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Author Topic: Joule Thief  (Read 6275651 times)

Dave45

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Re: Joule Thief
« Reply #16695 on: October 23, 2014, 02:40:40 PM »
Dave this latest drawing of yours does show correct electron flow convention current.
Im learning, Iv found through testing my former idea's were wrong.
But that being said we are missing something or we would have free energy and I will continue until I understand.
The pnp diode really had me stumped but I now understand so the exercises are not futile.
Hope you dont mind if I continue, I appreciate your feedback.

dave

So what happens if both pnp transistors turn on and off at the same time, does current continue to circulate in the coil? during the off cycle.

MarkE

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Re: Joule Thief
« Reply #16696 on: October 23, 2014, 04:28:49 PM »
Im learning, Iv found through testing my former idea's were wrong.
But that being said we are missing something or we would have free energy and I will continue until I understand.
The pnp diode really had me stumped but I now understand so the exercises are not futile.
Hope you dont mind if I continue, I appreciate your feedback.

dave

So what happens if both pnp transistors turn on and off at the same time, does current continue to circulate in the coil? during the off cycle.
Yes as it must.  The second transistor adds complexity but no change to the function.  If you had two diodes, one from the bottom transistor back to the positive sid eof the battery, and the other from the to transistor to the negative side, then when the transistors turn off the coil will discharge into the battery instead of just recirculating.  The current decay will be much faster.

Dave45

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Re: Joule Thief
« Reply #16697 on: October 24, 2014, 01:28:00 PM »
Wouldnt this allow the magnetic field to collapse......oscillate.

Dave45

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Re: Joule Thief
« Reply #16698 on: October 24, 2014, 01:48:08 PM »
Electron path

MarkE

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Re: Joule Thief
« Reply #16699 on: October 24, 2014, 01:48:21 PM »
Wouldnt this allow the magnetic field to collapse......oscillate.
The cirucit looks like it will oscillate.  The right hand section will develop voltages that depend on the relationship between the left hand oscillation frequency and the resonant frequency of the transformer primary inductance and the capacitor across it.

TinselKoala

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Re: Joule Thief
« Reply #16700 on: October 24, 2014, 04:27:58 PM »
PNP Joule Thief

The standard JT circuit using NPN transistor will also work with a PNP transistor, by reversing polarity of the Battery and the LED, and connecting the PNP collector to where the NPN emitter was connected, and the PNP emitter where the NPN collector was connected. I've just confirmed this on my Testbed JT, which uses a 1:1 toroid winding of 13+13 turns and a 2s12p series-parallel array of 24 LEDs, running on a depleted AAA battery.

I just got a package of 50 ea. BC556B transistors for about $3.50 US  and I need to find something to do with them, since I only needed one for the reason I ordered them.

Dave45

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Re: Joule Thief
« Reply #16701 on: October 25, 2014, 01:53:43 AM »
PNP Joule Thief

The standard JT circuit using NPN transistor will also work with a PNP transistor, by reversing polarity of the Battery and the LED, and connecting the PNP collector to where the NPN emitter was connected, and the PNP emitter where the NPN collector was connected. I've just confirmed this on my Testbed JT,
Interesting
I built the pnp joule thief as well but just changed out the transistors from npn to pnp and it worked well.
The transistor legs went back into the same positions, emitter to emitter, base to base and collector to collector.
Of course the battery and led were reversed.
I didnt think to turn the transistor around.

TinselKoala

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Re: Joule Thief
« Reply #16702 on: October 25, 2014, 09:07:27 AM »
I've found that some NPN transistors will also work "reversed" in the JT testbed with no other changes from the normal NPN configuration. Only some, not all. 

The BC337-25 is one such NPN that works both ways, although the "correct" way results in a brighter LED array and an audible singing sound.

The BC556B PNP also works both ways, I found, with the battery and LED polarities reversed from the normal NPN circuit.

Bear in mind that my testbed uses a 13+13 turn winding. It might not be the case that a reversed transistor will work with other winding ratios. I'm using a AAA battery that reads about 1.31 volts unloaded, and I'm driving a 24 LED array of white LEDs, 12 in parallel, in series with another 12 in parallel.

Dave45

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Re: Joule Thief
« Reply #16703 on: October 25, 2014, 03:27:01 PM »
I find it interesting that the high side setup is a start stop reaction whereas the low side setup continues to circulate the electron and current.
It would seem the low side setup would be much more efficient

Dave45

  • Guest
Re: Joule Thief
« Reply #16704 on: October 25, 2014, 03:48:08 PM »
Its not really the transistor type but the topology

MarkE

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Re: Joule Thief
« Reply #16705 on: October 25, 2014, 03:48:10 PM »
The PNP is a flyback topology.  The NPN is a boost topology.

In each current builds in the coil while the transistor is on.

In each when the transistor turns off the coil flies back until the LED latches on carrying the current.

Because the battery continues to supply current in the NPN boost case, the inductor current does not decay as fast as the PNP flyback, and it can generally deliver more power than the PNP flyback.

Dave45

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Re: Joule Thief
« Reply #16706 on: October 25, 2014, 04:10:48 PM »
The PNP is a flyback topology.  The NPN is a boost topology.

In each current builds in the coil while the transistor is on.

In each when the transistor turns off the coil flies back until the LED latches on carrying the current.

Because the battery continues to supply current in the NPN boost case, the inductor current does not decay as fast as the PNP flyback, and it can generally deliver more power than the PNP flyback.
Yes the high side driver delivers more power at the expense of the battery but the low side makes better use of the electrons in a short burst because of the electron recirculation.
It reminds me of tapping a wheel to make it spin.

MarkE

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Re: Joule Thief
« Reply #16707 on: October 25, 2014, 04:56:26 PM »
Yes the high side driver delivers more power at the expense of the battery but the low side makes better use of the electrons in a short burst because of the electron recirculation.
It reminds me of tapping a wheel to make it spin.
The efficiency of either configuration depends on several factors.  The most important are:

The inductor resistance.
The voltage drop of the transistor while on.
The turn on and turn off times of the transistor.
Transistor base current.

All things considered equal, the boost topology is usually more efficient.  Boost or flyback can be done with PNP or NPN transistors.  Faster and higher current handling NPN transistors are easier to make than PNP transistors. 

Dave45

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Re: Joule Thief
« Reply #16708 on: October 25, 2014, 05:41:12 PM »
Quote
All things considered equal, the boost topology is usually more efficient.
Is it because the high side driver oscillates whereas the low side doesnt.
It would seem the low side driver would be more efficient at driving leds.
The high side would be more efficient at driving a transformer.

Its interesting I really dont have the equipment to do proper measurements, no scope, but Im still going to setup the low side setup and see if I can get it to oscillate using a tank circuit.

MarkE

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Re: Joule Thief
« Reply #16709 on: October 25, 2014, 05:49:25 PM »
You've got to get yourself some sort of scope so that you can see what happens.  A Hantek 6022 that plugs into a USB port on a computer is $65.  Old analog scopes on eBay are $100.

LED or transformer, it does not matter.  You can get faster, lower voltage drop NPNs than you can PNPs..