Joule Thief

[resonanceman]

The 2K pot should be enough to limit current so you do not need another resistor. Just do not go to zero on the pot. But you could add a 2K resistor to be safe. You are going to need ten times larger base resistor with 12V than with 1.5V.

Xee

I have  a pot  for that .
I have tried it ....... eventually  I  turn the pot the wrong way or something and burn  up  my  transistor .
I just  think  it is time  to  work out the  right  resistor  size   and  use  a fixed  resistor.

If  I have to  I can  get  it working with the pot  then take the pot off  and measure it ........but I would  rather  know  how to calculate it .


gary

[jeanna]

Gary,
I was hoping for an easy answer from one of the brains here, but before that comes take a look at this page:

http://www.doctronics.co.uk/resistor.htm
About half way down the page is a computation of the resistor needed to protect a led from a 9v battery.
One point I caught is that all the components in series will add up to the battery input.

I am not sure what counts and what doesn't on a joule thief, but maybe if you start with a 2k and see then 1k then you can easily get close.

On my extended circuit with all the secondaries, the 33r resistor drops the incoming voltage in half and the current way down.
The 10r drops the incoming voltage to about 2/3 of the battery.

So, you may only need a 100r to take the 12v down enough, but start with 2k for safety then 1k to see if you need that.

Anyway that page goes through what you need to calculate this. It is more than ohms law, but makes use of it.

~~~~~~~~~~~~~~~~~

@All,

I just had a half of a breakthrough.
It looks little, but, I think it puts me into a new place.

In Tesla's drawings, he shows the generator, then a transformer various spark gaps and lights etc, then looking to the right in all his circuits, you can see another transformer.
This then is followed by more spark gaps and more lights etc.

I THINK I have finally been able to get past that second transformer.
I can only light 3 and not 4 leds, and...
there is still a series component to it rather than a coil straight across the ends, but I think it counts.

It behaves funny, too.
It lights up at about the same brightness as the rest of the lights, but when I unplug the battery and let the supercap drain out, these 3 are the last to go.
When this happened before, I was able to greatly extend the lights by adding more inductors.
So, this is what I will be doing.

I am still NOT able to make it work with a straight transformer across the ends without the series group.
If I try that then all the lights go out everywhere.
And the scope is not much help because it often puts out one or both groups of lights when it measures.

So, a little more progress. And 3 more lights which is 1 more than reported the other day using a different set-up.

So 27 leds.

jeanna

[crowclaw]

I am  math impaired .

Can  someone  help me  with  find  the  right  size resistor  for  using a  higher voltage battery ?

I checked on the  web ...... all  the  stuff I found about voltage drops was  for power lines .

I want to  be able to  run  a 3055  JT with a 12 V battery .

I want the  3055 to  run at LESS than  half  power  so  it  will stay relatively cool .

I was  thinking   of  dropping  about 9.5 V  with the  resistor  circled .

My  pot  has a minimum  of .5 ohms . 

Any help  will be appreciated

Thanks


gary

Hi, Garry calculating the exact value is not that easy as you are not dealing with steady state DC voltages.The transistor is switching off and on at the circuits resonant frequency. As already suggested experiment with a pott and connect a series resistor. You are controlling the collector current by adjusting the base current. Start with a high value and steadily reduce the pott resistance until you achieve your target. I don't know your level of technical knowledge but if you guy's are working with 2n3055 transistors it would be helpfull to down load a PDF of this device. The spec should give examples of the required base bias for relative collector output current   (hfe/gain) once you know the base bias required for your project you can then calculate a value for your base resistor. Hope this helps. Merv

[IotaYodi]

From what Ive read about the 2n3055 they are fairly old and rarely are not used without a heat sink. You may just have thermal runaway. You could try putting a to-3 heat sink on to see what happens using just the pot. They also can be made by different manufacturers with slightly different specs. As Crowclaw suggested you may want to download the manufacturers data sheet for their parameters.
Jenna gave a pretty good link on calculations. You would need to know how much current is needed at the transistor to make a resistance calculation easier.     

[jeanna]

Hi All,
I just added a post with a pic about today's successes on the ac thread.
It is getting pretty wild, all this success. 

I will be making a drawing and hopefully posting that tomorrow... or sometime...hopefully it will help?

jeanna