Wiring multiple 3v LED's to 12v DC

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steve70
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Joined: Sun Feb 12, 2012 5:19 pm

Wiring multiple 3v LED's to 12v DC

Postby steve70 » Thu Feb 11, 2016 9:20 pm

I'm currently building my Scalescenes Large Station Building and (luckily) before I got too far, I saw a video from DaveClass47 @ Dean Park Station building his.
He pointed out that it would be difficult to light once completed. I hadn't even thought about lighting mine so promptly ordered the same LED's and 680 Ohm resistors.

So I've got 6 x 3v LED's installed and gather that each one needs a resistor....Or does it?
Is it possible to wire say 3 LED's together and use one resistor?

Thanks

Steve

Suzie
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Re: Wiring multiple 3v LED's to 12v DC

Postby Suzie » Fri Feb 12, 2016 1:51 am

You might like to make the LEDs in to two strings of three and see how you get on with your 680R resistors (one per chain). It will give you around 4mA current through the LEDs which will probably be more than bright enough. If you need brighter use a 150R resistor in each chain instead to give you 20mA which will be full brightness.

Gordon H
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Re: Wiring multiple 3v LED's to 12v DC

Postby Gordon H » Sat Feb 13, 2016 9:41 am

Is there anything that can be done to stop retailers selling LEDs based on the notion of 'voltage rating'?
I guess it is because many of them still don't understand that these things are not like passive filament bulbs which are intended to run from a defined voltage supply. Alternatively, they may just see it as an easier way to pass LEDs off as direct bulb replacements - which they usually are not.
Anyway, LED operation is defined by the current that flows through them, not the voltage across them as such. The so-called 'voltage rating' is merely a consequence of passing this current through the LED, which is a non-linear semiconductor device. LEDs, like any other diode, exhibit a voltage drop when they are passing current. This voltage drop varies relatively little between the 'only just on' and 'far too much' states. As such it is far easier to control and vary the current that flows with a simple resistor than try to keep the voltage across it to such an exact ideal figure.
The main consideration that needs to be taken into account with the supply itself is that its voltage must be high enough to ensure that the LED will be able to switch on with it, and have a bit of extra voltage to spare for the resistor to drop, and thereby limit the current to the desired figure, typically about 10mA. Thereafter, it is just a matter of ensuring it can supply enough current for all the LEDs attached to it.
Of course, there are special cases, such as LEDs with resistors built in - usually for 12V or 5V operation. However, any LEDs you might see advertised purporting to have a 'voltage' of less than 5V is likely to be a bare unprotected LED, requiring the considerations outlined above.

Tricky Dicky
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Joined: Tue May 03, 2011 10:01 pm

Re: Wiring multiple 3v LED's to 12v DC

Postby Tricky Dicky » Sat Feb 13, 2016 8:10 pm

Gordon H wrote:Is there anything
Anyway, LED operation is defined by the current that flows through them, not the voltage across them as such. The so-called 'voltage rating' is merely a consequence of passing this current through the LED, which is a non-linear semiconductor device. LEDs, like any other diode, exhibit a voltage drop when they are passing current. This voltage drop varies relatively little between the 'only just on' and 'far too much' states. As such it is far easier to control and vary the current that flows with a simple resistor than try to keep the voltage across it to such an exact ideal figure.
The main consideration that needs to be taken into account with the supply itself is that its voltage must be high enough to ensure that the LED will be able to switch on with it, and have a bit of extra voltage to spare for the resistor to drop, and thereby limit the current to the desired figure, typically about 10mA. Thereafter, it is just a matter of ensuring it can supply enough current for all the LEDs attached to it.


Hear, hear Gordon, resistors used with LEDs are called "current limiting resistors" not voltage limiting resistors, setting the voltage drop across the LED is setting the current limit. The ideal power supply for a LED is a constant current supply, which makes even using a resistor a bit of a kludge, but in general it works.

Richard

steve70
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Joined: Sun Feb 12, 2012 5:19 pm

Re: Wiring multiple 3v LED's to 12v DC

Postby steve70 » Sat Feb 13, 2016 8:30 pm

As nobody actually answered my question I've actually tried it out for myself and found that YES.I can run multiple LED's off one resistor.

Thanks for all the mind boggling formulas and theories. I will bare them in mind.

Suzie
Posts: 327
Joined: Tue Dec 17, 2013 11:46 pm

Re: Wiring multiple 3v LED's to 12v DC

Postby Suzie » Sun Feb 14, 2016 2:17 am

You did as I suggested in the first response to your question. Well done.

