On Sat, 24 Dec 2016 10:01:48 +0000, Brian Gaff wrote:
Obviously? Why obviously? Most small LEDs turn on at less than one volt
forward voltage and it's the current through them rather than the voltage
across them that is the predominant consideration WRT brightness.
Consequently they may well run less brightly when powered by the non
alkaline cells, but that's due to the difference in the cells' inherent
characteristic internal *resistances* rather than their terminal voltages.
Hmm. I sort of know what you're saying, but:- It's the voltage across
the LED that will determine the current going through it.
If there is 1.35 volts across the LED that fixes the current going
through it, the LED doesn't know whether it's an alkaline cell or a
What I *think* you're saying is that the alkaline cell may well have
a higher internal resistance so that at (say) a current drain of 100mA
its output voltage is *less* that that of the NiMh cell at the same
On Sat, 24 Dec 2016 22:01:20 +0000, Chris Green wrote:
You're really not going to achieve anything useful here by focusing on
the voltage drop across the device which is a function of the diode's
characteristic *barrier height* as opposed to the applied voltage. An
avalanche effect occurs - a sudden rapid increase in current - at an
applied voltage well below the output of either type of cell (could be as
little as 0.6V) so *current* is the dominant criterion we must consider.
On Sun, 25 Dec 2016 05:44:07 +0200, The Natural Philosopher wrote:
Still pissed from xmas, NP?
If you'd been sober enough to read and understand the thread properly you
would realise I'm referencing small signal diodes here (0.55-0.72V BH)
for the sake of simplicity in making this comparison - NOT coloured LEDs
(exact same principle but an easier range of values to deal with).
But it's *still* the voltage across the device that determines the
current going through it. Yes, it's a non-linear device so there is a
'knee' in the current/voltage characteristic but this has no bearing
on the brightness from Alkaline versus NiMh batteries. At a given
voltage the brightness will be the same, if the Alkaline battery puts
a higher voltage across the LED then it will be brighter. The fact
(or not) that the NiMh battery *could* supply more current at a given
voltage makes no difference, the LED will only take the current it
takes at a given voltage.
On Sun, 25 Dec 2016 20:30:47 +0000, Chris Green wrote:
Yours is a hopeless approach that takes no account of the parameter
spread one encounters with different devices, even those from within the
same batch. This is a common problem we come across all the time in
electronic design. If you have a multimeter with a diode test function,
select a bunch of seemingly identical diodes and test them. You will find
there is a spread of results from maybe 0.65V to say 0.73. Hooking these
up to the same voltage will produce differing brightnesses and quite
possibly destroy those devices with a lower barrier height. OTOH, rigging
up series resistance in accordance with the device's datasheet to limit
the *current* to the same value will guarantee any variation in
brightness between devices to be so small as to be imperceptible and no
devices will be damaged or destroyed. This is the correct approach to
use. Try googling "monte carlo analysis" and you'll soon get the picture.
On Sun, 25 Dec 2016 21:26:19 +0000, Chris Green wrote:
A hopeless approach to understanding and predicting the behaviour of
these devices at a fundamental level. With much of electronics it
wouldn't matter, but with diodes and bipolar junction transistors you
really have to consider them from the current perspective, not voltage.
I am beiginning to wonder if tesco have put something in the Xmas sherry.
The number of post claiming outrageously erronous horseshit to be
perfectly true and valid has increased dramatically over the last few days.
That isn't what was being discussed. What originally made me comment
was the idea that somehow (because of its greater current capability)
a NiMh battery would push more current through the LED. I was just
pointing out that for a particular LED the current through the LED
will be the same for a given voltage across the LED, it makes no
difference what current the voltage source is capable of.
I agree that junction transistors and LEDs are 'current' devices, but
that wasn't particularly relevant to my original comment.
Connecting silicon diodes to anything does not produce any brightness.
LEDs are not the same thing as rectifing diodes. LED Vf is about 1.6v - 4v. The LED christmas lights I've used on NiMH instead of alkaline presumably have inbuilt resistance, and all the colours worked fine on both battery types.
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