Joule Thief

[jeanna]

              When I connect my DMM to my JT, I get a quick HV spike, then it go's down and settles,
              which is the most accurate one out of number 1 or 2
               
              1 - The high spike ? ...( I can quickly press the hold button to read this reading )
              2 - Or the lower result where the DMM reading settles ?

              I know I have been told that the DMM reading the volts from a JT is probably not accurate
              but still want to know out of number 1 or 2.  Thanks.

Not when it settles.
The capacitor after an augmented bridge is what tells the tale. It captures all of whatever it is.
also,
The reading speed of the dmm limits what it can see.

              One more question, if anyone knows... Would a low frequency radio scanner or frequency
              counter with a broad range of say 100Hz to 1Ghz  be able to read the output
              frequency of a JouleThief            

As for the frequency., I dunno
I think your scope can tell you but you need to get as 'close' as possible and count the lines.
You can expect this to be between 2khz and 250khz. It seems to me that the larger the toroid the lower the frequency.

I'm sorry I can't say better.

jeanna

[crowclaw]

@ Jeanna, I used my scope on my JT-power supply and I have posted a complete drawing of the scope shot, its exactly the same as on the screen, can you help me with what the frequency would be and volts

the very top of the green lines went a bit over the top of the grid when the JT is at its best.

Hi Freepow, youve had good answers to your question including a simple attenuator schematic. most scopes have a calibration reference output point if so use it to check your probe and scope settings, once set you can re check your waveform amplitude. Some attenuated probes also have a variable calibration adjustment!  regards. Merv

[freepow]

  Thanks all for your answers !  Does anyone know if a Spectrum Analyzer would work instead of a
    oscilloscope for a JouleThief, for checking frequency ?

     @ Jeanna, what do you think of digital oscilliscopes like yours ?

        I am thinking of getting one soon, I am looking at one that has a bandwidth of 10Hz to 1Mhz
        or the other one i'm thinking of is 10Hz to 5Mhz, which one would be best to measure frequency of
        most JouleThiefs
        1st picture = 10Hz to 1Mhz
        2nd picture = 10Hz to 5Mhz

        sorry about size of picture-1

[MrMag]

@Freepow

Personally, I would try to find a used 50-100Mhz scope on eBay. You should be able to pick one up for under $300.00. Also, I never did like the LCD scopes. The display is good for constant waveforms but you may loose some of the spikes due to the slow speed of the display. The other thing you may want to look into are the PC scope interfaces. I don't have any experience with them but they look pretty good.

[resonanceman]

Thanks all for your answers !  Does anyone know if a Spectrum Analyzer would work instead of a
    oscilloscope for a JouleThief, for checking frequency ?

     @ Jeanna, what do you think of digital oscilliscopes like yours ?

        I am thinking of getting one soon, I am looking at one that has a bandwidth of 10Hz to 1Mhz
        or the other one i'm thinking of is 10Hz to 5Mhz, which one would be best to measure frequency of
        most JouleThiefs
        1st picture = 10Hz to 1Mhz
        2nd picture = 10Hz to 5Mhz

        sorry about size of picture-1

Freepow

What is the maximum input voltage for  those scopes.
From what I have seen most of the cheaper scopes have fairly low input voltages........ It would not  be fun to fry a  scope  with spikes  on one of the  first times you use it.

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I had  an idea
I wonder if anyone else  wants to play around with it.
I am thinking  that  a resonant circuit in a JT could  be helpful
It may help  the JT run on much lower input voltages.

I have tried adding a cap in parallel  with the coil going to the base.
Although it can be done by connecting the cap to either end.....so far it looks to me like it works  better  if  I untwist  the ends of the JT primarys  and connect the cap there.

In theory...... once it is running (the secondary powering LEDs )  the LEDs should  be brighter with the cap than without it.

In my  first tests the only cap that did this  was a 1uF tantalium ......a 1uF polypropylene cap was no better  than any other cap I tried.
With the tantalium cap it was slightly brigher with the cap than without ......but only slightly

It is usually necessary to short  across the cap to get the JT started.

My first tests were done on a candy cane  coil.........4 turns through the  center for  primarys.........20 turns on each of  6 toroids.
I am not sure if candy cane coils  are ideal for this.....but it was what I had  handy.


I am thinking  more turns on a primary might  be helpful for  this particular circuit.

gary