Joule Thief 101

[MileHigh]

Anyway,we look at the base current in this video,and we can see the effects it has on the operation of the circuit.

https://www.youtube.com/watch?v=72BqF8bkk-k

Brad

Comments on your clip:

- Your pitch about currents through L1 and L2 adding up to put more magnetic energy into the core is dead wrong.  See my earlier posting.
- It's pretty obvious this time that reducing the base resistance barely increases the intensity of the LED and it can only be picked up by the light meter.  Changing the base resistance makes no change in the LED intensity for the naked eye.
- For the 10-ohm CVR setup, you are looking at a massive anomaly where the L1 coil is discharging straight into the L2 coil via the 1k resistor and sucking some life out of your Joule Thief and you are not saying anything about it.
- For the 10-ohm CVR setup, you can see that lowering the base resistance gives you more current through the base-emitter diode of the transistor.  The assumption is that this is excessive current that is doing nothing for you except wasting energy, just like I have said several times.
- I don't believe that putting a current limit on your bench power supply will emulate a nearly discharged battery's output impedance.  I would think that you should measure the actual output impedance of a nearly discharged battery of a given size and chemistry type, and then simply put the appropriate resistor in series with your bench power supply.

So, the variable base resistor is not doing much good at all for you in this clip, and then there is the discovery of the massive anomaly in your clip.  I have a few more minutes to watch to see if you have anything to say about it.

MileHigh

[minnie]

What transpires is that virtually nobody really knows what they're on about!
Just get yourself a decent PV panel and forget depleted AA cells, there's no
future in 'em whatever you do.
                   John.

[MileHigh]

Well Brad, you have a new elephant in the room, I am calling it the "Joule Thief death spike."  See attached.

How could you not say anything about this?  It's truly mind blowing when you get these absences of reaction when something truly unusual happens.  You have this huge spike of reverse current going through the base of the transistor and you say NOTHING?  LOL   Like a bloody lighting bolt it goes right up L2!  You are the Dr. Frankenstein of Joule Thieves!  LOL

Cat's got your tongue?  Zap the cat with a lightning bolt of current!

[MileHigh]

Brad:

Do you think that two different LED's will work in the very same way?
Do you think that two different core's will operate in the same way.
What about different transistor's?
What about different numbers of turns for each coil,or the wire size used?.

How you can say that the two different circuit's will/should operate very close to the same is beyond me.
Like i said MH,you really are not well versed in JT circuit's.
The fact that you do not know why the transistor stays on longer,and the frequency lower's when the base resistance is decreased,truly shows you have so much left to learn.

Facts are facts MH,and your fairy tails have been proven to be just that--fairy tails.

Presumably two different LEDs won't make much of a difference.
Assuming the cores are about the same size and both are high-permeability, not much difference.
Ditto for the transistors.

The number of turns for each coil will indeed make a difference.

There is no reason for your Joule Thief to run so fast and I think something is amiss.  You clearly have a serious design problem because you have the Joule Thief death spike.

The fact that you do not know why the transistor stays on longer,and the frequency lower's when the base resistance is decreased,truly shows you have so much left to learn.

Please go ahead and explain why it is happening.

MileHigh

[picowatt]

https://www.youtube.com/watch?v=72BqF8bkk-k

Tinman,

Moving the scope ground to the transistor base is not a very good way measure base current.  Any time the base voltage is less than the Vbe turn on voltage or is reverse biased, the base appears mostly capacitive (combined with a small amount of leakage current).  Moving the scope ground lead to the base changes the capacitance at the base.

A better way would have been to keep all scope grounds on the battery minus and use both scope channels and probes to measure both ends of the 10R base resistor.  With both scope channel vertical gains set identically (ca 20mv/div) invert one channel and then add them together (or use a math function to subtract one channel from the other).  This allows you to make a differential measurement across the 10R with minimal capacitive loading (particularly if you use 10X probes).

Also, you only have two channels so you should be taking advantage of the external trigger input.  It has limited "viewing" function, but if used smartly, it can be used to keep a marker on the screen at all times that indicates a particular time point in the waveform being observed.

That said, I do not believe the base current flowing thru L2 is acting in the manner you surmise.  In fact, I would think that current flowing thru the base (and L2) would generate a flux in opposition to that generated by current flowing thru L1.

You should consider measuring the Vce and Ic at the two base current extremes you are using.  You may find that Vce or Ic varies with Ib.  To do so, instead of making videos, consider creating six scope captures.

I would add a 1R resistor between the transistor collector and the L1/LED junction and connect the scope grounds to the emitter/battery junction.  I would then choose two base resistances to measure operation with such as 1K and 500R.  I would connect a third probe to the external trigger input and connect that probe to the L1/LED junction and trigger on the rising edge (which is the straightest edge in your waveform).

The first capture would be with the CH1 probe at the pot/10R junction in the base leg, and the CH2 probe at the collector of the transistor with the pot set to 1K (or using a fixed 1K).  The next capture would be identical, but with the pot set to 500R.  These two captures would let you see any change in Vce as the base current is changed between the two base currents.

The next capture would be with the CH1 probe moved to the L2/LED junction (i.e., the other end of the collector's 1R).  Using 1K as a base resistance, set both scope channels so that there is a good signal, and then invert and add (or subtract using math) to measure the differential voltage across the 1R, which is the collector current Ic.  The next capture would repeat this test using the 500R base resistance.

At this point you would have the collector to emitter voltage (Vce) and the collector current (Ic) when using the two base currents with 1K and 500R base resistors. 

We already know the base current is increased when the base resistor is changed from 1K to 500R, but it would be useful for further analysis of turn on, etc, to measure the base current under the same two operating conditions.

As was done to measure the collector current, make a capture using both probes to measure the difference voltage across the 10R resistor at the base using first the the 1K base resistance and then repeat this test making a second similar capture using the 500R base resistance.

You would now have a set of six scope captures.  Because the external trigger input was used, they are fairly closely time aligned.  The first pair of captures gives you Vce data, the second pair gives you Ic data, and the third pair gives you Ib data. Each pair provides data when the base resistance is set to 1K and 500R.

You might consider doing all these tests using your bench supply set to a known voltage as the battery voltage may change over the time period needed to make these six captures.

Although its just a joule thief, making these captures would be good measurement/scopology practice..

Just my 2 cents...
PW