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Author Topic: LED help needed(yep another one!)  (Read 510 times)

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Offline MalcolmAL

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LED help needed(yep another one!)
« on: September 22, 2017, 12:04:59 pm »
I have a direction of travel indicator on my diy powered railer which works but has a puzzling quirk :
When the loco is going backwards my Red Led lights - good !
When going forwards the Led does not light - as it is supposed not to do except !!! that ->

When the controller is raised from zero in the forward direction the led glows slightly until the loco begins to move - that shouldnt be ! should it !!

I think I have entered a mental black hole and desire your assistance :)
This is my wrong circuit



and this is what I did to put it right :


All good now, works perfectly,
So what is the problem ? you ask >
well, I thought that the simple (reverse bias protection) diode could be replaced by another led say a green or white one to give forward indication.
Nope nothing works then, neither forwards or backwards !

Why not ? Must be something to do with both Leds having a high forward volts whereas the protection diode D1 being an ordinary diode will have a lower Vf  but even so, I am puzzled :(
am I on the right lines so to speak ?



« Last Edit: September 22, 2017, 12:07:38 pm by MalcolmAL »

Offline MalcolmAL

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Re: LED help needed(yep another one!)
« Reply #1 on: September 22, 2017, 12:11:43 pm »
Extra :
I could of course use two steering diodes, two limit resistors and the two LEDs red and green with their two protection diodes, but cant get away from wanting fewer components :) !
« Last Edit: September 22, 2017, 12:19:25 pm by MalcolmAL »

Offline Only Me

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Re: LED help needed(yep another one!)
« Reply #2 on: September 22, 2017, 01:41:22 pm »
I've found on some LEDs I've had to add a zenner diode to stop back feeding which stops the white from lighting.  I add the zenner to the return leg of the red led before the return led of the opposite white led.



Offline dodger

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Re: LED help needed(yep another one!)
« Reply #3 on: September 22, 2017, 01:48:08 pm »
You've basically come up with a type pseudo AC network and I think you have arrived at the simplest solution for two separate LED's. You could try a single red/green LED that is configured back-back.

Dodger

Offline MalcolmAL

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Re: LED help needed(yep another one!)
« Reply #4 on: September 22, 2017, 02:44:26 pm »
Thanks for those thoughts guys, I thought I had seen two-leg packages of diode pairs to do this so I was surprised when it didnt work, so either I have done something incredibly stupid * on my birds-nesting or I cant do it with discretes , they may need to be well matched in the making.
Ah,  maybe they incorporate matching zenners or something clever :( inside the package that I am missing out.
Oh well I have loadsa small diodes on an ex-TV board to strip so I'll do cct3 until a proper bicolour comes via ebay :)

* hence my plea for help, felt sure someone would say "ya boo you silly ol'fool you forgot the ojimawhatsit" :) !

ummmm hang on tho' what if I move the steering diodes down the chain, with a common resistor at the top, ,, then that,, 
back in a while I feel another sketch is needed or an experiment >>>


Offline MalcolmAL

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Re: LED help needed(yep another one!)
« Reply #5 on: September 22, 2017, 03:43:46 pm »
Good news I think I have it :
If in cct3 above I rearrange in cct4 then


the two resistors that were commoned to the supply by link X can become one if I remove X and make a link Y instead. Then I can see how they mount two colours in one package, it isnt two leds but four or six diodes depending on if the reverse protection diodes for each led are still needed ? Assuming of course that I dont want to individually vary their brightness so the circled resister is redundant.

So I just jury-rigged it without the parallel protections (I have plenty of leds :) ) and it woiks ! yea !! bingo, and the leds didnt fail :) yet.



Offline dannyboy

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Re: LED help needed(yep another one!)
« Reply #6 on: September 22, 2017, 11:01:14 pm »
You lost me after "puzzling quirk" !! :confused1: You obviously have a good idea of what you are doing and I am sure a lot of others know what you are on about, but me, all I can say is that I know which way round to wire an LED - if it don't light, swap the wires!  ;D
David.
I used to be indecisive - now I'm not - I don't think.

