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Author Topic: Joule Thief  (Read 6276400 times)

Dave45

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Re: Joule Thief
« Reply #16680 on: October 19, 2014, 01:40:16 PM »
So yesterday I decided to give the little circuit some extra juice, since it didnt want to self run  ;D
I applied 12v and it blew the transistor into a couple of pieces and fried the large led,,,,,then it hit me I had to reverse the large diodes to get them to light with the low voltage circuit, this means the bemf wasnt charging the cap, the led's were in blocking mode.
Im going to get more led's from the shack and give it another go.
Persist to Persevere.

dave

Dave45

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Re: Joule Thief
« Reply #16681 on: October 19, 2014, 04:58:37 PM »
I can see the logic behind the npn setup
but the pnp setup ?

Dave45

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Re: Joule Thief
« Reply #16682 on: October 19, 2014, 04:59:48 PM »
In the pnp setup why does the bemf run through the diode at all.

MarkE

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Re: Joule Thief
« Reply #16683 on: October 19, 2014, 11:51:01 PM »
I can see the logic behind the npn setup
but the pnp setup ?
They work exactly the same way: In each positive convention current flows clockwise around the loop formed by the power source, the transistor and the coil when the transistor conducts, and counter clockwise through the loop formed by the transformer secondary and the LED when the transistor turns off.  Positive conention current flows from top to bottom through the transformer secondary in all four cases.  When the transistor turns-off, the coil voltage changes in the direction that resists change in current:  NPN low side switch, the coil voltage increases above the power supply positive terminal voltage.  In the  PNP high side switch case:  The coil voltage decreases below the power supply negative terminal voltage. 

Note that the orientation of the LED is always up.  The LED is always oriented so that it is reverse biased when the associated transistor is on, and the LED always connects to the opposite power supply terminal as the transistor.

MarkE

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Re: Joule Thief
« Reply #16684 on: October 20, 2014, 12:30:02 AM »
There is only One God and thats with a capital G.
The rest were created and yes I hope and pray He gives us the answer.
dave
Missing 'o'.

Dave45

  • Guest
Re: Joule Thief
« Reply #16685 on: October 20, 2014, 03:05:37 PM »
In the pnp setup I dont understand why it even goes through the led, it should run straight to ground.
Unless the coil is actually pulling, sucking.
And if it does pull,,,,well thats interesting.

MarkE

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Re: Joule Thief
« Reply #16686 on: October 20, 2014, 03:40:01 PM »
In the pnp setup I dont understand why it even goes through the led, it should run straight to ground.
Unless the coil is actually pulling, sucking.
And if it does pull,,,,well thats interesting.
Dave, referring to the drawings:  When the PNP transistor conducts, current builds up in the transformer secondary, lifting the coil voltage above the negative rail.  As the transistor turns off the inductor voltage changes in such a way as to try and maintain the current.  The inductor drives the voltage at the transistor collector down until the LED forward biases enough to carry the current.  The voltage is therefore below the bottom rail by the LED forward drop.  This is just the complement of what happens with the NPN.  There the collector voltage flies back above the top rail by the LED forward voltage.

tinman

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Re: Joule Thief
« Reply #16687 on: October 20, 2014, 04:14:18 PM »
In the pnp setup I dont understand why it even goes through the led, it should run straight to ground.
Unless the coil is actually pulling, sucking.
And if it does pull,,,,well thats interesting.
All you have to remember is-the current in the inductor always flows in the same direction,but the voltage reverses polarity when the transistor becomes open circuit(switches off)

Dave45

  • Guest
Re: Joule Thief
« Reply #16688 on: October 22, 2014, 10:58:13 PM »
I sure hate to conform

Dave45

  • Guest
Re: Joule Thief
« Reply #16689 on: October 23, 2014, 03:28:15 AM »
I guess this would be considered a shorted coil, the magnetic field would never collapse.

MarkE

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Re: Joule Thief
« Reply #16690 on: October 23, 2014, 04:19:05 AM »
I guess this would be considered a shorted coil, the magnetic field would never collapse.
No the coil is not shorted.  It works a lot like the other circuits.

Dave45

  • Guest
Re: Joule Thief
« Reply #16691 on: October 23, 2014, 01:01:42 PM »
I think we need to not forsake electron flow, I couldnt understand why the pnp setup didnt just run to ground, why it lit the led at all, now I understand the electron doesnt run to ground they continue to circulate in the coil.

Dave45

  • Guest
Re: Joule Thief
« Reply #16692 on: October 23, 2014, 02:02:57 PM »
The electron path, current may run to ground but the electron takes an alternate path

MarkE

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Re: Joule Thief
« Reply #16693 on: October 23, 2014, 02:05:29 PM »
I think we need to not forsake electron flow, I couldnt understand why the pnp setup didnt just run to ground, why it lit the led at all, now I understand the electron doesnt run to ground they continue to circulate in the coil.
Dave, you keep posting one circuit after another.  What are you looking for in comments when you do this?  I think you should at least say what problem you've found or are worried about with one circuit before moving to the next circuit.  This latest circuit doesn't make any sense at all.  What are you trying to do?  What specific idea do you have that you want to pursue that you think or hope this circuit will manage that the earlier circuits you've posted this week won't?

Things that you should keep in mind:

Inductors resist changes in current.  Wherever you connect an inductor to a transistor or other kind of switch you need to plan a path for the current to continue when the transistor or other form of switch turns off that will not develop a voltage that will harm the transistor or switch that you are trying to turn off.

This latest circuit will develop current in the middle winding when the PNP transistor is on.  When it turns off, the center winding will flyback without anything to maintain the current.  The voltage will swing negative until the PNP transistor avalanches.  Do that a few times and the PNP transistor will be destroyed.

If you turn the NPN transistor on, first it is configured as a common collector so it will drop at least 0.7V, more typically 1V.  Then more voltage will drop going through the diode and then the middle winding to the negative battery terminal.  When you try to shut off the NPN transistor the coil connection to the emitter will swing negative until the emitter is far enough below the base that the transistor continues to conduct.  As the energy in the coil's magnetic field dissipates, the NPN will turn off.

MarkE

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Re: Joule Thief
« Reply #16694 on: October 23, 2014, 02:08:12 PM »
The electron path, current may run to ground but the electron takes an alternate path
Dave this latest drawing of yours does show correct electron flow convention current.