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how to improve a joule thief ? will increase of no of coils help?


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How are you wanting to improve it?
The question asked:

"will increase of no of coils help?"

I would follow up with the question, "Help in what way?"  The question quoted above doesn't define "help".  Do you want more power to run more LEDs?  Or do you want to light more than one LED in series?  Or do you want to run the battery down to a lower voltage? (Less than 0.4V, which is typical for a silicon transistor.)  Or do you want to draw less current from the battery so that it will last longer?

iceng5 years ago
Sure, the thief is after all a transformer with a core magnetic material and
an auto transformer turns ratio that can be adjusted for what will work the
best in your application...

You will notice the thiefs in the related column on the right side of
this page have a few turns to a couple hundred.

i like this design, using a JT to charge an RC flasher.

A
WhiteLEDFlasher.gif
acmefixer iceng5 years ago
The transistor with no base connection is being used as a breakdown device, something transistors were never, ever made to do.  There is no specification for the breakdown, the makers will tell you that the transistor is never supposed to be subjected to voltages or currents that cause it to breakdown, and doing so can cause damage.  Once you've used a transistor in that circuit, the current gain will be permanently reduced to where it is no longer within its specifications.  It has been damaged.  But the important thing to remember is that even though the circuit works, you will never see this circuit in a piece of equipment because it cannot be depended upon to work, especially at temperature extremes.  One transistor may work okay, and another (or ten others) may not work at all.  It is an extremely poor choice to use in anything worth putting your name on as the designer. Go to quantsuff dot com for some flashing JTs that are simpler, more reliable and better.
iceng acmefixer5 years ago
Right you are but I'm old enough to enjoy quirky circuits.

If Building this for some one else I would use a Four-Layer-Diode
as the breakdown device from my stock.

A
acmefixer5 years ago
The conventional two winding JT coil is not a transformer because the second or feedback winding does not transform, it only reverses the polarity to keep the circuit oscillating.  The winding is not needed for the JT to work.  When you use the two transistor JT, there is only one winding and the feedback winding is not needed.  If you add a secondary winding, you can increase or decrease the turns ratio to get higher or lower voltage.  But the voltage and power is determined by the magnetic flux stored in the core, and the voltage will clime to excessively high voltage - a hundred volts or more - if you remove the load (LED).   So you really don't have to have a third winding, just a light load for high voltage.  But this high voltage can damage the transistor, so you must have some way of limiting the voltage on the transistor's collector and base (that's a whole 'nother story).

One big improvement is to use the proper transistor.  The 2N3904 is at the bottom of the list when it comes to proper trransistors.  Try a BC337 for a considerable improvement.  They're the same price.
rickharris5 years ago
there is only so much power available - Nothing short of magic can alter that.