Arfa Daily wrote:
"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


I suspect 78L12 & 79L12 for powering the board's 'opamp' module and 7805
(or maybe 78M05) for logic. What is a 78L series good for when its gone
into foldback limiting?

Now a *proper* school lab PSU is probably good for something like 25V
adjustable at well over 10A. I have fond memories of cutting tin plate
with a carbon arc using a 3B pencil, two such lab PSUs strapped in
parallel and a whopping great inductor wound with silk covered wire with
a bundle of soft iron wire as its core, mounted on a beautifull piece of
varnished mahogany, with *massive* brass thumbscrew terminals in series
to stabilise the arc. *AHH...* the joys of a physics lab supervised by
a newly qualified substitute teacher . . . . :-)

Didn't pay to get a 74xx logic chip in the breadboard backwards, the 5V
bench supply was rated for something like 200A (it was a 4 foot high
cabinet with fan cooling at the front of the room and each row of
benches had a 30A thermal circuit breaker). A dead short would melt
your hookup wire quite easily before the breaker tripped and a reversed
chip would melt itself into the breadboard. SMELL that smoldering PVC
insulation . . .


--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
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