What's the purpose of a joule thief?

Discussion in 'General Electronics Chat' started by tjohnson, Jun 16, 2015.

  1. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    I understand what the function of a joule thief is - to allow a device like an LED to be powered by a dying battery. But recently I've been wondering what is the purpose of one exactly?

    Isn't it possible to substitute a larger number of old batteries for a certain number of fresh ones? For example, if I needed a 3V power source, I could use 2 fresh AA batteries, or also 8 old ones that provide 0.375V each, right?

    The only benefit I can think of that a joule thief would provide is that it allows the power source to be much smaller. A single AA battery with a joule thief can fit in a pocket flashlight easily, while 4 old AAs can't. But I would think there must be more of a benefit than this?
     
  2. #12

    Expert

    Nov 30, 2010
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    That's all the benefit I need to lay my money down. I have been carrying a (1) AA LED flashlight in my pocket for over a year. I would never try to carry a (3) AAA torch...it's too bulky.
     
  3. MikeML

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    Oct 2, 2009
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    The Joule Thief gets the last few % of the available power out of a almost dead battery. Not worth the trouble...

    A Switch-Mode Power-Supply boost dc-to-dc Converter (the proper name for a Joule Thief) is beneficial if it is configured to drive a LED in constant-current mode while the input voltage can vary over a wide range...
     
  4. ErnieM

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    Apr 24, 2011
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    There one of these in all those inexpensive garden solar lights. To keep the cost down they use but a single AA NiCad battery and very few other parts ( inductor, the LED, the solar cell, and a tiny custom IC). It has a circuit advantage of using any voltage input (lower than the LED voltage) to produce exactly what current the LED needs, as determined by the inductance. No power is wasted with a dropping resistor.

    What you call a joule theif I call a switching set up converter.
     
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  5. #12

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    Get some in the local stores and expect to see my money arrive!
     
  6. ian field

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    You've pretty much got it as far as the purpose of the Joule thief - its basically for finishing off nadjered cells, its also a very simple circuit for beginners to experiment with.

    The most common type is the blocking oscillator, and its many variants have a multitude of uses. As others have pointed out, its used to run a white LED from the 1.2V nickel cell in older solar garden lights, a variant modified for high voltage is used in many mains/rechargeable shavers. Most of the 80s plastic cased home computers had flyback PSUs based on the blocking oscillator.

    Stacking a pile of worn out cells to get the original voltage won't work - the cells will all have varying amounts of internal resistance, the one with the highest resistance will be reverse 'charged' - that is a reverse voltage will be impressed across its terminals by other more lively cells. That can cancel out the voltage supplied by some other cells that are actually doing something.
     
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  7. wayneh

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    I'm with Ernie - by sheer number, most joule thief circuits are for powering LEDs from a single cell. These have a voltage below Vf and couldn't light the LED without the circuit. The joule thief is a cheap solution that provides current control at the same time.

    The joule thief is not so much about squeezing a little more power out, it's about getting the output voltage up over Vf of the LED and limiting the current. If the ONLY goal was more battery life, I think it would be cheaper for the OEM to just specify a 10% higher capacity in the first place.

    If the only goal was voltage boosting, it would be simple to use 2 cells in series, but then you'd also need current controller.
     
  8. ian field

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    The hobby use is for finishing off knackered cells and providing beginners with hours of experimentation.

    In its many various forms, its a simple flyback converter that has been popular in garden lights, shavers, early home computers and all sorts of other things.
     
  9. tjohnson

    Thread Starter Active Member

    Dec 23, 2014
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    Very interesting, I didn't know that. Why doesn't this also happen with fresh batteries, whose internal resistances probably vary by a small amount (just like resistors and capacitors have tolerance values)?

    It sounds like I should make my own joule thief, since I have literally dozens of old batteries sitting around. Do you think an old laptop would have a toroid bead inside?
     
  10. ian field

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    In the old carbon-zinc batteries, most of the guts inside a cell was manganese dioxide (the messy black stuff). That is the depolariser, most of the resistance is due to the insulating properties of tiny bubbles of evolved gas around the positive electrode. In a fresh cell the depolariser absorbs the gas so it doesn't insulate the electrode - once the depolariser has converted all the gas it can, the gas builds up around the electrode and insulates it - hence steadily increasing internal resistance.
     
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  11. tjohnson

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    I don't have any zinc-carbon batteries, but only alkaline ones. Is the same thing true of them?
     
  12. ian field

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    AFAIK: the gas bubble thing is common to most if not all battery chemistries.

    They just have different ways of mitigating its effects.
     
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  13. takao21203

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    Apr 28, 2012
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    most of your replies are not really correct.

    First of all Joule Thief is not recognized professionally and its not patented or trademarked.

    Its a blocking oscillator, which is the correct professional term.
    These can be found in literature since the 1960s but nowadays booster ICs are used for the same purpose.

    These are MOSFET based for instance MCP1640.

    YX8018 and QX5252 are current controlled voltage boosters also MOSFET based, and used for solar garden lights there are countless variants.

    Its also wrong a "joule thief" is limited to drive small LEDs from old batteries. Ive built at least 30 or so, the latest can light up neon lamps, orange from a single battery but needs 2 for green ones.
    Also 20 LED string is possible.

