Joule Thief 101

[MileHigh]

As expected

Was it good?

I found the posting and am looking at it.  #337 right?  Don't get too excited.

[MileHigh]

Brad's posting #337:

>>>>>>>>>>>>>>>>>>>>>>>>>>>>

As can be seen in the scope shot below,all of the flyback energy in L1 is dissipated before the transistor once again switches on. This is because the flyback energy from L1 is what is pulling the base of the transistor down(keeping it off).

You continually ignore the junction capacitance of the transistor MH,and this is why you cannot understand as to how the circuit actually work's. Current flows through L2 before any current flows through L1, so L2 is the coil that starts to create the magnetic field within the toroid core first-not L1. Current can flow in L2 before the emitter/collector junction starts to open,due to the junction capacitance in the transistor. This in turn creates a voltage potential in L1 that is opposite that to L2,and add's to the voltage being supplied to the base of the transistor via the base/collector junction capacitor/capacitance. Although very small in capacity,it is enough to get the emitter/collector junction to start to open. Once this happen's,then a stronger magnetic field starts to build in the toroid. Now you start to get your transformer action between L1 and L2,and this then starts to pull the transistor on hard. The magnetic field builds to a point where the available current can no longer keep the magnetic field amplitude rising,or the core reaches a point of saturation,and the induced current in L2 stop's. The magnetic field begins to collapse due to the transistor no longer receiving enough current,and begins to switch off. As the magnetic field is now decreasing in strength,a reverse current flow is produced in L2,and this pulls the transistor hard off--as can be seen in the scope shot below.Some of this stored energy in L1 is used to drive the LED,and the rest is used to pull the transistor down/off. Once all the stored energy in L1 has been depleted,and no longer can hold the transistor off,the cycle starts all over again.

This is why your JT circuit is not very efficient MH,as most of the stored energy in the magnetic field that we want to use to drive the LED, is fighting against the energy being supplied by the battery ,to keep the transistor switched off. So the battery is trying to switch the transistor on,and the flyback energy is trying to keep the transistor switch off. This is why i like to use circuit's that disconnect the battery during the flyback part of the cycle.


Brad.

[MileHigh]

In your original post #337, you don't actually identify it as "your" explanation of how the Joule Thief operates at low voltage, you just seamlessly transition into it.  And I did read it.

I'm not sure if your scope shot is for the standard Joule Thief running at 500 mV with a 1K resistor, or, to get it to run you tweaked the base resistor down really low.   Because for your comment about your scope captures in the next posting you made, #338, is the following comment:

Because MH,the pot was turned right down to it's lowest resistance,so it would make no difference to which side the scope probe was on.

I have no idea what the base resistance is for your original post #337 but I will push on.  The ideal case is to have a standard Joule Thief circuit running at a normal frequency, and then observe as the frequency jumps up and the waveforms change at very low voltages - without changing the base resistor value or anything else about the Joule Thief.

You continually ignore the junction capacitance of the transistor MH,and this is why you cannot understand as to how the circuit actually work's. Current flows through L2 before any current flows through L1, so L2 is the coil that starts to create the magnetic field within the toroid core first-not L1. Current can flow in L2 before the emitter/collector junction starts to open,due to the junction capacitance in the transistor. This in turn creates a voltage potential in L1 that is opposite that to L2,and add's to the voltage being supplied to the base of the transistor via the base/collector junction capacitor/capacitance. Although very small in capacity,it is enough to get the emitter/collector junction to start to open. Once this happen's,then a stronger magnetic field starts to build in the toroid. Now you start to get your transformer action between L1 and L2,and this then starts to pull the transistor on hard. The magnetic field builds to a point where the available current can no longer keep the magnetic field amplitude rising,or the core reaches a point of saturation,and the induced current in L2 stop's. The magnetic field begins to collapse due to the transistor no longer receiving enough current,and begins to switch off. As the magnetic field is now decreasing in strength,a reverse current flow is produced in L2,and this pulls the transistor hard off--as can be seen in the scope shot below.Some of this stored energy in L1 is used to drive the LED,and the rest is used to pull the transistor down/off. Once all the stored energy in L1 has been depleted,and no longer can hold the transistor off,the cycle starts all over again.

This sounds plausible but it didn't come from you.  You have probably been hunting around a long time for information on Joule Thieves, and in one of your links you found a good technical article that described how a Joule Thief operates at low voltages and basically copied that information and wrote it into your posting.  Is that a fair assessment?

