it for only the lights 

thanks very much again Hedley 

 

[Chrissaf]

The gauge makes no difference to LED resistor theory. The resistor is a theory of physics and electronics, not model railways.

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In an ideal world I would prefer to have a single resistor per LED, hence why I showed 4 resistors. However you can share a resistor (changing its ohmic value) amongst more than one LED with a caveat. And that caveat is that you don't mix LED colours sharing the same resistor.

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Different LED colours have different forward bias voltages. For example and typically, white LEDs have a forward bias voltage of between 2.7 and 3.2 volts, whilst red LEDs have a forward bias voltage of between 1.5 and 2.1 volts.

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This graphic has been taken off the Internet and shows an example of how different LED colours have different forward bias (Vf) voltages.

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/media/tinymce_upload/4d6317d88b01bd1ac465c503fd123dea.gif

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Now if you put a white LED and a red LED in parallel and they both share the same current limiting resistor. The voltage across BOTH the white and red LED will stabilise at the lowest forward bias voltage that turns on one or other of the LEDs. In this case the red LED. This would typically be about 1.7 to 2 volts. This is going to lock the voltage across the white LED to same voltage, but 2 volts is not high enough to reach the white LED switch on voltage. Thus in the circuit drawn below on the left, the red LED will light, but the white LED will remain off. Whilst the drawing on the right shows LEDs of the SAME colour sharing a resistor perfectly OK.

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To overcome this pure physics limitation, you have to group together LEDs that have basically the same forward bias voltages (taken from the LED data sheets) and they can then share the same current limiting resistor. But doing that, means that the resistor value needs to be amended to provide the same amount of current for the LEDs. Say for example, you are designing the LED circuit for 10mA per LED on a 12 volt supply. A white LED we assume will have a forward bias voltage of 3 volts. Therefore we want to drop 9 volts across the resistor at 10mA. Ohms Law would calculate the resistor value as R=V / I where I = current in Amps this equals R=9 volts / 0.01 Amps = 900 ohms, hence the fairly typical 1K Ohm resistor one sees used with LEDs. But put two white LEDs in parallel and we now need 20mA (10mA per LED) thus the resistor value now becomes R=9 volts / 0.02 Amps = 450 ohms. But if one of the LEDs goes open circuit, the remaining LED is now getting 20mA instead of 10mA. This starts to get even more complicated if the resistor is shared across more than two LEDs and its value reduced even further below 450 ohms, because if one or more LEDs in a shared group go 'open circuit' the remaining LEDs are taking more and more current, and could eventually burn out.

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Thus in conclusion, to eliminate all these complicated design decisions and Ohms Law calculations and potential damage to LEDs it is just easier to use an individual resistor per LED.

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PS - Notice how in the LED Vf graph graphic the LED Vf voltages below 20mA current are grouped together by colour. The blue, white & green LEDs to the right could potentially share a resistor and the red, amber & yellow LEDs on the left could potentially share a resistor. This might possibly account for your board having just two resistors.

[hedley_reynolds]

Hi Chris 

thanks very much again for your help and advice so will I need to put a resistor on each wire? on both ends but splitting the feed from the decoder into 2 I was never any good at school and I think that the teachers kept me away from this just in case I blew up the school so I need you to desperately put it in layman’s terms and let me know how to actually do it as at the minute I’m totally lost on how it goes but I’ll keep on reading your reply and I’ll see if it clicks 

but thanks very much again for everything Hedley 

[Chrissaf]

Hedley, I will help you if I can, but first I need to fully understand the issue. Can you expand on exactly what you meant when you wrote:

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"also there’s only one of each wire to ei end and they’re white at t top ie roof and yellow in the middle and blue at the bottom"

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So what I need to know is:

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What colour LEDs (I am assuming the loco lamps are LEDs and not GoW bulbs) and in what quantity are at each end of the loco?

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Are they all directional, that is to say, the different LEDs light up subject to loco direction?

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If some of the LEDs are not directional, what are they supposed to be representing. Such as 'Cab Lights' or Header code back lights etc?

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I appreciate that as it is not a working loco, you may not have the detail, but you need to give me clue as a fighting chance if I am going to be able to help. If at all possible can you post some photos of the loco interior, that will help a lot.

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EDIT: Read your text a few more times. Is this what you were trying to describe in the yellow quote text box?

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/media/tinymce_upload/ed4aa0ca172ea7d60675ca50ca86b8a4.jpg

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If yes, then I really need to know a bit more about this burnt out (original) circuit board you spoke of with the two resistors on it. Is it at the end(s) (both?) of the loco or somewhere in the middle. Are the end LEDs on their own little circuit boards, are there any tiny micro (SMD Surface Mount Device) resistors near the LED locations. Some close up photography will really help, particularly at the ends where the LEDs are mounted. As I don't have the benefit of being able to see things through your eyes.