    The schematic is very similar but different, the inductance ratio is also different and the inductance is rather high with 10 mH. you can see it here https://www.flickr.com/photos/takao21203 (not updated much anymore).

    Blocking oscillator still is used to generate small high voltage currents, there arent really many ICs for it and they arent common, the usual single cell converters dont go above 6 volts while some ics such as mt3608 can reach upto 20 volts.

    its not worth it if you only need a few mA at 30 volts and dont need max. efficiency.

    Joule thief isnt limited to old batteries and its just a coined term, others are LED torch circuit or voltage booster (professionally valid term).

    There are many variants such as using two coils instead a transformer or two transistors and one coil.

    Blocking oscillators have been around long before the joule thief and are mentioned in older professional literature, often to get higher voltage from 6v batteries (were common for cars a while ago).

    the main reason why they arent used so much anymore is the efficiency. 70% and more is possible but not easy to dimension and not as flexible as a booster circuit with oscillator and feedback loop.

    In the 1970s before that TRIAC supplies were around for a short time inside televisions.
     
  14. ian field

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    I've mentioned enough times to not have to keep on repeating it, that the most common type of Joule-thief circuit is the blocking oscillator, but there are other types as well - you can just as easily replace one of the collector load resistor in an astable multivibrator and use it to drive a white LED from a single cell.
     
  15. takao21203

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    well sure you are right, the problem is that "joule Thief" isnt found or recognized in professional literature.
    Its a specific variant of the blocking oscillator and has little commercial significance.

    is there are reference (besides wikipedia) defining what is a joule thief?

    a) its a blocking oscillator
    b) its astable multivibrator used to drive a coil

    Sure you can call both joule thief, i have no problem with it.

    The problem is to distinguish what is a joule thief and what not.
    Is it chosen by application, or the fact alone the circuit is used to drive LEDs from single battery?

    In the sense of wikipedia, I see as a joule thief a blocking oscillator using a real transformer and one transistor, and also allowing close variants.

    The MCP1640 isnt a joulethief ic or at least isnt sold as such, but can be used for the exact same purpose.

    Sure i know there are many google hits i have researched these circuits since some time.

    Its probably a marketing problem, would you buy a mini disc player with a label on it "joule thief technology"?
    these digital cameras have labels which peel off after a year or two.
    and this MD player used 1xAA in 2004 already.

    Heres the wikipedia definition

    A joule thief is a minimalist Armstrong[1] self-oscillating voltage booster

    and

    The circuit is variant of the blocking oscillator that forms an unregulated voltage boost converter.
     
  16. ian field

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    Was there some point you were trying to make?
     
  17. takao21203

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    besides what i already wrote, no.

    Definition of what is a joule thief not really clear besides the wikipedia entry (which I think is accurate).

    Is it a joule thief when it can drain batteries completely? For LED torch only?
    Or when the circuit is similar or even just stepping up voltage?

    I think its too broad to call these Joule Thief, I only count the original variant and very closely related kinds (for instance using 2 coils + capacitive divider).
     
  18. wayneh

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    Like a lot of things around here, confusion over terms comes from the distinction of the application versus the device or circuit used to serve the application. For instance "chokes" are inductors, but not all inductors are used as chokes. Joule thief circuits are usually voltage boosters, but not all boost circuits are meant to be joule thieves.

    To me, "joule thief" is an application that can be served by several circuits, same as "oscillator". From the name, the application is scavenging energy where it would otherwise be lost, particularly from batteries. Any circuit that accomplishes that might be called a joule-thief, although I think the term is reserved for only a few variants.

    I argued above - and I believe takao also pointed out – that this joule-thieving application is much less important than the voltage boost converter in most circuits where one is being used; flashlights, solar lights, etc.
    The circuit happens to accomplish both jobs, but the "main" application is the boost.
     
  19. ian field

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    Apparently LT do a few voltage booster chips designed to run from a single cell. The recent turn of this discussion made me wonder whether they qualify - although I don't recall seeing any mention of such in their literature.

    Another is the PR4401 LED driver chip, it looks just like a SMD transistor, you add an inductor, cell and white LED and you're good to go.
     
  20. takao21203

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    Apr 28, 2012
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    I think member wayneh pointed out the situation quite well, I agree.

    It is true a joule thief can run weeks from a D cell that isnt fit anymore for its original application, but since a standard LED isnt exactly very bright, these are merely gimmick applications.

    Some time I have made a JT using a tiny 6mm ringcore, russian germanium transistor, and hooked directly to a PIC 16f54 with LED chaser.
    Not exactly very bright but functional!

    The MCP1640 however works much longer from a single battery.
    Well I didnt compare to germanium variant, but actual battery energy is decreasing exponentially, also depends on the battery chemical composition, and believe it or not, brand.

    If you only wanted to drive a LED or LEDs, YX8018 etc are better I think, or my new JT variant using a ready made 10mH inductor and just 8 turns on top as primary + one transistor.

    But I think its more interesting to generate high voltages with minimal amount of parts, and at safe energy levels.
    The circuit has more in common with a Tesla coil than a Joule thief- its not meant to be to be used with drained batteries in fact it needs 3 volts to reach enough output voltage for green and blue neons.

    In that sense it isnt a Joule Thief, though, its actual possible to connect a LED parallel to the primary winding.
     
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