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Joule thief

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cduma

Sun Jan 04 2009, 10:43PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300

How does the device work? I measured the circuit with many voltmeters and I am getting the same voltage in and out. The leds I have turn on at 1.9v but, when I use the above circuit they turn on at .6v. I consulted an expert in person and he said that the circuit is impossible and would burn up.
Please help! The circuit appears to violate many laws!

Conundrum

Sun Jan 04 2009, 11:31PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

hi..

Seems that it works as a "buck" converter. You are probably not seeing the high current pulses so the average power seems very low.

It shouldn't burn up the transistor because the transistor turns off at saturation causing a high voltage "flyback" pulse which exceeds the turn on voltage of the LED allowing it to work on a lower voltage.

A more efficient less "noisy" circuit can be built using a MAX660 IC, only needs two capacitors and is small enough to fit inside a bulb casing so white LEDs can be used with a 3V torch.

Myke

Sun Jan 04 2009, 11:45PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

White LEDs work well with the joule thief. For LEDs that aren't white or blue, I have used a 10k resistor instead of 1k so that it doesn't draw as much current.

cduma

Mon Jan 05 2009, 01:10AM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

Why isnt the circuit a short circuit?
I placed a capacitor across the LED so that I could measure the peak voltage but, the circuit stopped working at all.

Marko

Mon Jan 05 2009, 01:50AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

cduma wrote ...

Why isnt the circuit a short circuit?

Why would it be a short circuit? Search for blocking oscillator for more information on this circuit.

cduma wrote ...
I placed a capacitor across the LED so that I could measure the peak voltage but, the circuit stopped working at all.

It is possible for a capacitor placed in parallel with LED to disrupt the oscillation, and you definitely won't be able to measure DC voltage that way neither. You need a diode between the collector and the capacitor to attain true DC output.

Marko

Firnagzen

Mon Jan 05 2009, 09:41AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

That page has everything you need to know about the circuit.

Myke

Mon Jan 05 2009, 02:33PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

You don't have to wind your own cores. I found that the line filters from monitors work well in this circuit.

Avi

Mon Jan 05 2009, 02:49PM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

A positive voltage enters the base thus turning on the transistor, creating a magnetic field, inducing a reverse voltage in the transformers secondary thus cancelling out the applied base voltage, turning off the transistor, stopping the induction, cycle repeats.
Also the primary coil generates a high reverse voltage across is when it is turned off as the magnetic field collapses rapidly (think moving a magnet in a coil the reverse way very fast), This reverse voltage (across the primary coil when the transistor has just turned off) is in series with the batteries own voltage. The sum of the 2 voltages is what powers the LED.

Dr. Dark Current

Mon Jan 05 2009, 03:03PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Avi wrote ...

A positive voltage enters the base thus turning on the transistor, creating a magnetic field, inducing a reverse voltage in the transformers secondary thus cancelling out the applied base voltage, turning off the transistor, stopping the induction, cycle repeats.

Well, actually it works exactly opposite way. A positive voltage enters the base turning the transistor on, but the windings are phased in a way so the feedback winding forces even more current through the base. When the core starts saturating, the base current starts falling, until it can't keep the transistor conducting. This results in a positive voltage pulse on the output, and negative pulse on the base.

If it was wired the other way, the collapsing field would induce a positive current through the base - the transistor would never really turn off.

kell

Tue Jan 06 2009, 01:42AM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

Read this:

Compare "torch circuit b" to the one you linked to.

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