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New to LEDs - help please!


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I'm putting together a small O gauge 1960's BLT, using standard DC, and thought it would be good to illuminate the buildings and station lamps etc.

I've never done any lighting before, so would appreciate a simple approach if possible.

I am about assemble the Peco station lamps LK-759 which have a tube cut out up the middle to fit an LED; this is my starting point.

I have 4 station lamps and 6 buildings, inluding signal box, waiting room, engine shed etc, so need 10 to 12 LEDs.

From doing searches around the intraweb and on here, and watching some technical vids on the 'Tube, I think I need to know:

 

Which LEDs do I buy, ie what voltage and which size?

Which resistors do I buy (if needed)?

What do I use to power them?

How do I wire them up?

I have an old Hornby 12v controller which I can use as a DC power supply, if that's any good?

I'm handy with a soldering iron, but have read it's best to buy the pre-wired LEDs - is that right?

 

If someone could point me to a specific link - 'go here, buy these, assemble this way' - that would be hugely appreciated, as I don't understand the whole voltage/resistor/amps thing!

 

Thank you!

 

David

 

 

 

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LEDs can be wired in Series or parallel. Old 12v controller should be fine for the number of leds you are intending to use. To calculate value of resistor, you need to know forward voltage and current of led. There are quite a few 'led calculators' online that do the hard part for you and show you how to wire them up. Really easy to do with practice. Good luck.

 

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5 hours ago, d00m said:

LEDs can be wired in Series or parallel. Old 12v controller should be fine for the number of leds you are intending to use. To calculate value of resistor, you need to know forward voltage and current of led. There are quite a few 'led calculators' online that do the hard part for you and show you how to wire them up. Really easy to do with practice. Good luck.

 

 

Thank you for your response.

Unfortunately, I don't understand how I discern the 'forward voltage' and 'current of led' from looking at the LEDs for sale (eg on Ebay or similar).  Typing 'LED Calculator' into Google brought up - you guessed it - loads of press-button calculators with LED readouts!

I did find one link though: https://ledcalculator.net/   but it's asking me for 'voltage drop' and 'rating' in mA's, whatever that is, so I can't do the sums required...not that I would understand the outcome unless it gave me a link to the LEDs and resistors I need.

I don't want to buy anything until I know what to buy...I just thought you buy 12v LEDs, plug them in to a 12v controller, and that's it, like wiring normal bulbs...clearly there's much more to it!

 

David

 

 

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Hi Grafarman.  Search for white 12 V pre wired 3mm LED. 

 

Lots of links , but they all look like this. Its a LED, a resistor and wires ready made. iu-1.jpeg.188c7d174565de75c86cb92ec9a67aea.jpeg

You can buy 50 for less than £10 . 

 

Connect a couple to your old Hornby controller, just in parallel. So red wires to one terminal, blacks to the other terminal. Turn the knob one way, they might light up and get brighter the more you turn the knob. If not turn it the other way. 

 

You can buy 50 for less than £10 . Ignore all the maths bit , at that sort of price just buy some and mess about a bit. 

 

 

 

 

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1 minute ago, Dave John said:

Hi Grafarman.  Search for white 12 V pre wired 3mm LED. 

 

Lots of links , but they all look like this. Its a LED, a resistor and wires ready made. iu-1.jpeg.188c7d174565de75c86cb92ec9a67aea.jpeg

You can buy 50 for less than £10 . 

 

Connect a couple to your old Hornby controller, just in parallel. So red wires to one terminal, blacks to the other terminal. Turn the knob one way, they might light up and get brighter the more you turn the knob. If not turn it the other way. 

 

You can buy 50 for less than £10 . Ignore all the maths bit , at that sort of price just buy some and mess about a bit. 

 

 

 

 

 

Thank you; that's great, I'll go and have a search...

 

David 

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It’s simple 

 

for single leds the forward voltage drop is typically 2.5V for coloured and 3v ( sometimes more ) for white 

 

current varies depending on type but these days 5 mA is s good starting point 

 

hence using ohms law 

 

(supply voltage - led forward voltage ) / current in amps 

 

For 12v 

(12-2.5) /.005 = 2500 ohms or nearest. Adjust the resistor value to get difference brightness. 

Edited by Junctionmad
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6 hours ago, Junctionmad said:

It’s simple 

 

for single leds the forward voltage drop is typically 2.5V for coloured and 3v ( sometimes more ) for white 

 

current varies depending on type but these days 5 mA is s good starting point 

 

hence using ohms law 

 

(supply voltage - led forward voltage ) / current in amps 

 

For 12v 

(12-2.5) /.005 = 2500 ohms or nearest. Adjust the resistor value to get difference brightness. 

 

Just my opinion but the way Ohm's Law is typically expressed (V over I equals R) doesn't quite get the message across. I think the problem is it does not bother to explain the basic algebra that allows the same law to be written three different ways. I'm tempted to think real problem has nothing to do with fundamental electronics; it's much more to do with fundamental math and lousy teaching methods.

 

There are a very few basic math rules to learn but if you don't grasp them many simple equations just look like a load of complete gobbledygook. I was fortunate to have a good teacher but I'm not sure I would ever have figured them out by myself had I not.

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I have no electrical skills, but have managed LED's OK so far.

First, they are polarity sensitive.  No problem in a station lighting situation, but would need a bridge rectifier on a loco to deal with forward/reverse.

12v is fine but if you use 3V you will need a resistor (3.3) with EACH LED.

Again, if in a loco a capacitor will prevent flashing.

I've put micro LED's in several Springside loco lamps, OK so far.

HTH

Edited by SuperD
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Thanks all; I have sent off for a bundle as Dave John suggested - they have the resistor wired in, so it makes things a lot easier!

