Adding an LED fed from a 6.3v DC supply

[Paul Bannister]

I thought this might be better as a new thread. Can anyone post a circuit (with values) for adding a on/off led fed from a 6.3v DC supply.

[Phil Y]

I'm afraid it's not as simple as that. You need to know the current required for a particular case. You need to know two things. Firstly the voltage. This is the supply voltage minus the voltage drop across the LED. The drop is different for different colours of LED. Secondly what current does your chosen LED need ? If it is a very low current one as often used in battery equipment, it might be as low as 2mA. If it is a super high brightness one it might be 50mA. It's simple to work out but you do need to know the spec. of the led you are using. If you wanted to do it by trial and error, you could assume a 2 volt drop and about 20mA but don't blame me if you blow up your LED. It is worth bearing in mind that LEDs can have some nasty chemicals in them, in the case of red, gallium arsenide

Regards, Phil.

[Paul Bannister]

Thanks for getting me started Phil. Any more advice gratefully received.

[Phil Y]

Example:

My (random) chosen LED.
http://uk.farnell.com/avago-technolo...amp/dp/9870776

6.3v - 2.2v=4.1v

R=V/I

4.1v/25mA=164 ohm

150 ohm will be fine.

Finally: power=VxI

4.1v x 25mA = 0.1025 watt.

A standard 1/4 watt resistor would be fine.


Cheers, Phil.

[Paul Bannister]

Phil,
That's a really clear explanation - thanks.