On 10/21/2015 11:00 AM, KenK wrote:
Don Y wrote in
:

On 10/21/2015 10:30 AM, KenK wrote:
I have a No-name 17-LED three D-Cell flashlight and only 7 of the
LEDs are lit, and another one goes on and off. I tried new batteries
but no change.

I assume the flashlight is kaput. Anything I may be overlooking I
could try before I replace it? Screw-in parts like front end are
tight.

Are you sure there aren't two "settings" available:
- 7 LED
- 17 LED

No such switch I know of. Just on/off pushbutton.

Often, the button is multipurpose: press once for "low", again
for "high" a third time for "off". Lather, rinse, repeat.
Or, other variations on that theme:
- 7 LED's (low)
- the other 10 LED's (medium)
- all 17 LED's (high)
- off

Note that if the connection from the switch (or its internal
mechanism) to those "other 10 LEDs" is defective, the behavior
would end up as:
- 7 LED's (low)
- the other DISCONNECTED 10 LED's (off)
- all 17 LED's -- of which 10 are disconnected (low)
- off

"Can't tell the players without a scorecard"

Is there a pattern to the LED's that are failing to illuminate?
I.e., left side, right side, outer ring, inner core, etc.

Seems random. Some much brighter than most.

And, in truth, the arrangement of the 17 LEDs is probably somewhat
"random" -- it's not like a nice square grid of 4x4 LED's, etc. :

Apologies if I'm oversimplifying in what follows -- or, not
simplifying enough! And, freely mixing metaphors/analogies... :

Internally, a 3C (alkaline) flashlight looks like a 4.5V battery
feeding a bunch of LEDs through a switch. (duh!) The battery
represents the available "pressure" in the system. The switch
is a valve (thinking in terms of a water analogy, here).

LED's are current (flow rate) driven -- the amount of electricity
flowing THROUGH the device determines how brightly it lights. Sort
of like the rate of flow over a water wheel determines how quickly
it will rotate.

But, there is a voltage (pressure) component as well. E.g., unless
you can get the water up to the *top* of that water wheel to start
with, doesn't matter how much of it is flowing as it won't be flowing
*over* the water wheel!

LED's typically start to turn on around 2V. But, this voltage varies
depending on how much current is flowing *through* them. As current
increases, voltage "drop" across the device also increases. So, at
a low current, it might "drop" 2V but 2.2V at a higher current, etc.

(at too high a current it simply burns out)

We have 4.5V available and need to LIMIT this to something around 2V,
based on what the LED's actual needs (at a given current) are.

So, there is a ballast resistor in series with the LED's (and the switch)
that "soaks up" (drops) some of this available pressure (voltage).
The pressure (voltage) drop across the resistor varies with the flow
rate (current) through it. So, as the LED wants more current, the
pressure (voltage) dropped by the resistor increases -- which cause
the LED to need less current (because it has less AVAILABLE pressure),
etc. Hence the term "ballast".

[Hopefully that makes *some* sense : Sorry, it's too early in the morning
for me to be thinking in these terms! : ]

Now, with 17 LED's, if you stacked them end to end and EACH ONE needed
~2V, you would need a 34V battery to cause *any* of them to illuminate.

So, instead of wiring them in series, they are wired in parallel. I.e.,
all 17 (or, perhaps a group of 7 and another group of 10?) side-by-side.
So, they *share* the flow (current) from the resistor.

But, not all LEDs are created equally (manufacturing variations called
"process variations"). So, some LEDs might START to light at 2.0V while
others don't start until 2.1. Some might get brighter much more quickly
with increases in available current while others less so. Etc.

It's sort of like putting a bunch of batteries side by side in a charger
and hoping that they all charge at the same rate and to the same final
state. In practice, some batteries will be lower than others and may
need more charge, etc.

[this is a bad analogy as batteries in parallel can charge each *other*;
LED's in parallel can't *light* each other!]

The ideal solution is to select specific LEDs for EACH FLASHLIGHT such that
they are identical to each other. Then, they will "share fairly" because
their individual characteristics will track regardless of operating conditions
(pressure/voltage, flow/current).

Ain't gonna happen! :

A more practical solution would be to associate a single "ballast" resistor
with *EACH* LED. Then, tweek the value of that resistor to compensate
for the specific characteristics of the LED that it supplies! But,
that adds lots of "unnecessary" resistors for the sole purpose of
getting the same amount of light out of each LED. Does the (price driven)
user really care if LED #8 emits less light than LED #12? The user is
just interested in the TOTAL light out of the flashlight -- regardless of
which LEDs happen to be carrying the load!

Take thisi a step further and you end up with the "consumer solution" -- put
all the LEDs in parallel with each other and use *one* resistor to save
the 16 micropennies that the other 16 LEDs would have cost. And, live with
the consequences!

That gives you an idea as to why there is LED-to-LED variation.

The fact that some are dark suggests a broken foil (if all were supposed to
be on one circuit -- not high/medium/low) or a wire. Or, a bad switch as
described above. Or, a different configuration (perhaps TWO resistors
used: one for this group of 7 and another for a group of 10).

Individual LEDs "flickering" may be caused by a poor ("cold") solder
joint and/or mechanical flexing of the circuit board.

Is the LED assembly accessible (to inspect)? Or, is it a sealed
assembly (discard when broken)?

Seems to be sealed, or at least I see no way to get at its insides.

"Discard when broken" :