When they arrive I'll connect them up in various ways to see what can be done.

I still don't understand the maths, or physics, and glaze over at equations and laws, but hopefully good old trial and error will suffice!

 

David

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I have tried to put some notes together to answer the kind of questions that come up for people who have no electronics experience.

LEDs and resistors are cheap. You can experiment without worrying what it will cost. Resistors are robust, mechanically and electrically, LEDs are not. LEDs can be destroyed by heat, don't solder too close to the body, and don't keep the iron there for too long. A good idea is to use chocolate blocks to try different resistor values, and solder once you're happy. LEDs are polarity sensitive, resistors are not. It does not matter a scrap which leg of the LED the resistor is in, BUT if you always put it in the same leg it makes life easier when wiring them up. LEDs with a resistor in series will not be harmed if connected the wrong way round, they just won't light up.

Resistors are measured in Ohms, symbol Ω. If using resistors in series simply add the values together, so a 10KΩ (ten thousand ohm) plus a 10KΩ in series equals 20KΩ. Putting resistors in parallel is more complicated, but for this simple example a 10KΩ and a 10KΩ in parallel equals 5KΩ. Resistors come in preferred values. So you will not find a 5KΩ resistor, you must use 4·7KΩ or 5·6KΩ, but don't worry, when it comes to LED brightness those difference will not make a huge difference. You will almost certainly only ever need ¼ watt, the power rating, as the current used by LEDs is very small.

Start off with high value resistors, the LED will be dim, reduce the resistance until it looks about right. Don't forget an LED sitting on the bench looks different once installed into a building.

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To discover which is the neg/pos legs/leads on a LED simply use a 3v coin battery. No danger of overload, and it will show if the LED is okay. As a basic rule-of-thumb a 1k resistor is often used as standard for a 12v supply. 

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Contrary to the information provided above, I would NOT use an old train controller to provide power for these LEDs.

The reason is because they are rarely, if ever, 12 Volts. On low loads such as a few LEDs, you'll find that the output voltage, is going to be more like 16 volts or even higher.

That will cause your LEDs to be too bright at best, assuming you've provided the correct resistor for 12 volts. Worst case is that your LEDs, will glow very brightly for a second, then permanently go DEAD!

 

Best off, to get yourself a plug pack type power supply, one that is regulated for 12 volts - make sure it says so on the packaging. That will give you a consistent base to work your resistors out on and get consistent brightness.

 

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17 hours ago, AndyID said:

 

Just my opinion but the way Ohm's Law is typically expressed (V over I equals R) doesn't quite get the message across. I think the problem is it does not bother to explain the basic algebra that allows the same law to be written three different ways. I'm tempted to think real problem has nothing to do with fundamental electronics; it's much more to do with fundamental math and lousy teaching methods.

 

There are a very few basic math rules to learn but if you don't grasp them many simple equations just look like a load of complete gobbledygook. I was fortunate to have a good teacher but I'm not sure I would ever have figured them out by myself had I not.

yes but the points is the computation for LEDS is simple the voltage dropped across the series resistor divided by the set current , NO laws , algebra etc 

 

if you can use a calculator you can get the resistor needed . ie Green led , 10V supply ( 10-2.5)/0.005  is no more difficult then that 

 

modern red green blue yellow leds will run down to 1mA these days  and upto about 30mA,  ( many will survive 60mA)  5mA is a good starting point, white leds are slightly different  

 

heres a quick summary of the major types 

0KPZt.png

Edited by Junctionmad
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Just join the third decade of the 21st century and buy your LEDs from Aldi ready wired in parallel in sets of 50 or 100 and powered by  AA batteries. (White Christmas Lights)  

Edited by DCB
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48 minutes ago, DCB said:

Just join the third decade of the 21st century and buy your LEDs from Aldi ready wired in parallel in sets of 50 or 100 and powered by  AA batteries. (White Christmas Lights)  

 

Well yes, but what if you want to tone them down a bit or make some signals that don't blind the poor driver? :sungum:

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6 hours ago, DCB said:

Just join the third decade of the 21st century and buy your LEDs from Aldi ready wired in parallel in sets of 50 or 100 and powered by  AA batteries. (White Christmas Lights)  

 

6 hours ago, AndyID said:

 

Well yes, but what if you want to tone them down a bit or make some signals that don't blind the poor driver? :sungum:

Or you want smaller LEDs? :rolleyes:

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1 hour ago, Junctionmad said:

There’s no right “ resistor “ it’s a range of values to suit the application 

Well I buy them from a seller in the model railways section of ebay, so he sells the LEDs with the right resistor for the application. Sorry I wasn't pedantic enough in my statement.

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42 minutes ago, F-UnitMad said:

Well I buy them from a seller in the model railways section of ebay, so he sells the LEDs with the right resistor for the application. Sorry I wasn't pedantic enough in my statement.

 

It's not about pedantry, it's about understanding what LEDs are, and how they work.

 

There's still no "right" resistor, only a compromise, even if you specify "model railways" as the application. What does "model railways" mean, anyway? LEDs for colour light signals, station lighting, loco lighting, or ...?

 

The buyer has decided to provide a somewhat arbitrary (within a certain range) value resistor with his LEDs. It may, by chance, be "right" for your particular application but for someone else it could result in too bright or too dim an LED.

 

It's a bit like buying a loco but letting the seller decide what scale they will supply. It will be perfect for some, not for others.

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12 hours ago, F-UnitMad said:

Well I buy them from a seller in the model railways section of ebay, so he sells the LEDs with the right resistor for the application. Sorry I wasn't pedantic enough in my statement.

See other replies to clarify that I meant the application of model railways. 
 

You  need to specify resistors. One value isn’t enough 

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