Wiring White & Red SMD LEDs

[RAF96]

I managed to put all the resistors on a bit of vero mounted amidships with three wires leading off to the socket, three each way to the leds at each end. The leds were also mounted on a bit of vero and assorted link wires soldered in to link the anodes and cathodes.

 

To get the regular leds to line up correctly with the body holes I pressed a piece of clear plastic packaging behind the holes and marked it with a pen, then drilled exact holes at these marks and used that to hold the leds accurately spaced whilst I bent their legs to suit the vero holes. After the leds and links were soldered to the vero the plastic was cut off to allow the leds to fit snug in the body holes.

 

I will try a similar trick at the other end using smd leds.

 

Tip - whilst you are buying a breadboard get a box of assorted link wires. Invaluable.

Rob

[ISW]

 

 

I managed to put all the resistors on a bit of vero mounted amidships with three wires leading off to the socket, three each way to the leds at each end. The leds were also mounted on a bit of vero and assorted link wires soldered in to link the anodes and cathodes.

 

To get the regular leds to line up correctly with the body holes I pressed a piece of clear plastic packaging behind the holes and marked it with a pen, then drilled exact holes at these marks and used that to hold the leds accurately spaced whilst I bent their legs to suit the vero holes. After the leds and links were soldered to the vero the plastic was cut off to allow the leds to fit snug in the body holes.

 

I will try a similar trick at the other end using smd leds.

 

Tip - whilst you are buying a breadboard get a box of assorted link wires. Invaluable.

Rob

Rob,

 

If you have a diagram or photo of your veroboard I'd like to see it. I'm sure I could 'steal' some ideas off it ...

 

"... get a box of assorted link wires. Invaluable." Hmmm, thought about that when I bought the breadboard, but the 'pack' was too big and too costly. Mistake? Anyway, I have a few metres of 6-core solid wire cable that I hope I can use instead.

 

Ian

Edited March 14, 2018 by ISW

[ISW]

It doesn't. The blue wire is directly connected to the positive of the rectifier on the chip, meaning that its voltage depends on how much the DCC set puts out on the track.

 

Decoder rectifiers are generally rated for at least 2 Amps, more if the decoder is aimed for larger motors. Remember that they need to handle all current used in the chip: motor, sound (if present) and all function outputs. That adds up, even if individual outputs don't offer that much current.

Thanks for the clarification. I'm 'sloooowly' getting the hang of the DCC thing!

 

Ian

[ISW]

If you want to modify an existing Vero board the following should do the trick

 

You will need to make 4 cuts in the Vero tracks & bridge these 4 gaps with 4 resistors, 1K or there about should be fine for proof of concept

 

You can remove & bridge out the 2 existing resistors or I suspect it will work ok with them in circuit (the LEDs will be a bit dimmer but this may be acceptable)

 

 

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a 5-10 minute job once the board is out of the loco

 

My previous Vero board layout was IMHO a better layout but modifying your existing board will only cost 4 resistors, a bit of time & use an existing board.

 

John

John,

 

That's brilliant - saved me making 'another' veroboard layout! But, before I get into that, I'm going to 'duplicate' it on a breadboard for my own peace of mind and learning experience.

 

Ian

[BWsTrains]

I would not recommend paralleling different coloured LEDs.

Why not? please explain your reasoning very clearly!    

Edited March 14, 2018 by BWsTrains

[Dagworth]

Why not? please explain your reasoning very clearly!

An LED and a resistor in series act as a voltage divider circuit, the potential difference across the LED itself when lit will be its voltage requirement, it has a proper name but I can’t think of it right now, I’m in the middle of a gig! The voltage across the resistor will be the supply voltage minus the LED voltage.

 

Different coloured LEDs have different forward voltage requirements. If they are in parallel, with only one resistor supplying both LEDs then only the one with the lowest voltage requirement will light, and it will bring the voltage across the others to its - and not theirs - voltage requirement. The other LEDs will not get enough voltage to bring them into conduction.

 

The only way to stop this is by having LEDs of different colours with a resistor for each colour. That way each LED/ resistor set acts as its own voltage divider across the full supply and gives each LED the voltage it needs to light.

[RAF96]

No picture of the resistor side of the smd board as it is stuck inside the body and the wire side is partly hidden by the window insert, but I have a hand-scrawled diagram of it. Note that the red wires swap ends to marry up with the whites at those ends.

 

Also pictures of the front and back of a regular led lighting board.

 

It may be of some help.

Rob

 

Edit - sorry but the iPad has inverted the pictures.

 

SMD Resistor board

 

Resistor board diagram

 

Front of lighting board

 

Back of lighting board

Edited March 15, 2018 by RAFHAAA96

[ISW]

Many thanks for all the assistance offered by the respondents to this topic, in particular John & Rob who finally lead me to the solution.

 

My first task was to prove it to myself that it worked, so I built the wiring on a breadboard as below.

 

I was most relieved to see both the red and white SMD LEDs lighting at the same time. The power supply was 12v (4xAA batteries).

 

With this success, I modified my veroboard as per the final diagram below.

 

My veroboard was modified accordingly and installed inside the Mainline Class 5 chassis as below. Yes, that is a layer of insulation (acrylic sheet) under the veroboard to avoid shorting on the steel 'ballast' weights.

