LED wiring on mimic panel

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dan8400
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LED wiring on mimic panel

Postby dan8400 » Sat Nov 19, 2016 8:28 pm

Hi everyone,

I am in the process of building my station mimic panel. I am using momentary (on)off(on) switches and have micro switches operated by the point motor. I am adding LED's to every route on the mimic panel. So when a route is set it shows you exactly where the train will go.

So, my question is: do I wire the LED's in parallel? And if I do, does each LED need a resistor?

I am going to use a wall wart. I have a box full. Mobile phone chargers, adaptors for obsolete electrical goods etc etc.

Just looking for suggestions.

If you need anything clarifying, please ask. I am not an electrical expert but a keen amature

Thanks
Dan
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End2end
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Re: LED wiring on mimic panel

Postby End2end » Sat Nov 19, 2016 8:46 pm

Although slightly different from yours (I am using the seep point motors aux switch to change what LED's are display on the mimic/control panel), I have used 1KOhm resisitors on every LED connected in parallel. If you wire them up in series the last LED in the chain won't get as much power as the first resulting in a dimmed LED.

Here is a diagram of my wiring for 1 point with the actual wire colours I will/have used. The R units are the resistors.
Image
Hope it helps. :)
Thanks
End2end
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fourtytwo
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Re: LED wiring on mimic panel

Postby fourtytwo » Sat Nov 19, 2016 8:57 pm

You can save on resistors as for each point only one of the two leds can be lit at a time hence they can share a common resistor.
For example assuming the motor switch grounds one or the other cathode, the anodes can be connected together and then connected to the positive supply using a single resistor!
I hope I have understood your question correctly :)
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dan8400
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Re: LED wiring on mimic panel

Postby dan8400 » Sun Nov 20, 2016 9:03 am

Thank you for the replies. I think I will have to use one resistor per LED. The LED's vary in numbers depending on route set. That's why I asked about wiring parallel. Don't want any dimming. For example, one mainline has 20 or so LED's but only 3 when the point is thrown.

Thanks
Dan
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fourtytwo
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Re: LED wiring on mimic panel

Postby fourtytwo » Sun Nov 20, 2016 9:19 am

dan8400 wrote:Thank you for the replies. I think I will have to use one resistor per LED. The LED's vary in numbers depending on route set. That's why I asked about wiring parallel. Don't want any dimming. For example, one mainline has 20 or so LED's but only 3 when the point is thrown.
Thanks
Dan

Hello Dan, now I understand a little more of your application, I had not realized you had strings of leds all lit at once, again you can reduce the resistor requirement by wiring them in series but within the constraints of the supply voltage. You should allow 2V per led and at least 2V across the resistor, so for example from a 12V supply you could string 5 leds in series together with the resistor.
The resistor value should be calculated according to your brightness requirements, usually somewhere between 10-20mA and the value is the voltage/current so in the above example 2/20mA = 100R.
I hope you get the idea and its useful, the exact led voltage depends on the color you can check the data sheet and its worth knowing how stable the power supply voltage is before doing your calculations.
Roger

PS when wiring leds in parallel different brightness levels are more likely to result due to the poor matching especially if different colors are involved.
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Teedoubleudee
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Re: LED wiring on mimic panel

Postby Teedoubleudee » Sun Nov 20, 2016 10:15 am

End2end wrote:Although slightly different from yours (I am using the seep point motors aux switch to change what LED's are display on the mimic/control panel), I have used 1KOhm resisitors on every LED connected in parallel. If you wire them up in series the last LED in the chain won't get as much power as the first resulting in a dimmed LED.

Here is a diagram of my wiring for 1 point with the actual wire colours I will/have used. The R units are the resistors.
Image
Hope it helps. :)
Thanks
End2end


Just a slight correction to End2end's reply, LEDs, or filament bulbs for that matter, connected in series have the same current passing through them so the brightness of them all is affected equally.

