Joule Thief 101

[MileHigh]

Brad:

I will go out on a limb now and honestly I didn't want to go here because I expect you to demonstrate your own innate capability and do a test that makes sense.  But since I got thrown off I will state what I have in mind and leave it at that.

When you decrease the base resistance the operating frequency lowers and thus the energizing time for the main L1 coil increases - obviously that can lead to a brighter LED.  I was expecting some effect on the frequency but not as much as is shown in your clip, with the caveat that I still don't have confidence in your setup because it is running too fast and the discharge time through the LED is way too short.  Nonetheless, a lower operating frequency means more time for the L1 coil to energize.  The conundrum is that the operating frequency is supposed to be governed by the L/R-type time constant to energize L1 and the L/R-type time constant for L1 to discharge through the LED.  The value of the base resistance doesn't really come into play for these two time constants, hence my expectation was that varying the base resistance would only have a marginal effect on the operating frequency.  I am not sure why lowering the base resistance lowers the operating frequency, but like I said before, if your Joule Thief was running at a more normal frequency it would be interesting to see what happens then.

Let's examine your premise for your experiment.  You said changing the value of the base resistor would compensate for a lowering battery voltage over time.  It's hard to tell from a clip, but the LED does not appear to be getting appreciably brighter as you change the base resistance.  Always the caveat being that I don't think your Joule Thief is operating at a normal frequency.  The light meter shows increasing intensity, but only at a level of 40 parts per thousand.  The eye is not very sensitive when it comes to detecting changes in brightness.

How will the LED intensity vary if you use a power supply and change the supply voltage from 1.5 volts to 0.75 volts?  Will it be noticeable?  How will the LED intensity vary if you run the Joule Thief at 0.75 volts and lower the base resistance?  Will that compensate for the presumed loss in brightness or not?  Again, the Joule Thief has to be running at a normal frequency.

That is the rough form of the testing that I think you should be doing.  To me it looks like varying the base resistance will only have a very marginal affect on the brightness of the LED because it slows down the operating frequency.  It is somehow affecting the positive-feedback triggering mechanism for the shutting off of the transistor.  I would not be surprised if lowering the base resistance only barely makes the LED perceptibly brighter.  In contrast, I believe that varying the supply voltage to the Joule Thief will have a very noticeable affect on the brightness of the LED.  And that is the crux of the issue.

MileHigh

[MileHigh]

Brad:

I know that you are going to jump around and gloat, but here is your posting #1076 verbatim where you give your "explanation" for how lowering the base resistance will increase the brightness of the LED:

MH
You need to have a good look at the basic JT circuit--the one you like,or call the JT circuit.
You have two coil's that provide current to build the magnetic field--not one.
As you decrease the resistance to the base,you increase the current flowing to the base,and this current is additive to the collector current due to the way the two coils are linked together,and thus the magnetic field strength can be maintained by reducing the base resistance value, so as to provide the same amount of power flowing through the LED from the kickback as the battery voltage drop's.

I will draw up a quick schematic of my test setup,and post it here. When you see the test setup,you will know that the measurements taken,and the statements i provided are correct.

Brad

So your "explanation" isn't even remotely close to what you are observing at all.  You are observing an increasing period in the energizing cycle for the L1 main coil, and that is what is making the LED only slightly brighter.  From your clip the brightness increase appears to be barely perceptible.

As you decrease the resistance to the base,you increase the current flowing to the base,and this current is additive to the collector current due to the way the two coils are linked together,and thus the magnetic field strength can be maintained

That statement looks pretty damn wonky to me.  The coils are opposite wound on the same toroid, right?  Looking at the schematic, let's say when current flows from top to bottom through L1 during the transistor ON cycle we are building up magnetic energy in the toroidal core.  During this time current is also flowing from top to bottom through L2 to keep the transistor switched ON.  L2 is opposite-wound to L1 and that means when current is flowing from top to bottom through L2 that it is producing magnetic flux that is opposite to the magnetic flux produced by L1.  So when current flows top to bottom through L2 it reduces the magnetic energy stored in the toroidal core, not increases it.  That would help to make the LED dimmer, not brighter.

So you can't claim any "victory" because your light meter can detect increasing brightness in the LED.  You just stumbled upon this effect, and your theory for why the LED was supposed to get brighter makes no sense at all.

MileHigh

[Pirate88179]

MH:


Quote:  "When you decrease the base resistance the operating frequency lowers and thus the energizing time for the main L1 coil increases - obviously that can lead to a brighter LED."


It has been my experience, as I have stated, that decreasing the resistance to the base increases the operating frequency as in the example of the low voltage battery causing the led flashes to be seen by the human eye...decrease the base resistance and now the led flashes on/off faster than you can see and appears "on".  I have seen this many times first hand on my own units.


Maybe I am not understanding you here?


Thanks,


Bill

[TinselKoala]

Here's something else to consider. In the standard JT, the toroidal windings actually form one continuous winding in the same direction, with a center tap. You can save yourselves a lot of trouble by simply taking one strand of wire, winding the toroid or ferrite rod etc. with one continuous single-layer, connecting the ends to Collector and Base resistor/capacitor (if used). Then use sandpaper to remove the insulation of the magnet wire in a "stripe" along the outside of the toroid or along the length of the rod, and locate the tap (which goes to positive rail) in the best place by experimenting with connecting to the uninsulated stripe. Once you have found the correct ratio location, solder the "center" tap to the bare stripe at that spot. This is similar to the way that a traditional inductively-tuned crystal set has taps located along the coil for tuning to particular stations.

[MileHigh]

Bill:

It's a great issue you raised and I don't have an answer, but only some follow-up questions to ponder:

When your Joule Thieves were running off of a very low battery, they might have been flashing the LED at 25 Hz, and then you lowered the base resistance and the flashing frequency increased.  What is the technical explanation for that?

How come Brad's Joule Thief is running at 20 kHz when one of yours might have run at 50 Hz?

Precisely why does the frequency in Brad's Joule Thief decrease when he decreases the base resistance?  What is the technical explanation?

Why is there apparently so little energy in the LED discharge from Brad's toroidal coil?  The discharge takes five microseconds, it's doesn't seem to make any sense.

Knowing Brad, who knows what the winding setup is like on his toroidal core.  It's possible that the effective number of turns for the L1 coil are very low, and the core is only storing a fraction of the magnetic energy that it can actually store.

What is implicit in the questions I am raising is that I am trying to push the boundaries past doing a setup and passively observing how it performs, and switching over to actively trying to understand how something works.  It's a huge shift from just observing something and pretending that you are experimenting and truly experimenting to test a hypothesis and actually understand the underlying processes that explain how something works.

Now, if I was in Brad's shoes, the first thing I would notice is that my decent-sized toroidal core is dumping a 5-microsecond pee-fart of energy into the LED when it discharges.  Something seems amiss there.  What goes hand-in-hand with that is the very high frequency that the setup is running at.  I would undertake to check the core material, check the winding configuration, clean up my overall wiring, and put my Joule Thief "in the shop" so that when it came out of the shop it was running at a frequency and generating waveforms to my satisfaction.  I wouldn't do anything until I was satisfied that my Joule Thief was operating in what I considered to be a normal fashion.  But I am not in Brad's shoes and I hold out no hope that he will pay any attention to these matters.

MileHigh