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Ruston
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Evening all,

 

I have bought some LEDS to make some colour light signals but have never had anything to to with LEDs, so before I accidentally blow them up I thought I ought to ask a few questions...

 

I understand that they only work when connected the correct way in that positive has to go to one particular terminal and negative the other, unlike a bulb which doesn't matter which way around. If I wire it the wrong way will it simply not work, or will it go pop? If it will go pop, how can I tell which way to wire it before actually doing so?

 

In addition to the simple green LEDs and the Red LEDS I also bought some that are clear but are supposed to be able to glow green or red in the same LED but there are only two terminals, so how does this work? How do I get it to change from one to the other?

 

Thanks

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LEDS are polarity sensitive but wiring them the wrong way doesn't damage them they just don't light.
The twin colour LED will light a different colour for each polarity. LEDS require about 1.5V DC so use either a low output transformer or resistors in a 12V feed.

You can use more than one LED per resistor but if a LED fails the others get more power and a chain of failures follows.

Wire all the - pins on the LEDS to a buswire and use a resistor in the feed to each + pin. (anode and cathode to give their correct names)

see http://www.electronics2000.co.uk/calc/led-series-resistor-calculator.php for a resistor value required. Go higher in value for longer life or lower for brighter light output but short life.

Edited by DCMarvel
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The leg that connects to the positive is the anode, which is normally the longer of the two legs on single colour LEDs. You must fit a resistor in line (series connection) to limit the current flowing through the LED. Normally a 1000 ohm (1k) resistor is a good starting point for 12 volt operation but you can increase the value if the LEDs are too bright. It doesn't matter which leg you connect the resistor to. I always fit them in the positive leg.

 

The 2 colour LEDs change colour depending on which way round they are connected. Again, you need to fit a current limiting resistor.

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  • RMweb Gold

I have bought some LEDS to make some colour light signals but have never had anything to to with LEDs, so before I accidentally blow them up I thought I ought to ask a few questions...

 

I understand that they only work when connected the correct way in that positive has to go to one particular terminal and negative the other, unlike a bulb which doesn't matter which way around. If I wire it the wrong way will it simply not work, or will it go pop? If it will go pop, how can I tell which way to wire it before actually doing so?

 

In addition to the simple green LEDs and the Red LEDS I also bought some that are clear but are supposed to be able to glow green or red in the same LED but there are only two terminals, so how does this work? How do I get it to change from one to the other?

 

As you say, LEDs need to be wired the correct way round.

 

They also don't like much in the way of current - or reverse voltage, for that matter.

 

This means that, when connected the right way round, they need a series resistor - if the supply voltage is somewhere between 10 and 20 Volts DC, a series resistor of about 1K is likely to be reasonably safe for initial test purposes.

 

 

One very easy way of working out which way round an LED should be connected on DC is to use an LED tester - I've personally found this one from Component Shop to be rather good: http://www.componentshop.co.uk/catalogsearch/result/?q=led+tester&cat=0

 

When LEDs are powered by AC (low voltage and limited current, of course), they also need another diode (or LED) wired in inverse parallel, to limit the reverse voltage to a safe value (typically 5V).

 

 

We now come to the 2 terminal bicolour LEDs you mention. These actually contain 2 LEDs, wired in inverse parallel (anode to cathode; cathode to anode) - so, connected one way round, you get one colour light - connected the other way, you get a different colour.

 

 

I hope this is of help - all the best with your electronics "adventure".

 

Regards,

 

Huw.

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Better not to think of them in terms of 'working off' a voltage like a filament bulb.

LEDs require a controlled current through them to operate correctly, typically about 10mA.

The voltage often quoted in LED specs is a feature of the LED when it is operating normally because it is a semiconductor device.

It 'drops' that voltage when powered from a current limited supply, so you need a bit more voltage than that in your supply to allow for the current limiter (usually a resistor) to operate.

LEDs should not be connected directly to an unlimited current supply such as a battery, in spite of any advice you might see to the contrary from certain people.

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Thanks for the replies. I was told that they will work on 1.5 to 3v and so I plan to power them with a pair of AA batteries.

Nope, you'll blow them , you either need a current limited supply , limited to approx 10-15 mA or more commonly a resistor.

 

A bulb develops a resistance that limits its current , an led has no such limit , and if fed directly by a power source ( inc batteries ) without a current limiting resistor , it will destroy itself

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Here's a diagram on how to connect your bipolar led:

 

attachicon.gifbipolar_led_UK.jpeg

GND means Ground or the negative pole of the battery/power supply. I've drawn the switch on the GND side, but in this particular case you can swap the position of the power wires. It just means the other colour will light up for a particular position of the switch. I forgot to mention in the drawing both resistors need to be approx. 1k in value. You can vary the value to experiment with brightness of the leds, a higher value will mean a lower brightness. Don't lower the value too much as the leds can (and will!) pop if you get it wrong. A sensible minimum would be 680 Ohm for a 12V power supply. If your power supply is lower, you can lower that value accordingly.

 

As stated above, a led ALWAYS needs a resistor so there's no reason to run your leds from a 3V battery unless your entire layout is run from that battery.

One resistor in the GND lead will suffice unless you need to tweak brightness separately

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In addition to the red/green two legged bi-polar leds, if you want red and green in the same led with the ability also to show yellow then go for the three legged bi-colour leds. You can get them in common anode (more friendly for DCC use) or the more often seen common cathode versions.

