this circuit is for a rechargeable flashlight. It charges the battery using a 220V power supply, after reducing the voltage. A capacitor and a resistor in this circuit are damaged. The capacity and voltage of the capacitor are known, but a resistor (marked with a pink circle) connected to the red LED has burned out and its color is not clear. Does anyone know the ohm value of that resistor? Thanks
It's impossible to tell the resistance of this resistor without being able to read its stripes – which is impossible after it's been damaged this much. The rest of the board, including the LED and at least half of the bridge rectifier diodes must be damaged as well. Replacing the LED would mean having to choose a different resistor anyways, and to know which resistor to choose, one would have to measure the bridge rectifier, which however must be damaged, otherwise you wouldn't see the catastrophic heat damage to the resistor.
So, really, nothing relevant of this board still works, and you need to replace it in whole.
While doing that, replace it with something less dangerous (and less prone to self-destruct) than this board. I don't know which voltage or current your flashlight takes, but it's probably possible to figure that out. Cheap power supplies that are actually safe and robust can be had at similar prices than you'd pay for shopping for 7 individual components, so trying to save this board makes no sense technologicially nor economically. Throw it away and safe yourself the chance of dying in a house fire or due to electrocution, caused by an unsafe supply design like this.
That circuit always was a threat to life.
Very importantly - the 400V rated capacitor used appears to be the one listed here which MAY be of reasonable quality in its intended role, BUT is unsuitable for main use.
Such capacitors MUST to be X or Y rated.
This one isn't.
Under most situations the capacitor will work well but under some surge conditions it will fail - as ,ay well have happened here. The capacitor may be short or open circuit, or walking wounded. (All of these are possible after a surge failure).
High voltage has appeared on the bridge output and destroyed the low voltage capacitor and LED resistor. The LED MAY now be open circuit.
Rebuilding the circuit may be possible but it is certainly not worthwhile and it will be lethally dangerous (again) if you do.
Best is to use a low voltage "plug pack" to supply the charge voltage.
The required voltage depends on battery voltage and type and charge circuitry. More details are needed to help in this area. IF this was connected to the charging circuit when it faiuled other circuitry may also be damaged.
Added:
Fault analysis, of sorts:
For the cap AND the LED resistor to have self dismantled it is very very very likely that a substantially higher than intended voltage was applied.
The LED would usually go open before a resistor had time to cook, BUT I have seen LEDs go short, and if this happened here the resistor then went OPEN.
You'd usually expect shorting an LED not to destroy its drive resistor so the voltage was probably substantial. The splattered cap suggests maybe 2x + voltage so maybe much more. My guess is that the non X or Y rated cap sustained spike damage and went short and applied mains to LED and CAP.
If so the diode bridge may be all you have that works - and maybe not that.
Using a wall wart externally, or even the internals of one internally if this is inside the torch would be much much much safer.
The electrolytic capacitor has spewed its guts all over the board, making it somewhat conductive. The board material is supposed to be dielectric.
Since the “guts” are ionic, you’ll need to wash and scrub the board with dishwashing soap and a toothbrush in the kitchen sink, and rinse it well afterwards. Then dry it out in the sun for a day or two.
The board material is pressed paper with phenolic resin. It is rather fragile and cracks easily. Be delicate when handling it. It will crack and break if you apply too much force to it, especially if you bend it.
I am on the fence whether repairing it makes sense or not. Sure, the design is barely adequate. But it should function fine after cleaning up the board and replacing the broken parts.
My biggest problem with this flashlight is that it is totally unprotected against fire due to component failures. If something in it fails well enough to draw say 50W or more from the mains, it will catch on fire rather nicely. As the failure progresses, it will start dissipating even more power, and will sustain fire even if all materials used in its construction were self-extinguishing.
So, if you insist on using and repairing it - only plug it in when you are nearby to supervise it. Don’t leave plugged it in unattended.
There is a simple fix that would vastly improve the safety of this flashlight. Instead of powering it from mains, power it from just about any DC power supply. The old Nokia 3.7V supply would do the job nicely. Of course the circuit would have to be modified, but at least I’d feel safe sleeping at night next to it.
I admit I have had a flashlight of the same design in my kitchen for 15 years. But at least it was made with higher quality components and has never failed - it still works. It also uses a properly rated mains dropper capacitor, and has a beefy Zener diode to further limit the internal voltage even if the battery would go open-circuit. It was random luck that I happened to buy one that wasn’t “cost optimized” down to nothing. I still am suspicious of that flashlight every time I look at it :)
