Joule thief efficiency evaluations - Page 1
Forum section: electronics / projects

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 04, 2019, 05:00:29 pm
content: A decade ago (yes it has been that long) I became interested in joule thieves, and their outstanding capabilities to light up LED from very low voltages.Although the basic circuit works fine, it can be significantly improved.To satisfy my curiosity I experimented a little, recorded my results, and wrote up a short article, to share in an online forum. Which I no longer remember which was. And I am not sure I ever shared.Cleaning old files from my computer, I found this old file.I am attaching it here, only FWIW and to share with whomever is interested in these circuits..

author: IanB
author_rank: Super Contributor
timestamp: February 04, 2019, 05:41:37 pm
content: Cool. That's an interesting read.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 04, 2019, 06:52:49 pm
content: Yeah, the basic circuit is dumb and sucks. It's an easily-built curiosity, but not practical or efficient in any meaningful way.Note that 600kHz iswaytoo high for a #26 core -- the #8 should be okayish though. These are often built with ferrite beads, which is just all kinds of wrong, but, there you have it.I made this one about five years ago,You can basically read the circuit off the layout. Selectable pull-up (infty, 1k, 100R) for adjustable light level. 22uF ceramic for main bypass, 1uF I think for base bypass. I think the primary is 2 x 28 AWG in parallel. The secondary I think is 2:3 or 1:2 ratio, so stepping up slightly. Core is #8 (yellow/red). There is no added resistance between the base bypass cap, secondary, and base pin. This can be added to soften switching, but it almost always only worsens performance. Transistor is a PBSS303NX or similar, low-Vce(sat) type, rated for several amperes.Input current is around 1A, and output is around 1W (4V pulsed, white LED). Forget what switching frequency is, maybe 200 or 500kHz?I evaluated efficiency by measuring input current, and measuring output voltage and current using a schottky diode. In that configuration I got 67%, not bad at all.If I ever made a bigger one, none of this would be necessary; for example, my 10W flashlight is build around an 18650 cell, so little conversion is necessary. As it happens, I chose to use series-connected LEDs, and an average-current-mode boost converter:Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 04, 2019, 07:01:17 pm
content: I believe that the JT's appeal is its inherent simplicity and that it just works.Almost any magnetic material will suffice, I remember seeing a "design" out there, where a chap could not be bothered to find a proper ferrite or powder toroid, and decided to use stack of steel washers.I am pretty sure that the efficiency sucked big time, but the LED lit up.................

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 04, 2019, 11:12:56 pm
content: Quote from: schmitt trigger on February 04, 2019, 07:01:17 pmIndeed! I built a couple of them for fun. Then made a 3D printed box for one...added a mercury switch and used a red led instead of white. Why?I am old. I need a beacon to a certain room at roughly 2am. I also like the fact that I can use batteries that have been taken out of service because they will not power respective device but still have charge in them. My JT takes them from ~1.3 volts down to about 0.7 volts. I am happy with that. The problem is that batteries are clogging up the input because I am not using them up fast enough with the JT.

author: mikerj
author_rank: Super Contributor
timestamp: February 05, 2019, 10:55:21 am
content: Quote from: Wimberleytech on February 04, 2019, 11:12:56 pmSolution: drink more before going to bed to increase number of bathroom trips

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 05, 2019, 04:47:17 pm
content: Quote from: Wimberleytech link=topic=167669.msg2177729#msg2177729 date=154932I am old. I need a beacon to a certain room at roughly 2am.[/quote

author: David Hess
author_rank: Super Contributor
timestamp: February 06, 2019, 05:15:12 pm
content: Texas Instruments made a battery module for their TI-30 type calculators which replaced the standard 9 volt battery with a rechargeable AA cell and joule thief type circuit which ran continuously. Operating time between charges was remarkably long so maybe it would be worth tracking one of these down to find out exactly what circuit they used. I have a couple of the BP5s as shown below somewhere.http://www.datamath.org/BP_Repair.htm