LED's

[NickD52]

How can you tell what size an led is without knowing the operating voltage please

[BoD]

What exactly do you mean by size? 

If you mean physical size that usually comes in the description of the LED or on it's datasheet which is normally available as a download at point of purchase.

If you are talking about anything else - current draw, resistor needed you will need to provide more information.

 

Also, if I may, can I suggest that you look more closely at the structure of the forum and the different topics/areas available.  Posting something like this in he electrics section is likely to get a better response.

[DCB]

No   There are all sorts of LEDs, some have built in resistors and work off 12 volts, most don't and imitate a super nova briefly before becoming a useless blob on 12 volts.  Some recent ones work on 1.5 volts and go pop on 2.5 volts.   It's a minefield

[NickD52]

So how do I tell what voltage an LED is without wasting loads just testing?

[Dagworth]

3 minutes ago, NickD52 said:

So how do I tell what voltage an LED is without wasting loads just testing?

Try it on 12v dc with a 1k resistor in series. No LED will blow at that. If it’s too dim then try a lower resistor, say 560ohms. If it’s too bright then try a 10k resistor. 
 

Andi

[PatB]

11 hours ago, Dagworth said:

Try it on 12v dc with a 1k resistor in series. No LED will blow at that. If it’s too dim then try a lower resistor, say 560ohms. If it’s too bright then try a 10k resistor. 
 

Andi

Agree with this. Playing around empirically will give you a bit of a "feel" for what's going on. I'd recommend buying an Ebay bargain bag of a zillion assorted ones at 1 or 2p each and practicing on those before risking cooking something specialist or expensive though.

[RAF96]

I have an old Maplins led test box powered by a 9 volt battery. No doubt these are still available from other places.

It has a series of internal resistors that limit the led operating  current.

It is simple to assess from the comparative brightness if an led is a regular 2-3 volt item or if it is a 12 volt with a built in resistor.

 

 

[AndyID]

On 09/02/2021 at 09:06, NickD52 said:

How can you tell what size an led is without knowing the operating voltage please

 

The light output of a LED is not determined by voltage. It's the current passing through it that determines the light output and the LED itself determines the voltage drop across it while it is emitting light. The voltage drop depends on the color of the light emitted. Red LED's are around 2 volts and blue/white LED's are around 4 volts.

 

E.G. - If you want to power a red LED from a 12 volt supply there will be 12 - 2 = 10 volts across the current limiting resistor. You might only need 5 milliamps to make the LED quite bright, so the resistor value would be:

 

voltage divided by current (Ohm's Law), or

 

10 / 0.005 = 2000 ohms.

 

You won't easily find a 2000 ohm resistor but you will find 2200 ohm (2K2) resistors which would likely be "close enough".