LED Lighting for dummies,

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Hi,

 

I have just about finished a station building in 7mm, and I want to add some lighting. The interior of the station building would of had gas lighting so I am after some form of soft lights to represent this, any help would be most appreciated.

 

Regards,

 

Martyn.

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Hi Dutch Master,

 

Thank you for the advice, I popped into my local craft shop today and brought a variable power supply to power the lights, this transformer runs from 3v to 12vdc in 7 settings it says on the box. So would it be better to run it at a lower voltage or rely on the resistors instead.

 

Regards,

 

Martyn.

[cliff park]

You don't have a choice. You must put a resistor in series. The size of the resistor will vary with the voltage, there is a formula if you really need it.

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It's not the Voltage doing the work, it's the Amps*. Lowering the voltage does have the added benefit that you can use a smaller (lower value) resistor and you have more options in experimenting at different settings with different resistor values.

 

*voltage is an indication of how many electrons are available, amperage is the (amount of) electrons actually moving down the wire into the circuit and doing the work. Due to the way an led is constructed, you want a low stream of electrons to emit just the right amount of light, and a resistor causes that by throttling the electrons through effectively a narrow passage, like a tap on a water line. But if you ensure there's not so many electrons available by lowering the voltage in the first place, the same trickle of electrons can be obtained with a lower value resistor.

 

Remember: if there's no electrons available, they can't do any work, so: without voltage no amps!

 

 

 

HTH!

 

PS: Ohm's Law: R=U/I, meaning the resistance R equals the Voltage U divided by the Amps I.

 

Hi Dutch Master,

 

Thank you so much for explaining the above in a more simplistic way, I will buy a few packs of resistors of different values and see if I can fine tune the leds.

 

Cheers,

 

Martyn.

[cliff park]

No disrespect to Martyn, but it seems to me that this query, or very similar ones, are on here week in week out. Those of us in the know, (and occasionally some who aren't) patiently explain it all and often refer them to Brian's website or similar. Is it possible to put some reference material somewhere on RMweb which can be easily found and will explain all this in laymen's terms. Probably with a simple chart giving supply voltage/LED power rating/ preferred values to avoid all the maths ? This would be useful across all scales, and provide a quick starting point if nothing else. It could be pinned to the top of this Non DCC section.

[Crosland]

No disrespect to Martyn, but it seems to me that this query, or very similar ones, are on here week in week out. Those of us in the know, (and occasionally some who aren't) patiently explain it all and often refer them to Brian's website or similar. Is it possible to put some reference material somewhere on RMweb which can be easily found and will explain all this in laymen's terms. Probably with a simple chart giving supply voltage/LED power rating/ preferred values to avoid all the maths ? This would be useful across all scales, and provide a quick starting point if nothing else. It could be pinned to the top of this Non DCC section.

 

There are already no end of websites that do that, better to link to one of them. Whatever, it will make no difference to the local village idiots who insist no resistor is needed.

[Junctionmad]

It's not complicated

 

An average led voltage drop is 2 volts. Current is typically 20 milliamperes , or maybe 2 mA if you have high efficiency ones.

 

Hence the series resistor is

 

Supply voltage - led voltage drop all divided by current consumption of the led

 

Eg on a 12 V supply and standard led (20ma)

 

12-2/ .02 = 1000/2 = 500 ohms ( 560 bring the nearest )

 

This will get you into the ballpark for the resistor

There are already no end of websites that do that, better to link to one of them. Whatever, it will make no difference to the local village idiots who insist no resistor is needed.

Of course no resistor is needed if you have a constant current power source !!!!

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Hi Guys,

 

Regarding Cliff Park's comment above, I personally think it's a great idea. I admit I am a total amateur with all things electronic ( although I can wire up a switch to change the point polarity ! ) .

 

I did have an albeit brief search on this forum for want of a better word " reassurance " before buying some lighting parts, the main reason for starting my own thread was the fact of how to try and replicate the glow of gas lamps, this was the one thing I could not locate.

 

Although I was amused at all the mud slinging when reading about whether you need resistors or not, not that i understood a word of the jargon. I mean the only " pot " I know is a cooking pot.

 

All the best Guys,

 

Martyn.

[Brian]

You will always need to know a few facts first... LED forward voltage (VF). LED forward current (IF) and the voltage of the supply.

LEDs have various ratings... Standard red can typically be 2 or up to 2.5 volts (known as VF or Volts Foward) while a white LED can be 4.0 volts or more VF. Many LEDs will have also varying maximum current ratings (IF). This is normally around 20 to 30 milliamp (0.02 to 0.03A) generally you can operate a LED at below this maximum IF rating without too much loss of light output. Low current LEDs can have an IF of 2 to 5 milliamp (0.002 or 0.005A). LED work best on DC power, but they can be used on AC or DCC with the addition of a diode either in series or inverse parrellel to the LED. AC LEDs are slowly becoming available now too.

 

I would always use a series resistor with any LED. The resistor can be fitted into either the Anode of Cathode lead of the LED. Note normally the longer lead of a standard LED is the Anode (Positive). Even if the supply volts equal the rated VF of the LED I would still use a 1R series resistor!

 

This LED resistor calculator web site is really useful to use and ideal to bookmark.... http://www.led.linear1.org/led.wiz

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Hi Guys,

 

Just a quick update to show you the fruits of my labour, I popped over to the Engine Shed ( Gaugemaster ) as Squires are not open on a Sunday and brought some yellow Leds and some resistors. No one at the shop seemed to know the values of the resistors, but they do the job.

 

The photos were taken in my kitchen which has a lot of natural light, but I think once the sun goes down it will look just how I hoped it would be.

 

 

 

Thanks once again,

 

Martyn.

