Author Topic: Joule thief with a twist  (Read 638 times)

Offline sorveltaja

  • Hero Member
  • *****
  • Posts: 561
Joule thief with a twist
« on: February 26, 2022, 04:54:14 PM »
Lately I've been playing around with different kind of leds, mostly with ones that are inside usual led bulbs. And when one led burns out, the whole lamp is useless - or is it?

I tested the rest of the smd-leds(in the bulb's pcb) with a simple voltage doubler, and yeah, it seems to be possible to run those 18-30V leds with lower voltage, like 9V. 

But as usual, things got out of hand, and I had to find out, if there are ways to use them at even lower voltages. That's why I started to look for joule thief.

It's basically simple circuit like this:


There are dozens upon dozens of versions of it, but generally one thing appears to be common between them: toroidal bifilar coil. And they are pita to wrap, especially if one would like to experiment with different sizes:


Simpler, air core axial coils also work with joule thief, as some builders have done. That seems to be the case, as I tested it too.

Current circuit on breadboard:
 

Then somewhere came the idea to test instead with two separate 180uH inductors side by side:


It's weird, but it seems to work. To make sure, that the oscillation happens between those two coils, I pulled them slightly apart, and it stopped.

The results with current settings so far, with 3V led at 0,8V 48mA:


Three 15V(or perhaps rated at 18-20V like similar size 2835 leds) leds at 0,8V 54mA:


In both cases, the leds aren't overly bright, though. I tried increasing the voltage to 1V and above, but it starts to use excess amount of current, like 100mA or about as much as the source provides, even when there are no leds connected.

For testing, I used a bench psu, as at the moment I don't have batteries flat enough that would go down to 0,8V.

Offline awemawson

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 8661
  • Country: gb
  • East Sussex, UK
Re: Joule thief with a twist
« Reply #1 on: February 26, 2022, 05:21:18 PM »
A few years back I equipped our letting holiday cottages with led lights as punters would leave lights on all day. In those days they were unreliable but also expensive, and I soon realized that several in the series chain could be shorted out when they failed with little detriment to the light output.

When a particular bulb had too many shorting links Id take it out of service and use it as a source of spares for future failures. I must have shorted / changed a few hundred leds over the years.

No longer have to use this approach as the price per bulb has plummeted and the reliability shot up to a satisfactory level.

People thought me tight, I probably was, but it was costing tens of pounds a month to keep them going, nowadays I rarely have to change a bulb, so some things are getting better.

Andrew Mawson
East Sussex

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6363
  • Country: us
  • Republic of Vermont
Re: Joule thief with a twist
« Reply #2 on: February 26, 2022, 06:22:10 PM »
Sorveltaja, cool!!!  :thumbup:
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline Sea.dog

  • Full Member
  • ***
  • Posts: 202
  • Country: gb
Re: Joule thief with a twist
« Reply #3 on: February 27, 2022, 05:26:39 AM »
This is weird. I still can't see any of Sorveltaja's pictures. If I look at the page source, copy the hyperlink and paste into my browser, I can view it (or just "open link in a new page").

I don't have this problem with any other poster  :scratch:

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 6363
  • Country: us
  • Republic of Vermont
Re: Joule thief with a twist
« Reply #4 on: February 27, 2022, 11:39:55 AM »
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline BillTodd

  • Hero Member
  • *****
  • Posts: 976
  • Country: 00
  • Colchester Essex (where the lathes were made)
Re: Joule thief with a twist
« Reply #5 on: March 01, 2022, 08:47:52 AM »
Quote
It's weird, but it seems to work. To make sure, that the oscillation happens between those two coils, I pulled them slightly apart, and it stopped.

Nothing weird :-) the two coils are magnetically coupled when close  just as if they were wound on the same former . If you place  pieces a permeable material on the top and bottom , you'd have a toroid .
Bill