"Ron(UK)" wrote in message
...



On Jan 26, 5:51 am, "Arfa Daily" wrote:
"Victory" wrote in message

...

I forgot to add - with the 12v, the LED went all the way to it
brightest and dimmest while turning the resistor. It was perfect, but
with the 5V, it peaked near the top of the resistors turn, and then
started to burn out. Not sure why with 5V it would burn out, but with
12V it did not.
And exactly what led you (no pun intended !) to this odd conclusion? I
can't
think of any practical or theoretical reason why this should have been
the
case. 12v and full range control, yes, no problem. 5v, and range of
control
bunched up one end. Yes, no problem. Then it was "burning out" ? What
was,
the pot or the LED ? If the pot, then I don't know why it should have
been
giving trouble at the lower voltage rather than the higher, but it does
rather neatly underline the problems that can occur when you use a
simple
pot to control DC, rather than adding a 'helper' transistor as I
suggested
*might* be a good idea.

Arfa

Victory wrote:

At the highschool it was on one of those 'tester' boards with
different voltages and such.

Top post clipped for clarity

Possibly the 12 volt output was current limited and the 5 v not so

Ron(UK)

Hmmm. It's a thought, Ron, but we're only talking a pretty small current
draw here, and I'd be surprised if any current limiting on a schools type
power supply would have the 'delicacy of touch' required to be able to limit
the current to something tha would not burn out a white LED i.e. less than
30mA perhaps ?

Arfa