2 Bi-Colour LEDs in parallel: One only works while the other is shorted.

[Lacathedrale]

I'm wiring up a pair of bi-colour LEDs to use in lamps. I know it's not technically correct but I want the LEDs to show red or white depending on the direction of travel, regardless of whether the loco is running light engine.

 

I'm using a remote control system which has two outputs - when the loco is in forward, the orange wire is at +3v and the green wire is at 0v. When in reverse, the opposite is true.

 

 

This is what is expected to happen.

 

What I am ACTUALLY getting though, is nothing at all unless I short out the contacts on the opposite LED. Both LEDs work as expected individually, changing colour as required, etc. when the other is shorted, but not together simultaneously.

 

I feel like this is a really obvious fix but my brain just can't work it out - can anyone help please?

[Mol_PMB]

You have wired them in series, in opposing directions. LEDs are diodes so they block reverse current, so if you wire two opposed in series then no current will get though.

I think you meant to wire them in parallel. Join a long leg to a short leg on each side.

 

Don't forget the resistor!

[Dungrange]

8 minutes ago, Lacathedrale said:

I feel like this is a really obvious fix but my brain just can't work it out - can anyone help please?

 

LEDs only conduct in one direction and the only electrical path that you have between Green and Orange passes through both one in the correct direction and the other in the opposite direction.  As such, I'm not surprised that it doesn't work if you've wired it as per your diagram.

 

I think you need to get rid of the blue wires and make sure that both LEDs are connected to both Orange and Green directly.

[Lacathedrale]

Resistor is there @Mol_PMB ! 330R

 

I see my mistake now thank you both @Dungrange, so now I have both LEDs connected to the orange/green  (orange to LED1 cathode and LED2 anode, and vice versa) and I'm getting the most peculiar result:

 

The red comes on and can be toggled between LEDs by direction switch, but the white doesn't come on at all. I'm getting 1.5v across the legs of the unlit LED (as expected, splitting 3v)

 

If I swap the wires over on one LED then I get both lights changing from white to red depending on the direction switch.

 

Am I being thick!?

Edited July 3 by Lacathedrale

[Mol_PMB]

White LEDs have a higher forward voltage than red ones, 1.5V (or even 3V) may not be enough.

[Edwin_m]

Are these the type of bi-colour LEDs which change colour with reverse current?  If so the circuit as drawn should work but would need more than 3V (it would need at least the total of the red and white voltage drops).  

 

I suggest you wire them in parallel but put a separate resistor in series with each LED.  

[Lacathedrale]

@Edwin_m yes - reverse current changes the colour.

 

 

If I switch BOTH wires around then I get a white LED which can be toggled by direction, @Mol_PMB - so not sure it can be that. The only permutation I can't get is one red and one white!

[Dagworth]

Do you have two resistors? Put one in series with each LED and the both resistor/led pairs in parallel. Then it will work as you want

 

Andi

[RAF96]

The red will swamp the white unless you use separate resistors of appropriate value. The simple way is to use 3-legged bi-colours leds making sure they are positive common anode if for DCC use.

Edit as above - senior moment, brain failure.

Edited July 6 by RAF96
To correct error.

[spamcan61]

2 hours ago, RAF96 said:

The red will swamp the white unless you use separate resistors of appropriate value. The simple way is to use 3-legged bi-colours leds making sure they are positive anode if for DCC use.

For the avoidance of doubt, I think you mean common anode?

Edited July 4 by spamcan61

[Lacathedrale]

@Dagworth they are currently in parallel:

 

 

Although, I wonder if I need to move the LEDs to be on the cathode side of the wire join? I can't see how that would matter though!

 

EDIT: I moved the resistors to the supply-side of the join and it seems to work fine now - my brain hurts.

Edited July 4 by Lacathedrale

[Jeremy Cumberland]

Each LED sees both resistors in series in your last drawing, which almost certainly isn't what you intended.

 

In your first post you wrote:

19 hours ago, Lacathedrale said:

Both LEDs work as expected individually, changing colour as required, etc. when the other is shorted, but not together simultaneously.

Did you have any resistors in circuit then? If not, then I suggest you remove the resistors in your latest circuit and see if this gives you enough voltage to power the white LEDs (or perhaps first remove one resistor, then if that doesn't work, remove the second one)

 

What voltage does the supplier recommend? Do they say anything about resistors?

 

The trouble with these two-leg dual colour LEDs is that you can't add separate resistors to the individual LEDs in each pair, which might be a problem if there is a large mismatch between the two colours. In fact, your original idea of connecting them in series wasn't a bad one, since the circuit would then always contain one red LED in series with one white LED, but you'd probably need something like a 5V power supply to drive it.

[MartinRS]

The solution is to wire them in parallel with one forward biased (for red) and the other reverses biased (for red). They should be controlled by a DPDT switch, which appears to be the case (OP: 'I'm using a remote control system which has two outputs - when the loco is in forward, the orange wire is at +3v and the green wire is at 0v. When in reverse, the opposite is true' so it looks like some sort of DPDT switch is already in circuit). Given that the voltage is 3v then there might not be a requirement for any resistor.

