Three-leg common-anode leds in series ?????

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Three-leg common-anode leds in series ?????

#1

Post by collectors »

Hi, is it possible to wire three-leg common-anode leds in series. I am going to use a Block signaling Points Position Indicator PPI4-DC for 3 things "i hope"
1. for 2 track signals bi colour 3 leg red/green & 1 mimic board with a bi colour r/g three leg led. Now i have tried contacting Block signaling, but not heard as yet. So i am thinking 3 x 3 leg led's in series as i notice it seems ok for 2 x 2 leg leds in series, but wasn't sure if i could push 3 leds.
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Re: Three-leg common-anode leds in series ?????

#2

Post by Tricky Dicky »

Firstly, am I reading it right that you want all the red LEDs on the signals and mimic board to emit light together and likewise for the green?

Bicolour two pin LEDs are simply two diodes reverse biased in one package therefore the current one way will switch on one and reversing the current switches the other LED on.

Three pin common anode (you can also get three pin common cathode) are a totally different beast. As their name implies each LED in the package shares the anode and each LED is switched on independently by connecting the cathode to the 0V rail either simply by individual switches or using an IC. That way you can either have the red or the green or both together emitting which will give you a third colour in the case of red/green a dirty yellow.

Assuming my first paragraph is what you want you could link all three red cathodes together likewise the green to get them to emit together and of course wiring all the anodes to the +V rail. They would therefore be paralleled you could even use a single resistor for each colour but would have to calculate the value carefully.

I am not familiar with the Block Signalling circuit you intend to use but using common anode LEDs suggests the circuit works by sinking the current, be aware that most ICs will only source or sink in the order of 10mA on their output pins so your calculations need to be quite accurate if you do not want to release the “magic smoke”. A workaround would be to include some transistors in the circuit.

Richard
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Re: Three-leg common-anode leds in series ?????

#3

Post by Brian »

Hi
Series LED wiring requires the Anode of one LED to connect to the previous LEDs Cathode. Problem is you have two Cathodes per LED on a 3 lead Bi colour LED so which one is to be used? Not possible to do IMO. :o

Why not reconfigure the LEDs so as each bi colour LED has its own feed and returns all coming from the same places if they are all to work together? That is, wire them in parallel. ;)

Ah I see TD posted while I was typing! :D
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Re: Three-leg common-anode leds in series ?????

#4

Post by collectors »

Hi, thanks for the replies. I basically wanted 3 red led's to light up when points are one way & the greens light up when switch the other way. Now simply wiring in parallel would be simple, but i wasn't sure if the Block signaling unit would take it with extra led's & was under the impression the load would be reduced if in series. "but not sure"
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Re: Three-leg common-anode leds in series ?????

#5

Post by Brian »

You are correct re load (almost!) Load is not reduced but is the same for all LEDs wired in the series chain.

If you don't get a definitive reply from BlockSignalling (Which would surprise me if you don't get one) How are you with some simple electronics?

Two option here spring to mind...
A) Use a LED Opto Isolator per output and then depending on the Opto isolator chosen its output feeds the three LEDs
B) Use a Transistor in place of the LED per output and a base resistor per transistor then the Transistor switches the LED load
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Re: Three-leg common-anode leds in series ?????

#6

Post by collectors »

Hi, the reason on the bi-led is that it made my mimic panel a lot neater & would be a real pain to change this part. Electrics i am fine with, electronics, not so good.
Now i think you have saved me some money as i was looking at using an "FET buffer" if this would of worked on the BS/outlets.
But your LED Opto Isolator seems a lot more reasonable if i end up needing it. Am i correct in that they seemed to be a type of relay with an input voltage to drive the on/off part & a seperate supply to power the led's. If this is so, do i need to know the output voltage of the BS/unit or do these LED Opto Isolator have a range of let's say 2-8 volt?
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Re: Three-leg common-anode leds in series ?????

#7

Post by Brian »

Hi
Opto Isolators are just an LED and a light receptive transistor or LED in a single package. The input LED connects where a normal LED would and the output can have a totally separate or the same supply pass via it and switch On its output when the input LED illuminates There is no electrical connection between input and output. See https://en.wikipedia.org/wiki/Opto-isolator
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Re: Three-leg common-anode leds in series ?????

#8

Post by collectors »

Have i got anywhere near whats needed & would i need 16 of the Opto Isolators to make it work for 8 sets of points. The 8 leds on the Block signaling Points Position Indicator have been left in the image for show & would be removed

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Re: Three-leg common-anode leds in series ?????

#9

Post by Brian »

Yes that's correct. However the LEDs then need to become common Cathode.
Opto isolators are fairly cheap and can be obtained for around £0.30p up. These can happily handle 50ma output Example https://www.rapidonline.com/isocom-tlp5 ... -4-63-1445

I would increase the Ohm value of the LED series resistors to 1K (1000 Ohm) minimum. Possibly 1K (1000 Ohm) and 1K5 (1500 Ohm) if the red and green are not similar on 1K each. Note; 445R and 500R there is no such resistor values in the E12/E24 range. Nearest are 470R and 510R

The other alternative is to use a cheap NPN transistor and wire its base via a 10K resistor to the Base. 2N2222 would be fine and I just received 50 from eBay for £3.40 (less than 7p each)

Both methods will need some sort of board to hold the components - Vero strip board is about the most commonly used for such a project.

But let hope you hear from Steven owner of BlockSignalling soon. :D
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Re: Three-leg common-anode leds in series ?????

#10

Post by collectors »

Hi Brian, thanks for all the info, but i am assuming with the change of led to common cathode i would have to switch 0v side of things on the anode like below. ??

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