Tricky Dicky
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Joined: Tue May 03, 2011 10:01 pm

Re: Wiring multiple 3v LED's to 12v DC

Postby Tricky Dicky » Sun Feb 14, 2016 2:45 pm

http://www.electronics2000.co.uk
steve70 wrote:As nobody actually answered my question I've actually tried it out for myself and found that YES.I can run multiple LED's off one resistor.


Suzie did and there was no further need to add to her excellent answer

steve70 wrote:Thanks for all the mind boggling formulas and theories. I will bare them in mind.


However, If you do not want to boggle your mind with formulas, the following site has a free to download electronics calculator that will work out the resistor values for single and multiple LEDs in series. If you have found by trial and error what current gives the appropriate light from your LEDs then you can specify that value rather than the default 20mA.

http://www.electronics2000.co.uk

Also included is a resistor colour code calculator to check you actually have the correct value resistors.

Richard

steve70
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Joined: Sun Feb 12, 2012 5:19 pm

Re: Wiring multiple 3v LED's to 12v DC

Postby steve70 » Sun Feb 14, 2016 9:11 pm

Thanks Richard..
And apologies to Suzy. I was after for a yes /no answer. I also sacrificed one of my LED's just to see what happens if I put 12V to it without a resistor :)

Suzie wrote:You might like to make the LEDs in to two strings of three and see how you get on with your 680R resistors (one per chain). It will give you around 4mA current through the LEDs which will probably be more than bright enough. If you need brighter use a 150R resistor in each chain instead to give you 20mA which will be full brightness.


OK but I'm still having a hard time understanding all this milliamp stuff.
I have 'warm white' LEDs which are fine/maybe a tad too bright straight off a 3v battery supply but they are close to what I want.
So I'm just going to buy a load of them and some 680 and 1000 Ohm resistors and have a play. I'm starting my building again from scratch so have a bit of time to source things. :)

Tricky Dicky
Posts: 205
Joined: Tue May 03, 2011 10:01 pm

Re: Wiring multiple 3v LED's to 12v DC

Postby Tricky Dicky » Mon Feb 15, 2016 1:48 am

steve70 wrote:
OK but I'm still having a hard time understanding all this milliamp stuff.


That is quite understandable, with bulbs being rated by voltage it is a simple halve the voltage and the bulb will dim. Put two 12V bulbs in series with a 12V supply and the voltage will be divided equally across each bulb. The bulbs will draw whatever current they require and is therefore a non-issue except that total draw does not exceed the max. output current of the PSU and if using batteries how soon they will be exhausted.

LEDs are different, they are current controlled and as Gordon pointed out simply stating a LED is 3V rated is not enough information to calculate resistance required other parameters are required. The only exception to rating LEDs by voltage are the types of LEDs that have built in resistors and these are most commonly rated at 12V and 5V. Parallel wiring of this type of LED is simple and adding further resistance in series with each LED will dim them. Series wiring is a problem because you have fixed resistor values being added to work with where under normal circumstances you would be reducing the resistance.

So why all this "milliamp stuff" ? To calculate resistance Ohm's Law is used which if you can remember from school days this relates voltage, current and resistance, know any two of those values and you can work out the third. Now you can see why just rating a LED by voltage is not entirely helpful.

So where do you get the information you require? Data sheets are the answer these will give typical forward voltages and typical and maximum forward currents plus other information. Each colour has different max. forward voltages and forward currents and as long as you do not exceed these parameters you do not need to be too precise with your resistor values.

Typically standard red and green LEDs have a forward voltage of about 2V and max. forward currents in the low 20-25mA whilst at the other end blue and white varieties can be 3.5V or there about and around 35mA. Typically a data sheet will for standard red or green will recommend achieving a current of 20mA this will be quite bright and OK for indicator use on electronic equipment, going down to 10mA will make little difference to the brightness. With white LEDs depends on what you want to achieve, for a lighting gantry to illuminate the layout as a whole you probably want maximum brightness so would tend towards the maximum current. To light up individual buildings or model street lamps then a much lower current is required for a more realistic effect so a lower current using a higher value resistor is required. LEDs do not dim proportionately so doubling the resistor value will not necessarily halve the brightness.

Anyway that's all the theory, using electronics calculators like the one I recommended in my previous post take all the hard work out of it for you and ensure that you at least do not use too low a resistor after that it is down to experimentation with higher value resistors until you find the appropriate light level.

Finally, like GordonH I recommend using a power supply with a voltage above that required for the LEDs and resistors to drop it down to the required voltage and thus setting the current. Using say a LED with a recommended forward voltage of say 3V with a 3V battery and no resistor is OK for a short test but can lead in the long term to a very shortened life for the LED. The theory behind that is a whole new can of worms and not something to go into now.

Richard


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