What's the best joule thief you've found that produces 3.3vdc?

Thread Starter

bulrush

Joined Jan 1, 2017
30
What's the best JT you've found that takes the lowest input vdc, but still produces 3.3vdc for LED lights? Some of them don't work well if the input voltage is below 1vdc.
 

ian field

Joined Oct 27, 2012
6,539
What's the best JT you've found that takes the lowest input vdc, but still produces 3.3vdc for LED lights? Some of them don't work well if the input voltage is below 1vdc.
The blocking oscillator variety is the most popular for running 1.5V cells down as low as they'll go. The downside is winding/appropriating the inductive component.

The astable type uses the simplest magnetics, but it can take some effort to get decent power out of it.

Solar garden lights use a variety of Joule thief type circuits to generate 3.4V for a white LED or 4V for the colour changing ones.

You can suck the cell a little more dry by using a germanium transistor, losses will be higher and suitable types are getting very hard to find.
 

DickCappels

Joined Aug 21, 2008
6,457
If you are referring to what has been called "Joul Thief" for the past 12 years or so, they are all similar being blocking oscillators that source current into the LED when the active device switches off. Peformance primarily a function of component selection.

The topic is covered lightly (no pun) but at some depth in the article at the link below)
http://cappels.org/dproj/ledpage/leddrv.htm
 

tcmtech

Joined Nov 4, 2013
2,867
I'd love to do some work with a thermoelectric generator.
Find a peltier cooler and run it backwards by transferring heat across it being all it is is a bunch of thermoelectric junctions (generators) in series so that they will work at a higher voltage. :rolleyes:
 

Thread Starter

bulrush

Joined Jan 1, 2017
30
@tcmtech : Thanks. I think I have 5 TEGs somewhere in a box.

@crutschow : I looked at the specs on the LT site and it doesn't mention the input voltage vs the output voltage. It just mentions the output voltage is settable to a fixed value. It doesn't mention the minimum input voltage.
 

ian field

Joined Oct 27, 2012
6,539
I've read that these chips are generally really inefficient. Especially ones from the dollar store.
They usually integrate a 2 transistor oscillator and probably only vary in minor detail from one type to another.

The chip also integrates light sensing to switch off during daylight. Originally they had an LDR and separate shutdown circuit, AFAIK: the chip uses a B/E junction to sense charging current from the solar cell - this doubles as the backflow blocking diode that stops the battery discharging into the solar cell during low light. They used to use a low Vf Shottky barrier diode, the B/E junction drops more than double the Vf.
 

ian field

Joined Oct 27, 2012
6,539
If you are referring to what has been called "Joul Thief" for the past 12 years or so, they are all similar being blocking oscillators that source current into the LED when the active device switches off. Peformance primarily a function of component selection.

The topic is covered lightly (no pun) but at some depth in the article at the link below)
http://cappels.org/dproj/ledpage/leddrv.htm
The term; "Joule thief" always makes me think of the blocking oscillator, but a lot of people also include any circuit type that can run a LED and suck a 1.5V cell dry.
 
@crutschow: Thanks! Yummy DC boosting goodness. I might try to buy one of those LTC3108s.

Wow. On Ebay the chip alone goes for $14-23. A working board with the chip is $40. Fun but pricey. I'd love to do some work with a thermoelectric generator. I might have to ask for an Ebay gift card for my birthday.
How about this $8 one over at ali: https://www.aliexpress.com/item/LTC3108-1-ultra-low-voltage-boost-converter-power-manager-development-board/32819094120.html
 

ro0ter

Joined Jul 9, 2013
14
@crutschow : I looked at the specs on the LT site and it doesn't mention the input voltage vs the output voltage. It just mentions the output voltage is settable to a fixed value. It doesn't mention the minimum input voltage.[/QUOTE]

min 20mV
max 500mV


Screenshot-2018-02-01--15-09-33.png
 

ro0ter

Joined Jul 9, 2013
14
However, I`d rather use the scavenger and have the AA/AAA batteries first in parallel then in series... use a dpdt switch, or some automatic tripping circuit for when the voltage of the cell goes under 0.9v for instance...

you`ll have 2 sets of cells. First both sets are connected in parallel. While they get discharged, their voltage will drop. When their voltage is under 0.7~0.9V, you should put them in series.

First batteries are connected in parallel:
Screenshot-2018-02-01--15-36-49.png

Secondly, when voltage drops (joule thieve or scavenger no longer delivering), connected in series:
Screenshot-2018-02-01--15-36-18.png
 

Audioguru

Joined Dec 20, 2007
11,249
I bought a bunch of solar garden lights for $1.00 each at Walmart. They use a Ni-MH AAA cell and a voltage stepup/charger IC. The solar panel has a glass face that lasts forever unlike the old ones with a plastic cover and LDR that got sunburned. Most have a white 3.2V LED but I replaced it with a colors changing LED in most of them.
 
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