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My joule thief Lights my LED for a second then dies, can anyone help?

My joule thief Lights my LED for a second then dies, I could only make it flicker by scratching two of the connections. I've used three different transistors but received the same result. The transistors were kn2222, pn2222 and 8050 (couldn't find Bc547,2n2222 etc). I had 18 loops around the toroid and a 100 ohm resistor, is there anything wrong with this and can I fix it?

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acmefixer6 years ago
A common error is the windings of the coil are not connected correctly. You can check this by disconnecting the two wires of one winding (only one winding) and swapping the two wires, then reconnecting them.

It is possible that the wires of the windings have been connected in other ways. To check that you will need some kind of way to check continuity, such as a DMM on the ohms range.

If this does not help, then it is likely that one of the parts has been damaged, typically the transistor, but the LED could be, too. However the LED usually shines extremely bright before it overheats and dies, so you should have seen that happening. Transistors don't glow, they just 'let the smoke out.' In either case, it's best to remove the coil and discard the rest of the circuit, which costs little to replace.
Zithe (author)  acmefixer6 years ago
When I wind the wires, do I have to wind it in a specific way or does that not matter?
acmefixer Zithe6 years ago
As I said above, swap the two wires of the winding that is connected to the base of the transistor. You can find out more on how to wind the coil by viewing the webpage here.
Zithe (author)  acmefixer6 years ago
Thank you very much. My Joule thief works now, turns out I was winding the wires incorrectly.
acmefixer6 years ago
Yes, there is something wrong. The 100 ohm resistor is much too low. Your JT may light up if the battery voltage is less than 1 volt, but the typical battery is 1.5V and for that voltage the resistor should be much higher value. The usual value for the resistor is 1000 ohms. Most of the transistors you gave will work well with 1000 ohms, but for the 8050 you will get a very bright LED and the battery current will be high. Use a higher value resistor in this case, say 2200 or 3300 ohms.
Zithe (author)  acmefixer6 years ago
Thank you for this tip. I have switched the resistor, however my joule thief still won't stay lit and I still don't understand how I could fix this.
acmefixer Zithe6 years ago
You do not indicate what the value of the new resistor is. Give all of the information, such as the colors of the bands on the new resistor.