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It's one thing to draw a circuit diagram for a circuit that is effectively 'new build' i.e adding lighting to something that never had it before. But a circuit diagram to repair an existing installation that has been damaged is a completely different thing. One needs to understand what is currently there and how it was originally wired before one can add additional new components to get it functioning again. That's hard enough when the model is on the workbench in front of one, it is 10 times harder when the model in question is not in one's possession.

 

[pidder]

Different colour LEDs can be put in series satisfactorily. I have done this to overcome the problem of parallel LEDs not working properly, specifically in some sets of red/green signal lights.

[Chrissaf]

Yes you are absolutely right Pidder, you can put them in series. But in the case of a loco directional light wiring, the white LEDs at the front would have to be in series with the red LEDs at the rear AND the white LEDs at the rear would have to be in series with the red LEDs at the front. It does make the loco wiring a little bit more convoluted to achieve.

[hedley_reynolds]

Hi Chris and Pidder 

thanks very much again right the wires went into a 3 terminal connector block under the circuit board and then the 3 wired plug went into a black square box ( I think that it’s the best way to describe it ) and the wires were soldered onto 3 very fine pins at the back of the block also I think that it could be an SMD and the wires were white at the top now this is where I have problems ( and another reason for not being allowed anywhere near the electric or electronics at school  ) the middle wire could be either red or orange in the middle and either blue ( dark blue ) or purple on the bottom  ( the reason for the colours is that I’m colour blind and my son’s aren’t here to help me ) also I had tried to join a 6 pin socket ( where the decoder plugs into ) onto the broken pins and I was using a blanking plug for to try it on DC before I tried the decoder and it started to smoke so I took it straight off the track and I think that I had shorted it over the tracks when I soldered it so I was going to hard wire the decoder after I tested  the motor for to see if it’s ok and I was going to wire in the lights as you have white going forward and red at the rear and then when you put it in reverse the lights automatically swaps over

but thanks very much again and it’s greatly appreciated Hedley 

[Chrissaf]

I really am going to need to see some photos Hedley, the text descriptions are just not working for me at all I'm afraid.

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One of the things about 6 pin decoders, is that they do not generate the Blue +ve return like the 8 pin decoders do. For a lighting circuit to work using a 6 pin decoder, you need to have some rectification diodes or a rectifier diode bridge installed externally to the decoder to generate the +ve return wire. Now these components may have been on the this damaged circuit board you spoke of, they may be part of this black square box. If I had the model in front of me, everything would probably be obvious to me and instantly recognisable, but I'm sorry, I just can't visualise what is what from the text descriptions.

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In the meantime, this drawing might help......or maybe even answer your question completely.

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This drawing will only be applicable if you were installing all the wiring and components completely afresh in a loco that didn't previously have lights. A 'Green Field Site' so to speak rather than your 'Brown' one. But hopefully, by comparing it to what you have got, it will help you to come up with a re-wiring plan. Trace back your existing Blue wire to see where it goes, it may go to existing components or board providing the function of the two 1N4001 diodes in my diagram. If it does, then use that part of the existing circuit instead. I have included a small photo of a 1N4001 to show the physical orientation of the component matched to the logical drawing representation of it.

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PS - As you say you are colour blind I have labelled all the colours individually.

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[hedley_reynolds]

Hi Chris 

thanks very much again and I’ll photograph the board top and bottom when I take it off but I’ll leave it connected to the wires that are underneath it and I’ll also send you the top shell of the loco showing you the box part that I’m going on about tomorrow afternoon also how do I post the photos I know that it’s in the panel just above where you put the text is it the one on the left of the smiley or the one of the tree 

thanks very much again for everything Hedley 

[Chrissaf]

The Black & White hill icon between the Tree and the Smiley. Note that photos are held back for authorisation and won't appear straight away, they are checked by Adam the Hornby administrator during normal UK office hours only. Photos need to be .JPG or .PNG file format and preferably not more than about 2MB.

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Note that I have now added a diagram as an edit to my previous post, so scroll up to review the changes.

[Chrissaf]

Addition to my previous drawing post:

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This is what it would look like if wired as per Pidder's suggestion.

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[hedley_reynolds]

Hi Chris 

thanks very much again and I can see the difference with both diagrams and I’ll send the photos of the circuit board on both sides and the light but I have just had a very good and close look and it looks like it’s a square piece of circuit board and would that help any ? but I’ll take a very close up photo and I’ll upload them to the post tomorrow on my laptop and I’ll use a proper camera instead of my phone and you’ll be able to go from there also the very fine wires came away from the plug that goes into the loco roof so I have wired another one on so I didn’t lose track of the connections until I sort out the decoder and board issues first also I have contacted dcc supplies for to see if they have a circuit board and then I can do a swap but if they don’t do them then I’ll do the hard wire with the decoder also how I can tell usually with the colours is by shades and the normal colours like red black blue yellow and white are ok it’s just like orange and purple is where I have to either ask for them or putting the 2 together and then I can usually tell and also the NMRA has helped by putting the orange next to the grey then the usual red then black so if I look at the wiring diagrams on the likes of this site and the NMRA and dcc concepts as they show which pins are which colours and I can work it out like that 

but thanks very much once again for your patience, time and help Hedley 

[Fishmanoz]

Hedley, just a quick note in this discussion I’m following with interest - if you use a modern camera, you’ll almost certainly exceed the 2MB size limit for an image on the site so may have to edit them down to post.