 

And, wonders will never cease, I had the power the correct polarity at the first attempt, and the white LED lit at the front with the 2x red LEDs at the rear. No need to swap the red / black wires from the track (yes, I know the black is yellow in the model).

 

Still plenty more to do on this model including; install the white LEDs in a 'defuser' behind the headcode boxes, cut out the headcode boxes in the body & install a '0000' headcode, fit the Craftsman 'detailing pack' for the bufferbeam and install kadee couplings. The pickups could also do with duplicating on the power bogie. I've already made cab interiors from styrene cutting the parts on a Cricuit die cutting machine.

 

I know it's a lot of faffing on a old Mainline model when I could just go out and buy a (better) Bachmann one, but where's the fun in that! The 'next' model will be so much easier having solved most of the problems on the Class 45.

 

Ian

[BWsTrains]

An LED and a resistor in series act as a voltage divider circuit, the potential difference across the LED itself when lit will be its voltage requirement, it has a proper name but I can’t think of it right now, I’m in the middle of a gig! The voltage across the resistor will be the supply voltage minus the LED voltage.

 

Different coloured LEDs have different forward voltage requirements. If they are in parallel, with only one resistor supplying both LEDs then only the one with the lowest voltage requirement will light, and it will bring the voltage across the others to its - and not theirs - voltage requirement. The other LEDs will not get enough voltage to bring them into conduction.

 

The only way to stop this is by having LEDs of different colours with a resistor for each colour. That way each LED/ resistor set acts as its own voltage divider across the full supply and gives each LED the voltage it needs to light.

Dagworth, spot on but if you re-read the sweeping statement I was pushing back against in the first place. To repeat Suzie's exact words

 

"I would not recommend paralleling different coloured LEDs." 

 

unqualified, unsubstantiated and incorrect. "paralleling" if such a word exists implies ANY parallel wiring arrangement not just specifically a branch formed off a single resistor. So statement is clearly not true for reasons you have explained.

 

My point to the original commentator was just a hopeless crusade against the airing of incorrect and unsubstantiated statements in this place. 

[Dagworth]

The power supply was 12v (4xAA batteries)

Ian

4x AA is 6 volts. You may find you need to adjust resistor values to suit track voltage

 

Andi

[Phil S]

Whenever wanting to make judgements about LED lighting levels, I would recommend buying a small, self contained LED tester with its own internal 9v battery.

 

Whilst it is available try getting one of these from maplins (£5) https://www.maplin.co.uk/p/led-tester-n71au. ...battery now extra

 

A small box barely larger than the 9v battery inside it, and with a graduated series of current terminals from 1mA to 50mA. With 2 10mA outputs to allow direct comparison of 2 LEDs.

 

I have found this to be more accurate than cheaper models in white plastic cases..... Each contact shows the chosen current and allows you to choose the brightness you want. ... (You are then left to calculate a suitable resistance for your voltage, of course.)

 

 

(The other, not so good type, has similar markings, but these do not seem to be accurate when using differing LEDs with higher vfd... Giving the impression that the box simply contains resistors and not a regulated current as labelled)

...

For use with smaller smd sizes it would need a small adapter board made from strip board , but larger smd LEDs will still work as the contacts are on the surface. ...again better than some other types

Edited March 16, 2018 by Phil S

[John ks]

Regarding the LED tester in the previous post I got one from Jacar in Australia about 10-20 years ago, it looks identical to the Maplin one

If I recall correctly it only contained resistors to limit the current

 

One of the best AU$10.00 I ever spent

 

John

[ISW]

4x AA is 6 volts. You may find you need to adjust resistor values to suit track voltage

 

Andi

Andi,

 

Oops. Couldn't agree more.

Thankfully, 12v supply isn't too bright and my 'test' was worth it.

 

Ian

[Suzie]

Why not? please explain your reasoning very clearly!    

 

It is very simple - they will have different forward voltages so will not share the current evenly.

[BWsTrains]

It is very simple - they will have different forward voltages so will not share the current evenly.

Suzie,

 

1) read Dagworth's post #32 which explains clearly why you're wrong. You need to understand the principles here before making blanket and incorrect comments.

 

Parallel means what it says, multiple lines from one common voltage source.

What happens on one line cannot and does not affect what happens on another in this configuration.

All you do is choose correct resistor for each line to get the required current across each LED

Voltage drop across each line is the same by definition as they share the same + and - connections

 

Sequential Voltage drop across Resistor then LED will be different each line according to the components connected .

[Crosland]

Suzie,

 

1) read Dagworth's post #32 which explains clearly why you're wrong. You need to understand the principles here before making blanket and incorrect comments.

 

Parallel means what it says, multiple lines from one common voltage source.

What happens on one line cannot and does not affect what happens on another in this configuration.

All you do is choose correct resistor for each line to get the required current across each LED

Voltage drop across each line is the same by definition as they share the same + and - connections

 

Sequential Voltage drop across Resistor then LED will be different each line according to the components connected .

 

What Suzie says is correct, when read in context.

 

Different colour LEDS in parallel, with a common resistor, will not work as expected. The one with the lower VF will clamp the voltage and prevent the one with the higher Vf from lighting.