Tech Forum

First, I would check the voltage coming from the crank generator, if there's no voltage there then that's the problem. Usually mechanical devices die before electronics. Assuming the generator is working and you have more than 3.6 V (you need 3.72 for Li-ion batteries usually), the transistor is probably a three-pin regulator and the added other parts are to keep it to 3.72 volts. Any three-pin regulator will work OR you can buy a complete Li-ion charging circuit from China for under $5. I think I bought five of them for around $5 at one time.

I've just signed up to the tech forum and noticed the question #2163 Cranky Flashlight; two things stand out.

1, The zenner diode ZD1 should not be a 1N4148 and needs to be replaced by a zenner diode as close as possible to 4.9V, since the voltage at Q1's emitter will be about 0.6 to 0.7 V lower than ZD1's cathode.

2,  The transistor Q1 is not connected correctly to work as a voltage regulator in conjunction with ZD1 and the 47 ohm resistor. Note - the full part number is 2SC8050.

The diagram shows the corrected flash light circuit with the base and emitter transposed.

The zener diode/transistor combination are a voltage regulator to provide an upper limit to the battery's charge voltage.  (see schematic) 

Somewhere on the internet a few years back, perhaps on Youtube, someone suggested replacing the failed battery with a 1 Farad memory back-up capacitor. At the time Jameco carried a 1.5 Farad capacitor of the same physical size, and with some minor board modification I was able to mount it in place of the battery. It now seems to work better than with the original battery.

Use a DMM to see if the Li-ion battery is being charged when the crank is being turned. The battery should have around 4.1 VDC across it if the charging circuit is working correctly. A replacement for the 2SC8050 is the SS8050 (try www.mouser.com/Semiconductors/Discrete-Semiconductors/Transistors/Bipolar-Transistors-BJT/_/N-ax1sh?P=1z0z63xZ1z0z60l&). The Li-ion battery may need to be replaced because it cannot hold a charge. So if you have the correct voltage across the battery with the crank turning, you probably need another battery.

I've drawn the attached schematic to show what I believe the design intent for this flashlight was.  It is a little different than the presented board, but works to explain what is probably wrong.

Since the lights come on when cranked, the full wave rectifier section is working and the transistor is conducting. I believe the intent of the ZD1, R4 and Q1 is to maintain the voltage at the battery at a safe maximum level. This would occur if ZD1 was actually a zener diode instead of a 1N4148. The zener diode would have a breakdown voltage somewhere between 4.3 and possibly as high as 5.1 volts. The battery would then be held below the zener voltage by the BE drop of the transistor, for the range of values mentioned 3.6 and 4.4 volts. 4.4 volts is really too high, but 5.1 volt zeners are cheap and widely available so the manufacturer may have cheated a bit.

This is not a very sophisticated charge control circuit. The spec sheet for the battery allows no more than 50 mA of charging current. Depending on the generator this could easily be exceeded by this design. The most likely problem is a dead (not just uncharged) battery. The battery could be dead because of age or overcharging. The overcharging may not only be because of the unsophisticated circuit. A shorted transistor would make the problem worse, as would an open zener diode. 

If the transistor is shorted virtually any silicon NPN transistor will work as a replacement, as long as it can handle at least 50 mA. It will be easier to use if it has the same ECB arrangement of leads as the SC8050. A quick look through catalogs gives 2SC2655, TS13003 and APT27H as candidates.

Finally, there may not be anything wrong at all. I have similar flashlights, and when left unused for a long time it takes a lot of cranking to get the battery charged enough to light the lights. If the generator is specified to not exceed the battery limits too much it should take almost half an hour to bring a completely discharged battery to full charge, and probably at least half that to get it to where it will light the LEDs.

It looks like part of the PCB track connecting the cathodes of the 3 LED’s is missing from the diagram. The zenner diode can not be a 1N4148 (a small signal diode), you need a 4.7 or 5.1V zenner for the simple voltage regulator to limit the voltage applied to the battery to about 4 to 4.4V. Check the transistor connections. I expect the 2SC8050 collector should go to C1’s positive terminal, the base to the junction of the 47 ohm resistor and the zenner cathode, the emitter then goes to the battery positive terminal.

Several things seem amiss. Diode 3 should connect to diode 4, probably under R1,R2, and R3. This would complete the negative part of the circuit. Zener diode 1 is not a zener if it is a 1N4148. This is a small signal diode with a reverse break down of 75 to 100 Volts and with E-B junction of Q1 probably protects the generator overvoltageing of the circuit Speaking of Q1, the SC#### is in the form a Japanese transistor just a 2 first. Search the internet for a 2SC8050. The picture of the switch does not appear an ON-ON-OFF but an ON-OFF-ON. Now to what wrong, since it works when cranked everything except the battery is working. When not cranking the circuit fails, then battery is not charging. A bad solder connection to the board, a bad connection to the battery, or just a bad battery. I would suspect the battery A voltmeter across the battery would give you the answer.