[Brian]

Cat looks happy!

 

The LEDs look good too.

[Crosland]

Cat looks happy!

 

A bit over scale to my eye.

 

Andrew

[DCB]

This will strike fear into the resistor fan club, just seeing my user name, however I think your LEDs are upside down.  Gas mantles leave a pool of darkness directly underneath them while electric lights do not.  If they are supposed to represent gas lights I think your LEDs should be anode and cathode down so the light goes upwards and reflects off the ceiling.  For the same reason I associate gas with wall lights.

 

Brightness is an issue, if you can see the lights in daylight they will probably be much too bright in darkness.  Mine are great when the room lights go out but apart from signalbox lights you can't tell if they are on or not with the room lights on.

 

Original question, I would go for 5 volts, you might get away with less as long as you don't try to use clear LEDs. The clear LEDs I have won't light on 2 X new Duracell AA batteries in series which is over 3 volts, Reds Yellow Green, and Orange light on 3 volts  I use pairs of tired AA batteries to power orange LED lights to light various 00 scale buildings outside.

[raymw]

 

This will strike fear into the resistor fan club

[dhjgreen]

There were 2 types of gas light. Originally they were a bare fan shaped flame, the light would clearly face upwards. Later mantles were developed which incandesce producing a whiter light. Generally this type had a reflector above the mantle giving a downward light. I have never seen either type ceiling mounted (edit to add: due to the heat) and as you are using yellow leds, these really should be wall mounted; rule 1 does however apply.

[Junctionmad]

I would always use a series resistor with any LED. The resistor can be fitted into either the Anode of Cathode lead of the LED. Note normally the longer lead of a standard LED is the Anode (Positive). Even if the supply volts equal the rated VF of the LED I would still use a 1R series resistor

 

 

Not to be pedantic , oh ok , I will, using a 1R resistor is meaningless. You can never just supply VF because of the tolerance on VF and the "Avalanche " style of the IF graph. The only way you can power a led at VF is in fact to use a constant current source where in effect a form of feedback controls the current through the led and hence by definition requlates the operational voltage across the led.

 

1R in effect does nothing and is misleading

 

All the rest of your original post does contain meaningful information

[Colin_McLeod]

There are already no end of websites that do that, better to link to one of them. Whatever, it will make no difference to the local village idiots who insist no resistor is needed.

There are LEDs with a built-in resistor and that may be causing those who did not realise to think that a resistor was not required. To call people a "local village idiot" us both unfair and unexpected from your good self.

Regards Colin

[TEAMYAKIMA]

I will admit that I am confused by LEDS

 

I am currently using GOW bulbs because I can easily dim them according to ambient lighting conditions in my shed and/or exhibition halls.

 

May I ask a slightly different question that confuses me .......................

 

I need to fit about 50 new lights in individual rooms/buildings and would prefer to use LEDS ... but are they dimmable? And by that I mean the brightness can be adjusted on a day by day basis.

[cliff park]

Problems arise trying to dim so many with a variable resistor. 50 LEDs at 25mA each equals 1.25 Amps. Let's assume a 2K pot in series and the power (I²​R) becomes in the order of 3 Watts. This is more than a normal carbon track pot can take. You could probably do it with say four or 5 pots and sharing the load. A simpler solution would be to experiment with fixed resistors (say 5 Watt rated) to determine suitable light levels and then use a rotary switch or similar to switch in different resistors. You wouldn't have the fine control, but 3 o4 different levels ought to suffice.

[Crosland]

You are starting form the wrong premise.

 

You would never try to control all the LEDs from a single resistor.

 

Modern LEDs don't need anything like 25 mA, even less when dimmed.

 

You use a higher voltage supply and connect a few LEDs in series. Four LEDs, for example, still only take 10mA or whatever operating current you choose.

 

Your 1.25 A quickly reduces to less than 200 mA (1250/4*10/25).

[TEAMYAKIMA]

Yes, ordinary leds can be dimmed. There's 2 methods: current limitation and PWM (pulse width modulation).

 

Presently I use GOW bulbs..... my only problem is the current draw is far higher than I had planned for which is the only reason I'm thinking of moving to LEDs.

 

This is what I currently use to control about 30 GOW bulbs (off one unit). I have more of these controllers and would have expected to control the LEDs with them.

 

[RAF96]

Against all expert advice and ignoring those on line calculators I use a standard 1K ohm resistor on all my led installations and also as an additional limiter in the blue wire when fitting Express Model lighting kits to keep them within a cheap decoder function current limit (100mA).

 

Should I need to balance red/green bicolour leds to make yellow/amber then of course I work out the necessary values, but hey special case needs special resistors.

 

I use an old anaolugue speed controller to dim my station and street light leds to suit the scenario.

 

I suppose I must therefore fit into the village idiot category as a result but so far in a decade or more of modelling I have not lost an led including those strapped directly across DCC tracks.

Rob

[Junctionmad]

Against all expert advice and ignoring those on line calculators I use a standard 1K ohm resistor on all my led installations and also as an additional limiter in the blue wire when fitting Express Model lighting kits to keep them within a cheap decoder function current limit (100mA).

Should I need to balance red/green bicolour leds to make yellow/amber then of course I work out the necessary values, but hey special case needs special resistors.

I use an old anaolugue speed controller to dim my station and street light leds to suit the scenario.

I suppose I must therefore fit into the village idiot category as a result but so far in a decade or more of modelling I have not lost an led including those strapped directly across DCC tracks.

Rob

12v and 1k gives an led current around 10mA which is right in the middle of most leds operating current range , even high efficiency leds can tolerate 10 mA.

 

That's why you haven't blown anything , buts that's no magic involved , just ohms law