 

The only 2 lead Red-White bi-colour LEDs I have been able to find on the web run at 12v, presumably because they have some sort of voltage dropping device built in. Do you have a any technical specification for the particular bi-colour LED you are using or a link to the supplier's site?

[Edwin_m]

43 minutes ago, MartinRS said:

The solution is to wire them in parallel with one forward biased (for red) and the other reverses biased (for red). They should be controlled by a DPDT switch, which appears to be the case (OP: 'I'm using a remote control system which has two outputs - when the loco is in forward, the orange wire is at +3v and the green wire is at 0v. When in reverse, the opposite is true' so it looks like some sort of DPDT switch is already in circuit). Given that the voltage is 3v then there might not be a requirement for any resistor.

 

The only 2 lead Red-White bi-colour LEDs I have been able to find on the web run at 12v, presumably because they have some sort of voltage dropping device built in. Do you have a any technical specification for the particular bi-colour LED you are using or a link to the supplier's site?

I think that will result in just the red lighting up, as it will light at a lower voltage than the white and prevent the voltage rising any higher. You need a resistor in series with each LED to deal with that (between one side of the LED and the connection to the blue wire).  

[Dagworth]

This is what it needs to be, one resistor in series with each LED and the each resistor/LED pair in parallel.

This prevents the lower voltage requirement of one colour from preventing the higher voltage colour reaching the voltage it needs.

 

Andi

 

[Suzie]

What you did originally would work with a bit more voltage.  If you have 6V available (rather than 3V) and a 40R resistor in series you will probably find that it will work.

[meil]

You are using the wrong sort of Bi-colour LED. You need to use a three legged type so that you can tune each colour to the intensity you want. Also you ALWAYS need a resistor in series. It's there to limit current as well as drop voltage.

[Crosland]

This is what happens when the OP is left open to interpretation.

 

Do you really mean two bi-colour LEDs or do you mean a red one and a white one?

 

Either way, 3V is not enough and one LED is always reverse biassed so no current will flow.

 

If they are bicolour LEDs that need 12 V then it will never work, unless you can arrange a higher supply voltage.

 

If they are individual LEDS then the crucial point is that the LEDs need to be connected in "reverse parallel" anode-cathode and cathode-anode.

 

One will light and the other will be reverse biased and will do nothing.

 

There is no problem with them having different forward voltages, and one preventing the other from working. The Vf of the both is less than the max reverse voltage of the other.

 

MartinRS has it, but omitted the resistors.

 

Dagworth almost gets the prize, except you will almost certainly want to use different resistors for each LED, due to the different Vf.

 

The final problem is that 3V is on the low side for a typical white LED.

[Dagworth]

6 hours ago, Crosland said:

Dagworth almost gets the prize, except you will almost certainly want to use different resistors for each LED, due to the different Vf.

I took it as being a pair of red/white bicolours.

 

Andi

[melmerby]

33 minutes ago, Dagworth said:

I took it as being a pair of red/white bicolours.

 

Andi

So did I. In which case you have two pairs of inversely connected LEDs

Edited July 5 by melmerby

[Lacathedrale]

I mean two bi-colour LEDs, - I already updated to say that switching the resistors to the cathode side of each split feed resulted in success.

 

the red/white colours in my diagram were just illustrative.

 

[RAF96]

On 04/07/2024 at 12:00, spamcan61 said:

For the avoidance of doubt, I think you mean common anode?

Brain fade - yes of course it should be common. I will amend my. post accordingly.

[Crosland]

On 06/07/2024 at 13:35, Lacathedrale said:

I mean two bi-colour LEDs, - I already updated to say that switching the resistors to the cathode side of each split feed resulted in success.

 

the red/white colours in my diagram were just illustrative.

 

 

Something like these https://www.rapidonline.com/truopto-osrwp23132a-3mm-red-white-bi-colour-led-2-pin-diffused-76-0018 ?

 

Note that the forward voltage for the white is 3.1V.

 

Your wiring diagram is correct, apart from lacking a current limiting resistor, with a red and a white in series in each direction. The problem is your 3V supply. If you short out the red in one, the white in the other may light but when the red is in series as well, the supply voltage is too low.

 

 

[MartinRS]

16 minutes ago, Crosland said:

 

Something like these https://www.rapidonline.com/truopto-osrwp23132a-3mm-red-white-bi-colour-led-2-pin-diffused-76-0018 ?

 

Note that the forward voltage for the white is 3.1V.

 

Your wiring diagram is correct, apart from lacking a current limiting resistor, with a red and a white in series in each direction. The problem is your 3V supply. If you short out the red in one, the white in the other may light but when the red is in series as well, the supply voltage is too low.

 

 

The OP posted a diagram last Thursday at 13:59  (edited) and reports that the problem is solved, something I missed.