Any LED Experts Here?

[tuanduong]

Hi all, it is some time since I visited this site,
but here I am looking for some guidance.

When I last worked seriously on electronics
they were all glass valves, so I know nothing.

I tried to convert a LED work light to infra-red
by removing the bright white lights and fitting
IR lights at 850 nm, there are 24 lamps, powered
by 3 X AAA batteries, when I switched on nothing
happened except the resistor got very hot.

I may have been sloppy in my soldering and left a
short circuit somewhere on the board, though I
like to think that I didn't, if not that can you tell
me why the circuit didn't work, I assume that the
different colours of LED have different electrical
values, but I have no idea what they may be.

There was a small calculation where I had to know
the voltage supply + the voltage of the lamps time
a large number that I can't remember, nor can I find
it again on the internet.

Any advice and guidance will be accepted with a lot
of gratitude.

Thank you in advance.

Mike.

 

[Simon47]

Dunno about IR LEDs, but white LEDs tend to have a higher forward voltage than other colours. So chances are that the IR LEDs were working, but the forward current was higher because of the higher current through the resistor - so resistor with higher current and higher voltage than as designed = hotter resistor.
You need to check the specs for the IR LEDs you used, and do the sums. Multiply forward voltage at the desired forward current by the number of LEDs in a string. Subtract that from the supply voltage - that gives you the voltage across the current limiting resistor. Then V=IR will give you the value of resistor needed to get the right current. You will also need to work out the power dissipation in the resistor, and you may find you need a higher power rating.
Depending on the arrangement, it may be more efficient to put more LEDs in series and fewer strings in parallel - but that might not be practical.

This is all assuming it's a very basic setup with a current controlling resistor as the only thing setting the LED current.