What you did not tie into all of this is the explanation for the high frequencies but it is probably there in the article.  If the very short transistor on and off times and short discharge pulses of the coil are because of small amounts of charge stored in the internal capacitors of the transistor, then I suppose the roughly one microsecond transistor-off-time/coil-discharge-times may make sense.

Without that article, you would be lost for a description and lost on the bench.  Without that article I would have to be on the bench for a very long time while I simultaneously reviewed transistor small signal and low voltage and transient behaviour.  I would have had to work on it myself, and if I found the same article that you found it would have helped me out tremendously.  Even without the article this would probably be child's play for people like Verpies and Picowatt.

So I missed your copy/paste because you didn't identify it outright.  I simply told you I wasn't going to discuss it because I was talking about a Joule Thief in normal operating mode at that point.

In the scope capture there appears to be an issue with the causality that I can't explain.  I think the transistor should still switch off before we see the voltage spike but I am not sure.  The scope probe placement issue may be a factor there.

MileHigh

[MileHigh]

Brad:

just like you refused to give me any specifications of the components in the JT circuit you wanted me to explain.  Like i said MH--it cant be done without these specifications. For example-if i make up a circuit,and use a set length of wire for each coil, and use a ferrite toroid,then the JT will oscillate at a certain frequency. If i do nothing other than change the ferrite torroid out for a steel laminated torroid(while everything else remains the same) then the frequency would drop by over 1000 %.

nor that stupid and incomplete question you asked EMJ. You try and trap people like this all the time MH--but they are all waking up to you.

As you could see in your no-link hidden copy/paste about the operation of a Joule Thief circuit at lower voltages, you can indeed fully describe the operation of a circuit without having to have component values.  This concept is freaking you out because it is outside your realm of experience and your way of thinking or capacity of thinking with respect to electronics.  The reality is that it's done all the time and it's the normal way that people describe the operation of a circuit.

You are really hung up on the EMJ question and accusing me of it being incomplete which of course is ridiculous again because I never even said what the question was.

Do you want to get the question so you can try to answer it?  You seem to be chomping at the bit so I can give it to you if you want it.

MileHigh

[tinman]

In your original post #337, you don't actually identify it as "your" explanation of how the Joule Thief operates at low voltage, you just seamlessly transition into it.  And I did read it.

I'm not sure if your scope shot is for the standard Joule Thief running at 500 mV with a 1K resistor, or, to get it to run you tweaked the base resistor down really low.   Because for your comment about your scope captures in the next posting you made, #338, is the following comment:

I have no idea what the base resistance is for your original post #337 but I will push on.  The ideal case is to have a standard Joule Thief circuit running at a normal frequency, and then observe as the frequency jumps up and the waveforms change at very low voltages - without changing the base resistor value or anything else about the Joule Thief.



What you did not tie into all of this is the explanation for the high frequencies but it is probably there in the article.  If the very short transistor on and off times and short discharge pulses of the coil are because of small amounts of charge stored in the internal capacitors of the transistor, then I suppose the roughly one microsecond transistor-off-time/coil-discharge-times may make sense.

Without that article, you would be lost for a description and lost on the bench.  Without that article I would have to be on the bench for a very long time while I simultaneously reviewed transistor small signal and low voltage and transient behaviour.  I would have had to work on it myself, and if I found the same article that you found it would have helped me out tremendously.  Even without the article this would probably be child's play for people like Verpies and Picowatt.

So I missed your copy/paste because you didn't identify it outright.  I simply told you I wasn't going to discuss it because I was talking about a Joule Thief in normal operating mode at that point.

In the scope capture there appears to be an issue with the causality that I can't explain.  I think the transistor should still switch off before we see the voltage spike but I am not sure.  The scope probe placement issue may be a factor there.

MileHigh

This sounds plausible but it didn't come from you.  You have probably been hunting around a long time for information on Joule Thieves, and in one of your links you found a good technical article that described how a Joule Thief operates at low voltages and basically copied that information and wrote it into your posting.  Is that a fair assessment?

This most certainly dose come from me MH,and is !!!NOT!!! a copy or paste,or re-edit from any other article.
Now that you can see that i do have an amount of sound knowledge ,and know what im talking about,you revert to your next tactic--trying to discredit the experimenter.
I have told you time and time again--i have had good teacher's,like Vortex1,Poynt-etc. I then take what i have learned,and apply it to devices that !do! work based around what i have been taught.

So dont try your crap with me MH. I had the balls to post what i knew,while you just say--im not even going to try and work it out.
I have put you in your place once again MH,and you need to suck it up princess.

And your welcome.


Brad