Colour light polarity

[reevesthecat]

Hi

I have started to put colour light signals on my layout but only have 2. They are by different manufacturers and are just basic red/green led lights.

I have set them up with a Hornby controller using the track feed as it is 12v and as far as I can tell wired as per instructions.

The issue is one works when the polarity is switched one way and the other when the polarity is switched the other way. I have tried switching feeds around etc with no effect. I have also tried a 12v power pack from some garden lights which is not able to be direction controlled with no success.

Can anyone say where I have gone wrong?

Thanks

Mark

[Suzie]

It sounds like you have signals with LEDs, and one is wired common cathode, while the other is common anode. Manufacturers do vary in how they wire their signals.

[reevesthecat]

Thanks Susie

Yes they are LED's, does that mean I cannot do anything about it and I should just standardise on 1 make as they should be wired the same

Thanks

Mark

[DCB]

If the signals have 3 wires as a bodge fit diodes in line with each signal  common wire so they work on DC and then use AC.

 

If they have two wires and rely on polarity reversal to change from Red to Green then there is not a lot you can do without a complete rebuild or one of the signals

 

Better idea use one manufacturers signals.  I have problems in that green LEDs need different resistors to Red LEDs but using ready made signals should solve this.

[Crosland]

I have problems in that green LEDs need different resistors to Red LEDs but using ready made signals should solve this.

 

The forward voltage (Vf) of typical red and green LEDs is so close (0.2V difference) that the same resistors can be used. What you are seeing is differences in percieved brightness that can just as easily occur between different red LEDs or different green LEDs, nothing to do with the colour, per se.

[reevesthecat]

Thanks for the replies will go for the self build train tech signals they are cheap enough at 8 quid

Cheers

Mark

[russ p]

I had a garden lighting transformer and it was 12AC is the AC affecting what you want to achieve?

[kevinlms]

Hi

I have started to put colour light signals on my layout but only have 2. They are by different manufacturers and are just basic red/green led lights.

I have set them up with a Hornby controller using the track feed as it is 12v and as far as I can tell wired as per instructions.

The issue is one works when the polarity is switched one way and the other when the polarity is switched the other way. I have tried switching feeds around etc with no effect. I have also tried a 12v power pack from some garden lights which is not able to be direction controlled with no success.

Can anyone say where I have gone wrong?

Thanks

Mark

I'm confused Mark. You state.

 

'The issue is one works when the polarity is switched one way and the other when the polarity is switched the other way.'

 

So exactly what happens with the red/green LED's on Signal 1, with the two different settings? Are these repeated on the 2nd signal?

 

As others have hinted at, are they two legged or 3 legged LED's?

[Suzie]

Sticking to one make will probably make life easier. There is no reason why you cannot wire one up one way, and the other one the other way if you are just controlling them with switches.

[reevesthecat]

I'm confused Mark. You state.

 

'The issue is one works when the polarity is switched one way and the other when the polarity is switched the other way.'

 

So exactly what happens with the red/green LED's on Signal 1, with the two different settings? Are these repeated on the 2nd signal?

 

As others have hinted at, are they two legged or 3 legged LED's?

Hi thanks for your interest.

Both signals have 3 wires (2 for the switch and 1 for the negative feed). When the polarity is switched the other signal is dead no matter which way it is switched one of them is always dead. I have tried swapping around the wires with no change in behaviour. It's really strange. Have ordered another train tech signal to see if that works with the other and if so will standardise on these.

Thanks again

Mark

[reevesthecat]

I had a garden lighting transformer and it was 12AC is the AC affecting what you want to achieve?

Hi Russ

Have tried both a Hornby controller and the garden transformer with the same results

Cheers

Mark

[smokebox]

Hi thanks for your interest.

Both signals have 3 wires (2 for the switch and 1 for the negative feed). 

Mark

I think that statement is the cause of your problem.

 

On the signals that work properly, 2 for the switch and 1 for the negative feed is correct but on those that don't work properly you need to reverse the polarity so that you have two wires supplying a negative feed from the switches and a single positive return wire.  That's the difference between common positive and common negative.

[Izzy]

If they have two wires and rely on polarity reversal to change from Red to Green then there is not a lot you can do without a complete rebuild or one of the signals

 

Where just two inputs are needed and reversed polarity a crosswired DPDT switch as used for direction control on DC controllers is all you need.

 

Izzy

[Suzie]

One has a common negative (common cathode) and you have to switch the positive, while the other has a common positive (common anode) and you have to switch the negative, that is how they differ.

[kevinlms]

Hi

I have started to put colour light signals on my layout but only have 2. They are by different manufacturers and are just basic red/green led lights.