 

The common leg allows you to arrange your circuit to switch the other legs separately to show the red and green colours or switch both on together for yellow, but to get this right you need to have separate resistors in the colour legs as the forward voltage and currents of the colours are different and the resistors must be the correct values to achieve yellow.

 

Other colour combinations are available like red/white or red/yellow for combined loco lights.

Rob

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It looks as though you can't use 2-legged bi-polars with an Arduino then, unless someone knows different.

Why not, it can source and sink upto 40mA per pin , ( remaining within Chip power dissipation of course )

 

You use either a single pin , or a dual pin approach

Edited by Junctionmad
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It looks as though you can't use 2-legged bi-polars with an Arduino then, unless someone knows different.

 

I have never done this or know if it will work but is seems that it should, if anybody knows differently then please correct me

The outputs of the microcontroller must be able to source or sink current

With output 1 high & output 2 low current should flow from out1 High to out2 Low via the top half of the LED

With output 2 high & output 1 low current should flow from out2 High to out1 Low via the lower half of the LED

 

post-28417-0-30346700-1504444840_thumb.jpg

 

If you feed a Bi-colour LED with AC then you will get orange/yellow

Tweak the values of resistors1 & 2 to get the best colour

 

post-28417-0-33344100-1504445557_thumb.jpg

 

John

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  • RMweb Gold

I have used the circuit below quite successfully with a couple of PIC Micro Controller circuits described here on RMWeb.

(The PIC can sink OR source 25mA but I had to tweak the 'green' resister down to 75 ohms to have it match the Red's brightness.)

 

The advantage of this circuit is that it requires only 1 I/O line for an LED that has 4 different states - Green, Red, Orange or off. Another advantage is that a simple 0 to 5v supply (USB/Phone Charger) will quite happily run this all day.

The disadvantage of this circuit is that it is current hungry - and in the case of orange, processor power hungry (but you don't have to use orange - or any shade from red to green using PWM!)

 

 

Nothing particularly clever about this but other members may benefit from applying this design to their circuits – expecially if they have limited I/O left.

 

post-12815-0-21049600-1392720443.png

 

One thing to note is that when the LED is OFF then the Potential Divider will have a current of 28.5mA!

This will add up, and in my case if I have all 8 Bi-LEDs off then the quiescent current will be 228mA for no light at all!

 

So my circuit (8*LEDs) takes a quarter of an Amp normally! Not a problem, I just need to be aware of it.

 

 

Kev.

 

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Nope, you'll blow them , you either need a current limited supply , limited to approx 10-15 mA or more commonly a resistor.

 

A bulb develops a resistance that limits its current , an led has no such limit , and if fed directly by a power source ( inc batteries ) without a current limiting resistor , it will destroy itself

 Is a current limiting resistor different from a resistor that drops voltage and exactly what resistor do I need to use for running the LEDs from 3v battery?

 

Do I need one resistor for each LED or will one resistor do for all of them if I put it between the battery + and the switches? There will be no more than 3 LEDs lit at any time.

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I've just included TinkerAD in the 3D forum, this browser based program includes a virtual Arduino simulator - no more destroyed kit.... (been there!)

Arduino have a forum that includes masses of useful info - see https://www.arduino.cc, I did a very quick search for bipolar LED but failed to find any hits.

I think that John ks is correct in post 15.

 

Andy

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Is a current limiting resistor different from a resistor that drops voltage and exactly what resistor do I need to use for running the LEDs from 3v battery?

 

Do I need one resistor for each LED or will one resistor do for all of them if I put it between the battery + and the switches? There will be no more than 3 LEDs lit at any time.

The purpose of the series resistor in a led , is to act as a current limiter, the voltage drop is then a function of ohms law

 

For a single led , on a 3V battery, anywhere from about 50 to 100 ohms , will give 10-20 mA , assuming standard leds. For high efficiency leds ( 2ma type ) you would be looking at 470 ohms

 

I would recommend you put a separate resistor in each led to be switched

Edited by Junctionmad
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I just got some 0603 (1.6 x 0.8mm) leds to fettle my self-harmed Class 56 built in lights. They are quite small enough for my old eyes and podgy fingers to solder.

Rob

My MO with small LEDs is drill two small holes in something insulated the same distance apart as the led wires. Poke a feed wire down each hole. poke the led wires down the same hole as the feed wire. No soldering, easily replaced LED.

I also slip insulated sleeves over the LED wires and prod the feed wires down the sleeves again to avoid the need for soldering.

The right size holes should make detachable LED headlights for 00 steam locos do able if not exactly easy.

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I just got some 0603 (1.6 x 0.8mm) leds to fettle my self-harmed Class 56 built in lights. They are quite small enough for my old eyes and podgy fingers to solder.

Rob

My MO with small LEDs is drill two small holes in something insulated the same distance apart as the led wires. Poke a feed wire down each hole. poke the led wires down the same hole as the feed wire. No soldering, easily replaced LED.

I also slip insulated sleeves over the LED wires and prod the feed wires down the sleeves again to avoid the need for soldering.

The right size holes should make detachable LED headlights for 00 steam locos do able if not exactly easy.

 

 

 

You seem to be missing the point that the LEDs refrred to do not have wires.

Edited by Crosland
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