Flashlight problem

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.
TIA
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On 10/21/2015 10:30 AM, KenK wrote:

Are you sure there aren't two "settings" available: - 7 LED - 17 LED
Is there a pattern to the LED's that are failing to illuminate? I.e., left side, right side, outer ring, inner core, etc.
Is the LED assembly accessible (to inspect)? Or, is it a sealed assembly (discard when broken)?
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No such switch I know of. Just on/off pushbutton.

Seems random. Some much brighter than most.

Seems to be sealed, or at least I see no way to get at its insides.
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On Wednesday, October 21, 2015 at 2:00:09 PM UTC-4, KenK wrote:

I'm not saying you are wrong, just that an "on/off push button" can do more than just turn the device "on/off". It can indeed control 2 modes.
I bought some no-name LED lights with a single "on/off push button".
One push turns a set of 15(?) LEDs on the face of the unit as a work light. The next push turns the work light off. The next push turns on 5(?) LEDs on the end of the unit as a flash light. The next push turns the flash light off. Wash, rinse, repeat.
I wanted to use them in a linen closet, with one attached to the underside of each shelf to provide light for the shelf below, but I didn't want to have to push the button 3 times to get to the work light function.
I opened each one up and moved a wire so the switch is now an "on/off push button" that controls only the 15 LED's on the face. Internally, the switch is still toggling between the 2 modes (flash light and work light) but externally, the work light just turns on or off with each push.
Maybe yours is supposed to be a 2 function device but the switch is screwed up and not making the correct contact any more.
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Intersting theory, but I don't think so. In the years I've had this light the switch never did more than turn in on and off, always all 17 LEDS.
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On 10/21/2015 11:00 AM, KenK wrote:

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"

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.

"Discard when broken" :>
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On Wed, 21 Oct 2015 12:05:32 -0700, Don Y

On a lot of the cheap "no name" stuff they don't even use a ballast resistor. They simply wire pairs of LEDs in series, and then parallel them. Depending on the LED forward voltage, they may wire triplets in series instead of pairs.. So if one LED fails, 2 or 3 go out. If one shorts, another 1 or 2 get over-bright and shortly after also fail. The GOOD ones will use current regulator chips for each LED, and will continue to put out consistent light until the battery is totally dead (or at least until the battery voltage drops below the forward voltage of the LED. There are also "regulators" that will run a 3.5V LED off a 1.2 volt battery, at full brightness. These are called buck/boost regulators.
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On 10/21/2015 2:13 PM, snipped-for-privacy@snyder.on.ca wrote:

A typical way is to use the internal series resistance of the battery to limit the current. Works great if you use crap "heavy-duty" AAA batteries. By the time you get up to D-Cells, that ain't enough resistance.
I modified one of the free Harbor Freight 9-led 3-AAA lights to accept one 18650 lithium ion battery to see how long it would last. 4.2V is less than 4.5V, right? Well, it was quite bright for a while. Even when used less than a minute at a time, the lights began to flicker and die. There's zero heat sinking.
I did another one with 1.5 ohms in series. That one is still going strong.
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On 10/21/2015 2:00 PM, KenK wrote:

Please try pressing the button several more times, let us know what happens.
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Tried that. Same light display each time.
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On 10/22/2015 1:53 PM, KenK wrote:

I'd listen to the others who mention possible bad trace on circuit board. Some times lights will unscrew so you can get at the board.
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wrote:

Don't appear to be as best I can tell.
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Cross-threaded ends would not make some LEDs work and others not. The LEDs in a 17 LED array At least in most "cheap" arrays) are wired in a series-parallel matrix, and if one fails open, all in the series section shared by that LED will fail. If one LED fails shorted, the other LEDS in that series string will light brighter - and will eventially fail from overcurrent.
I have had really bad luck with some Chinese LED lamps failing both ways.
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Glad to know that I'm not the only one who destructs broken stuff before tossing it ! I like the term " educational destructive autopsy " thanks ! I agree - poor solder connections are common < on the little 3 x AAA 9 / 21 LED flashlights > The other common problem was a bad battery - that didn't test bad at first .. but putting in 3 good brand-name batteries - made the difference .. Not often the switch - unless it was obviously broken. John T.
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There is probably a break in the printed circuit board they are mounted to. If you got nothing better to do, remove the board and start looking with a magnifying glass, or jumper them if you know the basic pattern of the board. It's easier to just buy another flashlight, but I too have torn stuff like that apart and sometimes can fix it.
A local rock band has all LED colored stage lights. Each color has an individual controller. There are hundreds of LEDs in each light. One of the lights has a bad segment, whereas about 10% of the lights dont light up. All of them are in one section, and all colors are affected. That tells me that the neutral is not going to that section. Probably a break on the board. I know the guys in the band and asked the guy that runs the lights about it. He said he dont work on that stuff, and was told it would be expensive to repair by the store that sells that stuff. He said that eventually he will just replace that light if it gets worse. But he said they are costly, so he will just uses it as-is, until it gets real bad.
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I agree. Probably a cracked ciruit board or cold-solder joint. Or shorted ot open LED as someone else suggested. I'm just trying to decide if it's worth the time to tear the bulb assembly all apart. I'm retired so I have more time than money, but I'm incredibly lazy.
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Get off your lazy ass and consider it an adventure!
If you can get the board out, it should not be that hard to find the crack or open circuit board trace or had solder joint. An open LED will only affect one LED. A short will not allow ANY of the LEDs to light.
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On Thu, 22 Oct 2015 14:05:04 -0500, snipped-for-privacy@none.com wrote:

Not true. An open LED will affect all LEDs in the series string, if they are wired series parallel - which virtually all COB (Chip On Board) Arrays are. If one is shorted it will over-drive the rest of the LEDs in that string, causing them to fail in short order.
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After having bad luck with multiple led cheap flashlights, I stay away. LEDs had poor connections flickering. Pretty much throw aways.
Greg
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On 10/23/2015 12:39 AM, gregz wrote:

Most of those are 10 year old technology. At the time one LED was puny so the idea was to cram as many as possible into the flashlight.
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