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LED Primer needed, please!

Posted: Fri Feb 09, 2018 12:19 pm
by Papyrus
Returning to the hobby after 50-plus years, a few things have changed! And I have quite a few learning curves to ascend, Electrics being just one. I'd thought that LV LEDs would relate to old-fashioned LV bulbs just as domestic LEDs relate to incandescents, but it seems to be more complicated than that.

I want these particular LEDs on a mimic diagram, which will also carry (12v DC) cab-control switches and point motor actuation studs. So some LEDs to indicate section status and some to indicate turnout status - four colours for preference. I've sorted out the switching arrangements.

So my first questions are -

1. I gather that LV LEDs have an anode/cathode, and will only work with the correct polarity. So am I right that I need a LV DC supply (12v, or perhaps lower)? Or could I use 16v AC and just use that half of the current that's flowing the right way?

2. So if I have to have a 12v DC supply, does that mean a dedicated transformer - in which case what's recommended at a reasonable price? Or is it best to use a battery pack? (My Gaugemaster Combi doesn't have a suitable output.)

3. Resistors. It seems generally agreed that a 1000 ohm resistor is needed in series. But is that one resistor for one LED (or at least one resistor for one LED lit at a time), or one resistor for any number of LEDs? I've seen statements (e.g. on the Gaugemaster website) that the latter is the case, but that seems to be contrary to my (admittedly rusty) understanding of Ohm's Law.

Then there's the problem of sourcing suitable panel clips, but that seems to be more trial and error.

I'd be most grateful for any advice! Oh, and are there any good web resources I should be reading?

Alan

Re: LED Primer needed, please!

Posted: Fri Feb 09, 2018 12:32 pm
by Flashbang
Hi
Work on the principle of one lit LED equals on series resistor. So for a point indication where one LED lights for Normal (Straight) and the other for Reverse (Turnout) positions one resistor can be shared between the two LEDs as only one is lit ant any one time. See some examples here... link to item

You're correct re LEDs only working one way around on a DC supply. The longer lead is the Anode or Positive.

Use this web site to calculate the resistor minimum value need for both single or series wired LEDs... http://led.linear1.org/led.wiz

Never connect a standard LED to a supply without a series resistor. The resistor can be fitted into either leg of the LED but do try and keep a standard throughout where you fit resistors onto say the LEDs Cathode (Negative) leads.

Some LEDs are pre fitted with a series resistor and are voltage rated such as being 12 volt rated. These when on a regulated 12v DC power supply do not require any further resistance. Though it you wish to dim them adding a series resistor will reduce their brightness.

You can run LEDs on AC or DCC power, but I recommend adding either a series standard diode 1N4148 or 1N4001 etc or wire a diode so as its across the LED in inverse parallel

Re: LED Primer needed, please!

Posted: Fri Feb 09, 2018 12:34 pm
by mahoganydog
Hi Alan,

Be careful which type of LED you get. For a panel you really want diffuse LED's because many are hyper bright and if you accidentally look into them it will hurt your eyes. Dedicated transformer is the best way, Gaugemaster do a dual output 12V transformer or if you feel you can do it modify a computer power supply and use the 5V output (advanced option and not easy).

This site will help you get the right resistors; http://ledcalc.com/

Jim

Re: LED Primer needed, please!

Posted: Fri Feb 09, 2018 11:33 pm
by D605Eagle
These might help you.
This is an LED power supply, it doesn't use a conventional transformer that you would be used to. Its specially designed for use with household 12V LED lighting that you see in fancy kitchens etc. To use this you will need for each LED to have a 680 or 1k ohm resistor. Just a 1/4 watt one will do. Don't try to daisy lots of LEDs onto one resistor, its false economy, but as you will see from these links, prices are stupidly low .
LED 2.5 amp power supply.
clicky
LEDs, packs of 50. These are not over bright and defuse nicely for your display panel with a wide viewing angle. Go for 5 mm LEDs Avoid blue, they are vile to look at!
clicky again!
1k ohm resistors
more clickyness!
Finally LED bezels. Makes them look much neater
final click!
Cheers
Jim

Re: LED Primer needed, please!

Posted: Sat Feb 10, 2018 7:12 am
by RAFHAAA96
If using 3-legged bi-colour LEDs for simple points indication i.e. red and green you can get away with one resistor on the middle leg, but if you want that LED to show yellow/amber to indicate a point not set correctly then you need two resistors one on each outer leg but of different values as the LED forward voltage and currents vary for different colours and you are trying to balance them.

I use two of these dual colour LEDs on each point of a crossing loop, such that if the points are correctly then each will show green for way and red for blocked, but if the points are set contra i.e. one each way then both sides of both LEDs are powered and the indicators show yellow/amber alerting me of a problem.

Rob

Re: LED Primer needed, please!