I have set them up with a Hornby controller using the track feed as it is 12v and as far as I can tell wired as per instructions.

The issue is one works when the polarity is switched one way and the other when the polarity is switched the other way. I have tried switching feeds around etc with no effect. I have also tried a 12v power pack from some garden lights which is not able to be direction controlled with no success.

Can anyone say where I have gone wrong?

Thanks

Mark

Is this the instructions you are attempting to follow for your wiring?

 

http://ccgi.dcpmicro.plus.com/traintech/pdf_manuals/sk2_manual.pdf

 

Also what sort of switch have you used?

[reevesthecat]

Is this the instructions you are attempting to follow for your wiring?

 

http://ccgi.dcpmicro.plus.com/traintech/pdf_manuals/sk2_manual.pdf

 

Also what sort of switch have you used?

Hi

Thanks yes they are the instructions the switch is a expo A28012 on/on switch

Thanks

Mark

[RAF96]

The forward voltage (Vf) of typical red and green LEDs is so close (0.2V difference) that the same resistors can be used. What you are seeing is differences in percieved brightness that can just as easily occur between different red LEDs or different green LEDs, nothing to do with the colour, per se.

 

 

I use bi-colour LEDs as points indicators and when these were used at a cross-over (a pair of points in opposition) I expected to see both show amber if the pair of points failed to synch, due to both sides of the leds being powered at the same time, but until I gave each side of the led its own resistor this didn't happen, so the forward voltage is important in this scenario, although a single common resistor will work fine for just red or green selection.

[Brian]

Hi

 

Not all signals have the same connection requirements..

Train-Tech SK2 or SK3 signals use common Cathode (Negative) wiring, but some other makes can use common Anode (Positive) wiring.

 

For wiring the Train-Tech signal take a 12 volt DC power source and connect the Positive output to the toggle switches central tag.   Connect the outer two tags of the switch to the Train-Tech signals bottom and middle solder pads.   Then solder one end of the series resistor to the top Common Return pad on the signal.. Connect a wire to the other end of the resistor and it goes to the 12v Negative connection.  By flipping the toggle switch one way the red (or yellow) aspect will be displayed and in the opposite direction the Green aspect will be displayed.. 

Do not wire the signal to 12 volts DC without the series resistor.

Ideally Do not use AC for the feed.

 

Does this one signal now work?  

Yes, it works correctly... All is OK

No, it still doesn't light, you may have blown the LEDs or you have a poor connections somewhere. Recheck wiring. 

 

If you have a second signal from another manufacturer you will need to find out if that signal uses Common Anode or Common Cathode wiring?    

To prove, connect the series resistor to the signals common wire/pad.. Then try on the 12v DC the signal by temporarily connecting the other end of the resistor to the Positive of the 12v supply and either one of the red/green wires onto the Negative. Does the LED light?

Yes, your signal is Common Anode (Positive).

No it doesn't light. Swap the resistor wire that is on the 12v supply positive over to the 12v supplies negative terminal and try the red or green wires onto the Positive terminal. Does each LED light now? If yes its Common Cathode.

 

You cannot operate a common Cathode signal and common Anode signal from the same Single Pole Double Throw (SPDT) switch!  

But you can operate the two if they are the same style.  Simply then connect each signal to the switches outer tags and each of the signals return wire/tag to its own series resistor. The other ends of the resistors runs back to the 12v power supply negative terminal.

 

You can by using A DPDT On/On toggle switch wire it so as one half of the switch operates a common Cathode signal and the other half of the switch works a common Anode signal, but more care will be need in getting the wiring correct.

[Crosland]

I use bi-colour LEDs as points indicators and when these were used at a cross-over (a pair of points in opposition) I expected to see both show amber if the pair of points failed to synch, due to both sides of the leds being powered at the same time, but until I gave each side of the led its own resistor this didn't happen, so the forward voltage is important in this scenario, although a single common resistor will work fine for just red or green selection.

 

Different problem. If you use a single common resistor and then apply power to both LEDs then there are two issues:

 

(1) You have more current flowing through the resistior (the combined current from both LEDs) and thus the voltage drop across the resistor increases, reducing the voltage available for the LEDs.

 

(2) One LED is clamping the voltage across the other which may cause one LED to be dimmer.

 

The best practice [1] with LEDs connected in parallel is is to ALWAYS have a resistor per LED. This is a good example.