Offline MalcolmAL

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Re: LED help needed(yep another one!)
« Reply #7 on: September 22, 2017, 11:48:24 pm »
You lost me after "puzzling quirk" !! :confused1: You obviously have a good idea

 if it don't light, swap the wires!  ;D
Thanks very kind of you to say :) but it is all make-believe ;) I am still puzzled by that quirk, if it was current flowing because of reverse breakdown I'd have thort it would have packed up totally! and does a band-gap even apply backwards to cause the same freq. light. But tomorrow is another day to learn things.

I can do better than that !! > > check the switch is on first  >:D :)

Offline Malc

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Re: LED help needed(yep another one!)
« Reply #8 on: September 23, 2017, 02:53:52 pm »
I expect the DC from your controller is not totally smoothed and has an element of AC ripple superimposed on it.
I would have thought 2 LEDs back to back would work. Maybe the series resistor was wrong for the green LED.
I'm not sure if life is passing me by, or trying to run me over.

Offline dannyboy

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Re: LED help needed(yep another one!)
« Reply #9 on: September 23, 2017, 04:06:28 pm »
Stop it Malc ! The only two ripples I know are Strawberry and them that you get on water!  :)
David.
I used to be indecisive - now I'm not - I don't think.

Offline MalcolmAL

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Re: LED help needed(yep another one!)
« Reply #10 on: September 23, 2017, 05:13:27 pm »
Hehee I could murder a strawberry one of they right now :) Dont forget ripples in the fabric of space-time caused by colliding black holes, LIGO  ;D

Good thinking Malc, I really must get round to fixing my 'scope
Resistor ok, common to both diodes. I expect I did something really stupid with the wiring.
This latest incarnation seems to be doing fine and I am thinking that parallel protection diodes may not be needed, because the reverse volts across either when the other is lit should be no more than about 3.5 to 4 ish ? ? ?

Offline carderrail

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Re: LED help needed(yep another one!)
« Reply #11 on: September 23, 2017, 09:03:40 pm »
The protection diodes have no real function (except dropping 0.6V), two LED wired back-back with a single resistor will work. Yes red/green LED have different forward voltages (i.e. the voltage the light) as long as you have a reasonable resistor on a 16V power supply you should be ok (depending on the rating of the LEDs). I always start with about 1k ohms and work up or down a bit... if the supply is AC you can reduce the resistor.

Regards

Tony 

Offline MalcolmAL

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Re: LED help needed(yep another one!)
« Reply #12 on: October 08, 2017, 01:28:07 pm »
I would have thought 2 LEDs back to back would work. Maybe the series resistor was wrong
You are, and everyone else, quite right, all is well with the universe and physics you be pleased to know.  It does indeed work, it was me that was wrong (well demented actually !)
It is time to fess up, I had spent some time previously messing with flickering LEDs ( arc, bonfire, burning building, gas lighting) that when I picked up two diodes for my rerailer direction indicator it did not occur to me to wonder if they should be flickering !! Doh !

Of course now that I use two proper LEDs without the built-in flickering chips, all is well.  :thumbsup:

And y'all will not be surprised to know that I am the world's most complete idiot :( :dunce:

 

Offline dannyboy

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Re: LED help needed(yep another one!)
« Reply #13 on: October 08, 2017, 01:57:00 pm »
So now it's on to the strawberry ripples  :thumbsup:
David.
I used to be indecisive - now I'm not - I don't think.

Offline MalcolmAL

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Re: LED help needed(yep another one!)
« Reply #14 on: October 08, 2017, 02:48:11 pm »
So now it's on to the strawberry ripples  :thumbsup:
Ha! I wish, but I think dry bread and water on the naughty step is more appropriate :(
I thought it was odd just now when I measured the forward and reverse current of one of them, there shouldnt have been much , unless, , oooh err , , ! Interesting though that the internal chip has not had any smoke let out by reversing it (still works).

My gran had a saying "old age doesn't come alone "   >:(

 

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