Posted: Sat Feb 10, 2018 10:41 am
by Flashbang
WARNING... On no account should that power supply linked to above as suggested by Jim for an LED power supply, be used without an additional insulating case, which ideally is a fully secured lid plastic style. It has exposed 240 volt terminals! Then you will also have to consider its ventilation too.
There is a serious danger of electrocution if its not properly insulated in a cased!

You're actually much safer using a 'plug in' style PSU with a low voltage lead and connector. 2.0Amp is more than ample and would power around 190 lit LEDs with 1K resistors. This is an example... Link to suggested type of LED PSU

Re: LED Primer needed, please!

Posted: Sat Feb 10, 2018 12:58 pm
by Papyrus
Many thanks all of you for some really useful info and links, not all of which I have followed as yet. You've saved me some heartache, abortive purchases and web research! Though if I want four colours of LED I may have difficulty avoiding blue! :D

I've also just realised that I have a 40-year old Hornby mains power pack with a 14.3V DC output. "1.1VA" it says - but can that be right? Just 0.08 amps? Anyway, it seems that suitable power supplies are easily obtained on eBay.

Cheers, Alan

Re: LED Primer needed, please!

Posted: Sat Feb 10, 2018 1:20 pm
by Bufferstop
It might just about power the small 0-4-0 chassis if not working too hard. I suspect it would have been running overloaded for some of the time. Provided it never got hot enough to melt or catch fire, with the light usage they usually receive when working in a starter set Hornby may have considered it adequate. It would be the first item that I would upgrade.

Re: LED Primer needed, please!

Posted: Sat Feb 10, 2018 3:02 pm
by Papyrus
Ah well, at least it will do to test my LEDs. Until I get around to ordering something better.

(I grew up in a house backing onto the North end of the B&NW line ...) :wink:

Alan

Re: LED Primer needed, please!

Posted: Sat Feb 10, 2018 3:25 pm
by TimberSurf
Papyrus wrote: Oh, and are there any good web resources I should be reading?


Yes, mine! lol
Not that I am biased! or late!

Lumsdonia

Re: LED Primer needed, please!

Posted: Sun Feb 11, 2018 1:32 am
by D605Eagle
This site is brilliant for electronics and wiring
Brian Lambert

Re: LED Primer needed, please!

Posted: Sun Feb 11, 2018 1:40 am
by D605Eagle
Papyrus wrote:Many thanks all of you for some really useful info and links, not all of which I have followed as yet. You've saved me some heartache, abortive purchases and web research! Though if I want four colours of LED I may have difficulty avoiding blue! :D

I've also just realised that I have a 40-year old Hornby mains power pack with a 14.3V DC output. "1.1VA" it says - but can that be right? Just 0.08 amps? Anyway, it seems that suitable power supplies are easily obtained on eBay.

Cheers, Alan

Ones from the 70s were rated at 1/2 amp upwards depending on which one it was. The big blue one with a red lable on it was 2.5 amp IIRC. Trouble with these older transformers is they rarely had adequate smoothing if any at all, not that that matters for running LEDs or DC trains.

Re: LED Primer needed, please!

Posted: Sun Feb 11, 2018 12:39 pm
by MickleoverTestTrack
I've used a cheap 1 amp plug in transformer with variable voltage. The advantage over a fixed voltage one being that if it turns out too bright you can turn it down or if you are playing during the day with sun coming through a window or something you can turn it up. You'd get the same flexibility using an old DC controller. I'd definitely recommend having some kind of variable intensity unless you can guarantee your back lighting will be the same all of the time, I like to turn the lights off and watch trains in the dark so having variable intensity is essential to me.

I would say that LEDs have come on in recent years and seem to be brighter now for a given level of current, so if you use a fixed supply you might want to experiment with your resistor values first.
There are now LED bike lights that can give the same power as a traditional 50w car lamp from a tiny Lithium ion cell and they last for hours on a single charge.

Re: LED Primer needed, please!

Posted: Sun Feb 11, 2018 2:46 pm
by D605Eagle
Good point. I used this very simple variable voltage circuit to convert some of wifeys battery operated Christmas tree lights to mains supplied. You don't need to make the more fancy circuits as your not operating sensitive audio equipment. If you use a 15V power supply with it, you can vary the output from 0 to about 12 volts. If you use LEDs with a 680 ohm resistor they will be adjustable from nice and bright to nothing. If you end up using a large number of LEDs you might need a heat sink on the LM 317 regulator.
clicky

Re: LED Primer needed, please!

Posted: Mon Feb 12, 2018 11:13 am
by Flashbang
If overall LED dimming is required, or just fine voltage regulation of a DC supply, I highly recommend something like these units, which offer a variable output and are ideal for use with LEDs.
From China Link 1
Or UK supplier..
Link 2
Or even one without voltage indication ...
Link 3
There are many other variations obtainable too. Just needs a DC input with a DC input voltage greater than output required. e.g. 12v DC in = variable to maximum 10ish volts out