 

 

 

[1] Those who eschew "best practice", and want to accuse others of "having a go" or "scaring and confusing", when promoting it, need not reply

[RAF96]

Different problem. If you use a single common resistor and then apply power to both LEDs then there are two issues:

 

(1) You have more current flowing through the resistior (the combined current from both LEDs) and thus the voltage drop across the resistor increases, reducing the voltage available for the LEDs.

 

(2) One LED is clamping the voltage across the other which may cause one LED to be dimmer.

 

The best practice [1] with LEDs connected in parallel is is to ALWAYS have a resistor per LED. This is a good example.

 

 

 

[1] Those who eschew "best practice", and want to accuse others of "having a go" or "scaring and confusing", when promoting it, need not reply

 

Agreed Xland - as I found out by trial and error.

Using a common resistor hoping to produce an amber/yellow meant I actually got a red it being the 'stronger' led element.

 

To be spot for colour and resistor value for this, it would mean using a pair of pots in the outer legs to trim resistor values to exactly those required to give a proper amber/yellow in crossed points conditions, then pick the nearest standard value from the range.

 

Hence I always use 2 resistors on 3 legged leds.

[dhjgreen]

(1) You have more current flowing through the resistior (the combined current from both LEDs) and thus the voltage drop across the resistor increases, reducing the voltage available for the LEDs.

Sorry Crosland, I tried to leave this but it is not correct.  

 

The nominal volt drop across an LED is 2v, if we are using a normal 12 volt DC supply, that leaves 10 volts to drop across the series resistor.  R=V/I so for a 10mA flow we need 10/.01=1000ohms.  A second LED in parallel with the first will not change any of this ( the second LED still being nominally 2v, but as has been stated, not exactly the same).

 

From your contributions elsewhere on here I think you already know this, but for other users I was just clarifying.

[Crosland]

Sorry Crosland, I tried to leave this but it is not correct.  

 

The nominal volt drop across an LED is 2v, if we are using a normal 12 volt DC supply, that leaves 10 volts to drop across the series resistor.  R=V/I so for a 10mA flow we need 10/.01=1000ohms.  A second LED in parallel with the first will not change any of this ( the second LED still being nominally 2v, but as has been stated, not exactly the same).

 

From your contributions elsewhere on here I think you already know this, but for other users I was just clarifying.

 

Just testing if anyone was awake

 

Yes, my description is wrong (not sure what I was thinking of) but the nett effect is the same (change in brightness of two LEDs v either LED alone).

 

The current will be split between between the two LEDs, each LED has half the current (5mA) and the brightness will halve (from a fairly typical LED data sheet). The forward voltage of each LED will reduce slightly (approx 0.1V).

 

Andrew

[DCB]

The light output (mcd) at the same vf of a typical red and a typical green LED from the same range varies dramatically, between 10X and 100X so you are looking at a vf of 2 for red and 2.2 for green. While this is not a wide difference between resistors on a "12 volt" supply (often actually 19 volt) the alternative is an excessively bright red and a dim green.  Personally I like to start with 2.5 volts which makes for a larger difference between the two resistors. 

[Gordon H]

The light output (mcd) at the same vf of a typical red and a typical green LED from the same range varies dramatically, between 10X and 100X so you are looking at a vf of 2 for red and 2.2 for green. While this is not a wide difference between resistors on a "12 volt" supply (often actually 19 volt) the alternative is an excessively bright red and a dim green.  Personally I like to start with 2.5 volts which makes for a larger difference between the two resistors. 

 

I suspect you mean at the same 'If', not 'Vf', as 'Vf' is determined by the LED chemistry to produce the various colours. From a 12V supply, the 'If' difference between a LED with 2.0V 'Vf' and 2.2V 'Vf' is virtually irrelevant if they are fed independently via their own identical resistors. Their perceived brightness may well be different for the same 'If', not only because one type might be more efficient than the other, but the eye's perception of brightness varies with colour too. As has been said many times before, with LEDs you need to be considering the current they pass, not the voltage across them.

[Crosland]

The light output (mcd) at the same vf of a typical red and a typical green LED from the same range varies dramatically, between 10X and 100X so you

are looking at a vf of 2 for red and 2.2 for green.

This is demonstrably wrong from checking a few LED data sheets. 1.5x to 2x is more likely.

While this is not a wide difference between resistors on a "12 volt" supply (often actually 19 volt)

Using a crappy unregulated (and probably unsmoothed) output from an old DC controller is probably the cause of your confusion. My 12V supplies are 12V +/- a few percent.

 

the alternative is an excessively bright red and a dim green.  Personally I like to start with 2.5 volts which makes for a larger difference between the two resistors.

You have also told us how you like to use LEDs with no resistors so any advice you give on the use of LEDs can safely be ignored