Joule Thief 101

[Magluvin]

In fact, no led at all, 114v across EC.  33ohm resistor.

Mags

[picowatt]

The reduction in base current increases pulse width,and increases LED light output

In your video, it appeared that increasing the base current increased the on time pulse width.  Perhaps you misspoke.

Note that a bipolar transistor is a current operated device.  The forward voltage of the base emitter junction is typically one diode drop (600 to 800 mv for silicon) and varies only slightly as base current is changed (or junction temp varies).

In your video, we might "assume" that the base current increases as you decrease the value of the potentiometer connected to the transistor's base.  However, because the base voltage remains constant, there is no way to see (or know) what change is actually occurring to the base current as you adjust the pot with the tests you made in the video. 

Just like measuring any other current with a 'scope, to measure the base current you need to measure Vdrop across some kind of CSR and then calculate the current (or use a current probe).

For example, you could preset the pot to 1K and then use both scope channels to measure the Vdrop across the pot and then calculate the base current using Vdrop and the 1K value.  You could then preset the pot to 500R and repeat the measurements and calculation.  You would then know the base current for those two base resistances at the given battery voltage. 

PW

     

[picowatt]

Taking note of the frequency at the bottom of your scope,

Do not confuse a waveform's repetition rate with its frequency content.

Although Tinman's video shows a waveform repeating at around 31KHz, it is obvious from the observed rise and fall times that the waveform contains frequency components out to 200KHz or more. 

This confusion seems to happen quite frequently, as in this example with regard to the coil's SRF, or as occurred further back in this thread with respect to discussions of "subharmonics" and the "pushing a swing" analogy.

PW

[tinman]

PW

   

In your video, it appeared that increasing the base current increased the on time pulse width.  Perhaps you misspoke.

Yes-it was suppose to read reduction in base resistance= an increase in base current.

In your video, we might "assume" that the base current increases as you decrease the value of the potentiometer connected to the transistor's base.  However, because the base voltage remains constant, there is no way to see (or know) what change is actually occurring to the base current as you adjust the pot with the tests you made in the video.

Not entirely correct.
As the current flow in L1 is set once the transistor is fully switched on,then the only way the LED can receive more current from the inductive kickback is by way of a stronger magnetic field being produced during each on time pulse,and as L1's current is set,then the only way to increase that magnetic field is by way of L2. For this to happen,then L2 must be receiving more current flowing through it,and we know this would be the case if we reduce the base resistance value..

Just like measuring any other current with a 'scope, to measure the base current you need to measure Vdrop across some kind of CSR and then calculate the current (or use a current probe).

I would do this,only MH had issues with the fact that i had a CVR in my first test setup,and so he said it was not a JT circuit-->it is hard to please everyone

For example, you could preset the pot to 1K and then use both scope channels to measure the Vdrop across the pot and then calculate the base current using Vdrop and the 1K value.  You could then preset the pot to 500R and repeat the measurements and calculation.  You would then know the base current for those two base resistances at the given battery voltage.

I would be happy to put a 10ohm CVR inline with the 1kVR,and we could get a clear picture of the current flow through to the base. But as i stated above MH had a tanty,and said by having the 10OHM CVR in the circuit,it was no longer the JT circuit. This just gives him room to make more dismissive comments,so i stuck with the basic circuit in the first test.

Now that i have proven my point,then i will be doing such test,so as we can see everything that is taking place with the reduction of the base resistance.


Brad

[picowatt]

Yes-it was suppose to read reduction in base resistance= an increase in base current.

Not entirely correct.
As the current flow in L1 is set once the transistor is fully switched on,then the only way the LED can receive more current from the inductive kickback is by way of a stronger magnetic field being produced during each on time pulse,and as L1's current is set,then the only way to increase that magnetic field is by way of L2. For this to happen,then L2 must be receiving more current flowing through it,and we know this would be the case if we reduce the base resistance value..

Yes, one can "assume" that is what is happening and use what is observed as a proxy for base current.  My point was that nowhere is base current itself being directly measured or observed in the video.  As well, it is incorrect to state that you are varying the base voltage when adjusting the VR.

I would do this,only MH had issues with the fact that i had a CVR in my first test setup,and so he said it was not a JT circuit-->it is hard to please everyone

I would be happy to put a 10ohm CVR inline with the 1kVR,and we could get a clear picture of the current flow through to the base. But as i stated above MH had a tanty,and said by having the 10OHM CVR in the circuit,it was no longer the JT circuit. This just gives him room to make more dismissive comments,so i stuck with the basic circuit in the first test.

Now that i have proven my point,then i will be doing such test,so as we can see everything that is taking place with the reduction of the base resistance.

Brad

You originally used a rather large 10R in the emitter leg as a CSR for measuring emitter current.  However, resistance inserted into the emitter leg will produce "degeneration" or "negative feedback" which effectively reduces the transistor's gain.  If your goal is to stabilize the LED current as Vbatt varies, you might consider experimenting with increased degeneration (i.e., a larger emitter resistor).  If maximum efficiency or minimum Vbatt operation is the goal, this would be less desirable.  To measure emitter current, use of a smaller value emitter resistor is advised to reduce the effects of degeneration.

The base current, however, must be measured in the base leg circuit and does not require a separate CSR (although you could indeed use a separate 10R in series with the base as you suggest).

All you need to know is what the Vdrop across the VR is and the resistance value the VR is set to.  The use of the separate 10R as a CSR as you suggest would, however, allow you to measure the base current real time without needing to know the VR's set value.

PW