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Bufferstop
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Re: LED wiring on mimic panel

Postby Bufferstop » Sun Nov 20, 2016 10:39 am

Just a thought off the top of my head.- Could you make use of the "LEDs on a roll" that are now available. Some of the wider spaced ones would be about right for a mimic panel, and it doesn't matter how many you have in the string, you just apply 9-12V DC to the end. You would probably need to make a sandwich construction with the spaces between the strips of LEDs filled with card of the same thickness and a translucent top sheet with the spaces around the tracks painted black (perhaps on both back and front to stop light leakage.
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dan8400
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Re: LED wiring on mimic panel

Postby dan8400 » Sun Nov 20, 2016 10:51 am

I wish I had thought of that first bufferstop. I am almost finished drilling and fitting all the LED's one by one. Ah well.


I am using green LED's if that makes a difference. A quick look at the eBay data sheet shows:

Forward voltage: 3.2 - 3.8v
Wavelength: 520nm
Brightness 10000mcd

Thanks
Dan
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dan8400
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Re: LED wiring on mimic panel

Postby dan8400 » Sun Nov 20, 2016 10:57 am

So if I already have this power supply:

http://m.ebay.co.uk/itm/172137174287?_mwBanner=1


All I really need to know is what resistors I need and is series or parallel best? Just trying to get a fairly even brightness across the whole panel, whether 10 LED's are lit or 50

Thanks
Dan
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End2end
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Re: LED wiring on mimic panel

Postby End2end » Sun Nov 20, 2016 11:00 am

Teedoubleudee wrote:Just a slight correction to End2end's reply, LEDs, or filament bulbs for that matter, connected in series have the same current passing through them so the brightness of them all is affected equally.

Just to clarify on Teedoubledee's point (for myself as well as others)
From The BBC bitesize website: -
If you put more lamps into a series circuit, the lamps will be dimmer than before.

In a parallel circuit, unlike a series circuit, the lamps stay bright if you add more lamps in parallel.

So ALL lights will be dimmer in a SERIES circuit the more bulbs you add.
ALL lights will be the same brightness in a PARALLEL circuit
Thanks
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BorderShed
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Re: LED wiring on mimic panel

Postby BorderShed » Sun Nov 20, 2016 11:46 am

Morning all, a word of caution, I'm not happy with that BBC thingy !

"BBC bitesize website: ALL lights will be the same* brightness in a PARALLEL circuit"

That will only be approx. true if each LED has its own current limiting resistor and then only true if all LEDs and all resistors are well matched. (same batch from same manufacturer, up to manufacturers regular spec. maybe ? how 'well matched' is up to the eyeball observing !)
In other words - *same in this context should be read as each being the same brightness as it would have been if it were alone, single, on the power supply, not the same as ALL the other LEDs in a parallel circut.

Using only one resistor to feed several LEDs is not 'best practice' ** and may result in some LEDs not lighting at all. If for example - -
Dan : Forward voltage: 3.2 - 3.8v
If several are at 3.2 and fed from a single resistor and one at 3.8 is added that 3.8 one will probably not light at all !?

It would be a good idea to try them out first in a test circuit before wiring up a panel only to unwire it later to add extra resistors.

edit ** and that is all without mentioning 'cascade failure mode' with one common feed resistor to several parallel LEDs :)

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Re: LED wiring on mimic panel

Postby Bufferstop » Sun Nov 20, 2016 1:20 pm

I would say that if you are looking to fault tolerance, ie one LED failing without affecting the rest then one resistor for each LED is the most straightforward solution. Which it is no surprise to find that is exactly what you get with them on a cut to length roll. It's what you will find in LED cluster lamps for traffic lights (and signal aspect lamps I guess). It means a fair few can fail before the lamp needs changing.
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dan8400
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Re: LED wiring on mimic panel

Postby dan8400 » Sun Nov 20, 2016 2:52 pm

It seems like a belt and braces approach then to use a resistor per LED and wire in parallel.

Would you agree?

I don't mind buying the extra resistors and doing the work if it'll save me time and heartache with a failure in future

Thanks
Dan
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Re: LED wiring on mimic panel

Postby End2end » Sun Nov 20, 2016 6:08 pm

All my LED's have 1K resistors (including the building lights) each but I just wanted to post a note of thanks to all members for the clarification. :)
Thanks
End2end
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Re: LED wiring on mimic panel

Postby dan8400 » Mon Nov 21, 2016 8:38 pm

Quick question. Can I use a variable resistor like a dimmer switch for the whole panel so I can set the brightness?

Just a thought

Thanks
Dan
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