In fact, by direct observation of a JT circuit in my lab upstairs, lowering the base input resistor (from 1000 ohms to 500 ohms) resulted in a DECREASE of the operating frequency of the JT by a large amount -- 461 kHz to 361 kHz.
I say this not to embarrass you, MH, but rather to show (once again) that one's theoretical understanding of the JT circuit -- or the one I just test upstairs, a very simply JT circuit (without the collector LED) -- is in the OPPOSITE direction from what you predicted, and the change is large. I invite .99 to make the same test and check if this result is more general. Quick and easy test.
Also, MH, we see again the importance of testing with an actual apparatus -- experiment trumps theory... as previously noted throughout the history of science.
And again I encourage you to set up a JT to play with... not difficult.
Actually I am not embarrassed at all. There are limits to what can be done when you are just commenting and you haven't worked with one on the bench, nor have I seen your actual circuit. In most of my postings along these lines I make a disclaimer that I am not actually on the bench myself and therefore I can't be 100% certain of what I am stating. The process of exploring like this is part of the learning experience.
Also, MH, we see again the importance of testing with an actual apparatus -- experiment trumps theory.
You are missing the point PhysicsProf. I was wrong with respect to your test. The empirical evidence on your particular Joule Thief shows large changes in frequency and a decrease in frequency when you lower the base resistance, the opposite of what I said. There are too many unknowns on both sides to draw any conclusions at all between what I stated and what you tested.
Do you know if your transistor was fully on or in partial conduction mode when you used a 1K base resistor? What about when you switched to a 500-ohm base resistor? My statement was based on the presumption that for both the 1K and 500-ohm case that the transistor would be functioning as a switch. Can you explain why the frequency dropped when you changed the base resistor? It would be fun to work together one day, time permitting, to figure out why the frequency changed.
There is a logical reason for the decrease in the operating frequency of your Joule Thief that is backed up by theory. So the challenge for those that are interested in pursuing the research is to explain your observations with sound theory. That's where the fun comes in as part of the intellectual journey.
Let's go back to the Wikipedia formula for the operating frequency of the Joule Thief:
F = ((V_batt x R_batt) / L_mutual). Note R_batt is the output impedance of the battery. This formula presumes that the transistor is acting like a switch.
On the the other hand, supposing the transistor is in partial conduction mode when the base input resistor is 1K ohm. That would effectively make R_batt become (R_batt + R_transistor) which is higher, which according to the formula would
increase the JT operating frequency.
Supposing the transistor is in full conduction mode and acting like a switch when the base input resistor is 500 ohms. That would effectively make R_batt lower, which according to the formula would
decrease the JT operating frequency.
So, based on the limited amount of information that I have on hand, it would appear to me that when you have a 1K base input resistor the transistor is operating in partial conduction mode. That would explain the decrease in frequency when you switch the base input resistor to 500 ohms.
As far as I understand, the desire is for the Joule Thief circuit to operate the transistor as a switching device because this then limits the power dissipated across the collector-emitter junction to a minimum.
This suggests an interesting experiment: As you lower the base input resistance, you should see the operating frequency of the Joule Thief decrease. Eventually you should get to a point where lowering the base input resistance even further does not change the operating frequency (i.e.; the basis for my original comments). That point where the frequency stops changing would be your ideal base input resistance. I am assuming that a standard Joule Thief circuit assumes that the transistor should be operating as a switching device.
MileHigh
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Topic: LTJT - poynt99 Tests #2 (Read 106250 times)
