View Single Post
  #5  
Old October 12th 17, 07:35 AM posted to uk.tech.digital-tv
Woody[_5_]
external usenet poster
 
Posts: 1,800
Default [OT] LED polarity etc


"Pinnerite" wrote in message
news
It is not often that I point a soldering iron at anything but I
wanted to
add a switch and an LED to a battery powered function generator
about the
size of a fat credit card.

I decided a strip of Vero board would do and had a few scraps from
the early
'70s.

We use to use lead/tin solder but it appears 'elf-n-safety abolished
that.
The first thing was which way round to connect an LED. I have a
substantial
stock of components but no idea of the spec of the LEDs.

Calculating the value of a suitable current limiting serial resistor
was
easy enough but which way round should the leads go?

A quick web surf and I appeared to have the answer, the internal
flat plate
should be negative and the longer lead positive. Except on mine it
was the
opposite.

Anyway I had to drill the tiny board to mount the toggle switch
before
soldering it to the board, plus the two components and the leads to
the PP3
battery. It should have been easy. The solder would not flow. There
was no
flux on the board and it looks like the solder, purchased from
Maplin did
not have the cores of the Multicore that I was used to.

I'm beginning to dislike progress. I must have got old when I wasn't
looking.



The cathode always has a flat on the edge of the LED case against it.

In terms of current calculations, take your supply voltage minus 2V
and multiply it by 100 to give you the value of resistor taking the
next lower preferred value. Why? Most resistors work quite well on
10mA of current but typically anything in the range 10-20mA will do.

15-2=13 x100=1300, next preferred value down is 1K2.

Simples?


--
Woody

harrogate3 at ntlworld dot com