Joule Thief 101

[seychelles]

this is just one transistor circuit what the fuck..you guys should deal in uhf circuitry running at 450 mhz.. you guys make me fucking sick of your petty childish fucking arguments..

[TinselKoala]

@MH

Below is my test setup schematic.
I have added a diode and cap to the JT circuit,so as the LED see's only a stable DC current at all time's.
The LED is a 2.6 volt LED,and i have used a 2.5k VR instead of the 1k resistor. This is needed so as i can get the voltage across the cap down to the rated 2.6 volts of the LED,when the circuit is supplied with 1.5 volt's. If i use a 1k pot,the cap will charge up to 3.1 volt's,and the LED passes out. At 1.2 volt's,we are very close to the 1k ohm resistance value,and so very close to your standard JT circuit.

I have a 10 ohm CVR,as a 1 ohm is far to noisy at these low power levels,and the 10 ohm CVR allows for very clear scope traces.

I have the LED across a 4700uf cap,and this gives us a very clear/clean voltage value across the LED.

With this setup,i can clearly show a maintained voltage across the LED as the supply voltage is reduced. This can only mean that the magnetic field strength is being maintained as the input voltage is dropped.

With the 10ohm CVR,i can also clearly show the pulse width increase-and there for the current increase, as the base resistance is reduced along with the input voltage--all while maintaining 2.6 volts across the LED, with a smooth DC current flow through the LED.


Brad

Have you thought about using a suitable PNP transistor, with the proper feedback to its base,  in place of the variable resistor?   

[MileHigh]

Brad:

Oops, I notice a much more subdued Brad and much less trash talk after my two postings.

Pre my two postings:

MH
It is painfully obvious that you really do not know how a transistor operates

I am starting to think some brainless twat has hacked your account

Now, your words come back to haunt you.  You don't really know how a transistor works.  You also don't know how to analyze how a transistor works when doing a basic switching function like you see in a Joule Thief.  Think about that.  You have been doing this stuff for six years, and all this time you have never reviewed basic transistor switching circuits to understand them and use them properly.  You don't know how to determine the proper base resistor for a given switching function.  And you clearly have some serious gaps in your understanding of how a transistor works.

You have heard me complain about the "continuous affirmation" environment that you guys set up for yourselves, and the "Straitjacket of Agreement" where you are all paralyzed and can only agree with each other like a bunch of bobbing rubber ducks in a pond.

And look at the results.  Combine the "continuous affirmation" and the "Straitjacket of Agreement" and the bobbing rubber duckies and your "I am Brad and I am never wrong" and "I am Brad and I take the lazy route when I can" attitudes and here you are six years later and you can't properly analyze a basic switching function in a five-component circuit like a Joule Thief, nor do you truly understand how a Joule Thief works..

Instead, you play this ridiculous trash talk game and you are as fake-ass as a three-dollar bill when you assume the role of a bad actor and play your ridiculous "MileHigh you are making mistakes everywhere" game.

Now that that has been said, I will comment on the technical in my next posting.  However, I am not going to spoon feed you anything.  You can go online or order a few books about electronics from Amazon and undertake to educate yourself.

MileHigh

[EMJunkie]

this is just one transistor circuit what the fuck..you guys should deal in uhf circuitry running at 450 mhz.. you guys make me fucking sick of your petty childish fucking arguments..

Nicely said Seychells!!!

MileHigh is wrong again - No one takes him seriously anymore anyway!!!

   Chris Sykes
       hyiq.org

[MileHigh]

Brad:

You have two coil's that provide current to build the magnetic field--not one.

Take a look at the attached schematic for a standard Joule Thief.  L1 is the power coil that gets energized and then illuminates the LED.  L2 is the feedback coil and is essentially an EMF source to switch the transistor ON and OFF via the base resistor.  Your statement above is nonsensical.  L2 does not provide any energy towards the lighting of the LED.  Only one coil, L1, "builds up a magnetic field" with energy that gets discharged into the LED.

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.

As you increase the current flowing into the base input of the transistor because of a decreased value of base resistor, that represents expending more energy to switch on the transistor than you have to.  If you put more current than you need to though the L2 coil, than that means that the battery can supply less current to the L1 coil (when factoring in a higher battery output impedance for a nearly dead battery), and that translates into less energy available to light the LED.  An excessively low value of base resistor just makes the Joule Thief less efficient.  If you ignore the issue of the battery output impedance then you are just siphoning extra energy away from the battery to switch on the transistor.  That extra energy could be put to better use by saving it to light the LED via L1.

I have already stated that when the battery voltage has dropped, you can't escape the simple V/R limiting factor for the amount of current that can be induced to flow through L1, and that means less current to light the LED.  Even if somehow the transistor stays on longer, the V/R current limiting factor is what really counts and the LED will be dimmer.

With respect to your setup, as far as I am concerned it deviates too far away from a standard Joule Thief to extract any useful information about the behaviour of a standard Joule Thief.  Your 10-ohm CVR may be introducing a voltage bounce to the whole circuit that throws off the feedback.  The filtering capacitor is not needed and also is probably throwing off the feedback.  If you know what you are doing, just probing a standard Joule Thief and trying varying the base resistor should be enough.

1.2v/1kohms is 1.2mA. This is not enough current to fully switch on the transistor,as the transistor is a current device,unlike the FET which requires very little current,but a higher voltage.

One kohm is in the standard Joule Thief schematic and I will take their word that it is a correct value based on the indicated recipe for winding the Joule Thief transformer and the associated resistance of L1.  You completely forgot the base-emitter voltage drop in your comment.  Saying "XX current is not enough current to fully switch on the transistor" is just you revealing that you don't understand the issues around how a transistor switching circuit works like I already stated.  Perhaps go to Amazon and do some online shopping.

MileHigh