Cheap LEDs

[28XX]

I bought a bag of 10-of-each 3mm red, yellow and green LEDs, cheap off eBay when I was getting some other components. (No data sheets). I have used some of the red and green ones on 12v DC with 560R resistors to allow around 20mA. They look fine and quite bright. That's ok because they indicate "fault" and "running" conditions.

 

I've used some yellow ones tonight in a "power on OK" circuit and thought, since they will be on all the time, a lower brightness would be preferable. I kept doubling the resistor value up to 18K, 0.25mA @6v and they are still too bright!

 

Question. 1. Are all LEDs that efficient these days? Even cheap ones?

 

Question 2. Do I need to up the resistance to the red and green ones to avoid failures, or will they cope?

[nigelb]

different colour LEDS need different value resistors  to get a similar light level 

[Huw Griffiths]

Most LEDs can take 20mA - but, speaking personally, I'd probably aim for a significantly lower current - perhaps about 5mA - even lower (like 1 - 2mA) if the LEDs give enough light to be seen clearly.

 

This isn't just about saving energy. When you pass electric current through any component, some heat is also generated - and I'd expect it to take some toll over time.

 

LEDs are also reputed to degrade over time - a process I'd expect to be speeded up if they get "hammered" - I've always taken the line that a cool component is a happy component.

 

I understand that a number of LED manufacturers' data sheets quote forward current limits in the order of 20 - 30 mA. I've also come across some "low current" LEDs, with forward current limits in the region of 2 - 5 mA.

 

It's worth bearing in mind that these aren't target figures - merely maximum limits (under ideal conditions) above which manufacturers aren't prepared to warrant that these components will work for a reasonable length of time. They're effectively saying you exceeed these limits at your own risk.

 

 

Turning to what value series resistors to use, with 12 V DC, I'd probably suggest something like 2K2, 3K3 or 4K7 as sensible starting points - although some people might opt for 1K or 10K (multiply these resistances by a factor of 5 or 10 for the low current ones).

 

 

Here's another thought. You might have come across LED testers - effectively small plastic boxes with lines of small sockets on the front - and a 9V battery and resistors to limit current to something like a figure quoted on the front panel.

 

A number of electronic component suppliers sell these things - as do some traders at model railway and model engineering shows. You could also build your own - but you're probably better off buying one ready made, as they're not particularly expensive.

 

Although some people might view the things as an extravagance, I find mine extremely useful for checking and matching LEDs - and for trying out different current levels.

 

(It's also useful because, over the years, some LEDs have been supplied wired the opposite way round to usual - if there's any doubt, I'd prefer to know before I solder them into place!)

 

These testers are very easy to use - and I'd personally recommend them, especially if you use lots of LEDs (particularly cheap ones, with only sketchy data available).

 

 

These are, of course, my personal views - and I'm sure that some people would disagree with me.

 

 

Regards,

 

Huw.

[SHMD]

Is the LED light "very directional"?

Are the LED lenses clear or defuse?

What colours are on the resister bands - (and what is the voltage)? - Just checking!     

 

 

Kev.

[cliff park]

A very simple way is to wire a variable resistor (potentiometer) in series with the LED, say 20K to 50K, then vary the resistance until you have the brightness you want. Measure the resistance and pick a preferred value somewhere near that. 

[DCB]

Something is very wrong.  

 

Generally Red LEDs take less voltage for full brightness than other colours.  Red usually light around 1.8 volts and are bright at 2.2, then Green Yellow etc in increasing voltage in sequence until the Clear needs over 3 volts to light at all. if your Red and green LEDs work with the same resistors I would suspect they are from different manufacturers or ranges.

 

Check your power unit output voltage, many 12 volt railway units are 22 volts off load.   Your resistors may be duff and marked wrongly.  

 

It sounds very strange.  doubling the resistance should make a real difference. You could try a potentiometer as suggested but 1 per LED.

[Gordon H]

Generally Red LEDs take less voltage for full brightness than other colours.  Red usually light around 1.8 volts and are bright at 2.2, then Green Yellow etc in increasing voltage in sequence until the Clear needs over 3 volts to light at all. if your Red and green LEDs work with the same resistors I would suspect they are from different manufacturers or ranges.

Please ignore any advice on LEDs from this source if misinformation and flawed theory. Concern yourself only with controlling the current that flows through the LEDs, not the voltage they drop.

[Suzie]

It is always handy to have the data sheet for the LEDs if possible. Some LEDs will give little more than a pleasant glow at 20mA, and there are others that will blind you at 1mA. It sounds like you have LEDs from different ranges or manufacturers. The very bright yellow ones are probably a type normally described as 'High Efficiency' or 'Superbright' and you might need a very high value resistor to tame them.

 

Brightness of LEDs can range nowadays from 10mcd all the way up to 4000mcd for the same current, and they all look the same! As an example I used some LEDs recently on 12V and while the red, yellow and green ones were all quite happy with 1K resistors, the white and blue ones needed 4K7 resistors for a similar brightness and these were all from the same range. There are a lot of LEDs available nowadays and we are spoiled for choice looking at what is available from electronics distributors.

[SHMD]

I try to avoid the "Kingbright" ones!!!

 

 

Kev.

(Actually not true)