[hedley_reynolds]

Hi Fishy

I was going to use my camera and then edit them in my photoshop software down to the 2mb and then upload them to the forum at the same time unless I can work out how to do it again on my I pad and then upload them as I did that last year when I had to send a copy of a receipt when I had problems with my laptop  and I’m sure that was 2mb as well but either way I’ll figure it out 

but thanks again Hedley 

[96RAF]

Hedley

On the iPad open the photo and use the square with an up arrow icon to send it to the cloud.

Then when uploading to here via the black and white icon above, select choose picture then browse from the dialogue instead of library and go to the cloud to get that picture.

 

Easier to actually do than it is to say.

In the best Blue Peter tradition here’s one I did previously

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Rob

[2e0dtoeric]

RAF - that's not Blue Peter!  😛

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As Hedley is really struggling, perhaps, if we knew where in the country he is, someone can arrange via Adam to get his address and call round?

[hedley_reynolds]

Hi Rob and Eric 

Rob what happened to the step by step numbers like the ones you get on the painting by numbers also Eric you have a better idea for Adam to come to Geordie land and do it for me anyway thanks Rob I’m in the process of going to do it now with your steps so fingers crossed I’ll get them on this afternoon Hedley 

[hedley_reynolds]

Hi Chris, Rob and Eric 

here goes anyway Chris I had noticed that when I had took the board off one of the wires was off but I don’t know if that’s why I had bother unless it came away when I had lifted the board up but it doesn’t look like it had been soldered to the board but thanks very much again to you all Hedley 

[hedley_reynolds]

Hi Adam I’m sorry I have hit the wrong photo can you please delete it thanks Hedley 

[hedley_reynolds]

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[hedley_reynolds]

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[hedley_reynolds]

Hi all

I have just managed to upload the photos and fingers crossed Adam will put them on as I don't know if I have done it right thanks very much again Hedley

[hedley_reynolds]

Hi Rob and Adam

it wouldn’t let me do it the way you had said Rob it kept on saying that the file was not allowed so I went back to basics and used my kindle and that worked also thanks very much to Adam for putting them on so quickly as it was nearly finishing time for him when I eventualy got them to send 

but thanks to everyone for your help and advice Hedley 

[hedley_reynolds]

Hi all

I have just realised that when I had problems with programming a loco and I was asking about the ESU decoder tester this was the one that I had problems with from the beginning and I wonder if the circuit board and the decoder socket was faulty which was causing me the problems Hedley 

[Chrissaf]

Looks like you had a fire in that loco 😮 what a load of damage. That PCB has got so many scorch marks on it, I doubt it is salvageable. But it doesn't look to contain anything super critical on it.

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Diodes D1 and D2 (just pure guesswork) are most likely performing the function of my 2 x 1N4001 diodes in my last two circuit diagrams. They are generating the positive return voltage for the lights. L1 & L2 are most likely motor suppression or power smoothing inductors and should not be super critical to retain or replace in a new circuit. The four 1,000 ohm resistors are most likely doing the same function as my four resistors shown in my original 6 pin decoder drawing. What the two 2,200 Ohm resistors are doing is anybodies guess, but my guess is that they are part of the lighting circuit.

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Given the state of the board in general, and the melted state of the plastic around the end cabs giving the impression of soldering with a blow torch 😆. I appreciate that N gauge locos require microscopic tools, steady hands and better than perfect eyesight. Then personally in your position, I would remove the PCB completely, and wire a completely new circuit as per my first 6 pin decoder drawing on Page 1 and implement all aspects of my design including the 2 x 1N4001 diodes. Just wire in a 6 pin decoder socket in a harness.

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Now with regard to the 3 pin white connector at each end of the board. Given that they have Blue, Yellow & White wires on them, they are almost certainly the connectors that provide wiring to the LEDs in the end cabs. Being on a plug and socket means that the body of the loco can be unplugged from the chassis when dismantled.

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If you want to replace them, then these white connectors are bog standard JST connectors. These ones in this link are pre-wired. They are the same connectors as used in your loco, but pre-wired with wire colours for radio controlled model servos. You can buy them un-terminated, I have some, but you need special crimping pliers to fit the terminal pins to the wires. It will just be easier for you to buy the pre-terminated variety and just work within the constraints of the different colours. Red, Black & Yellow. The socket part could go on any circuit board you make to replace the damaged one, the new board could have the new resistors on it too. To keep components small, resistors with a 1/8 watt power rating will be fine in this particular circuit.

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