Joule Thief Steals Power For A Clock

A common project among electronics tinkerers is the joule thief, a self-oscillating circuit that can “steal” the remaining energy in a battery after the voltage has dropped so low that most devices would stop working. Typically the circuit powers an LED until almost all of the energy is extracted from the battery, but [Lionel Sears] has created a specialized joule theif that uses the “extra” energy to power a clock.

The circuit uses four coils instead of the usual two to extract energy from the battery. The circuit charges a large capacitor which provides the higher current pulses needed to drive the clock’s mechanism. It can power the clock from a single AA battery, and will run until the voltage on the battery is only 0.5 volts.

Normally the clock would stop running well before the voltage drops this low, despite the fact that there’s still a little chemical energy left in the batteries. The circuit can drive the clock for an extended time with a new battery, or could use old “dead” batteries to run the clock for a brief time while the final little bit of energy is drawn from them. If you’re so inclined, you could even use hot and cold water with a joule thief to run your clock! Thanks to [Steven] for the tip.

13 thoughts on “Joule Thief Steals Power For A Clock

  1. The “extra” energy part is delusional. Joule thief is a very poorly design boost converter with low efficiency. In the lifetime of the battery, the joule thief is stealing more energy than that last tiny bit it is claiming to get. Near the final moments of a battery, the internal resistance of the battery increases. As you try to draw current from it, there is more I*R drop resulting in a lower voltage at the battery terminal. So that last little might not be as much as you think.

    http://rustybolt.info/wordpress/?p=7796 “When And When Not To Use A Joule Thief”
    – If the supply voltage is higher than 1.5V, then the Joule Thief may not be the best solution.
    – Solar Photovoltaic Cell And Other Low V Sources.[…] Even so, using a Joule Thief will lower the efficiency to about 50%

    If you are concerned with getting the most energy out of the battery, you are much better off with a properly designed boost converter that is efficient in the first place. There are tons of very simple boost converter chips that are specifically designed for the job.

    As for working on low voltage, high current supplies such as solar cells etc. There are chips from TI, Linear Tech (others?) that are specifically designed for high efficiency energy harvesting.

    1. In My Opinion, The joule thief is usable in these cases:

      -To power a led with a single Alkaline battery, without spending much money in a decent Boost converter.

      -To make some use of those alkaline batteries you’re stocking aroung because they still got some energy (1V-1.2V) but the device where you pulled these from cant manage to work with such a low voltage.

      -When you need a simple and rapid-to-build Boost converter for something where efficiency is not an issue.

    2. Agree with your points.

      By the way, $2 boost modules (0.9V to 5V regulated @ 200mA) from China have very good efficiency (they claim 96%). They use a PWM regulator! You are good to go for your projects.

      Something to think about even the cut corner PCB from China do not use Joule Thief design.

  2. Ugh, good thing it’s Friday. I read “for” as “from”, even after re-reading it, and thought to myself “what a stupid thing to do”… 5 more hours of work go go go.

    1. Gotta pick a nit: Energy (joules) = Power (watts) x Time (seconds)
      Power (watts) = Vout (volts) x Iout (amps)

      A boost converter will work as follows:
      Power Out (watts) = Power In (watts) / Efficiency (unitless)

      Example:
      80% efficiency
      Vout = 10V
      Iout = 0.001Amps
      Pout = Vout x Iout = 0.01Watts
      Vin = 1V battery

      Pin = Pout / efficiency
      Pin = 0.01Watts / efficiency
      Pin = 0.0125 Watts
      Iin = Pin / Vin
      Iin = 0.0125Watts / 1V

      So in this case the battery has to supply 12.5mA at 1Vin to the boost converter in order to drive just 1mA at 10Vout.

      Replace the battery in the above with the ratings of your solar panel.

  3. This is not “stealing” power because I paid for the batteries!

    I wanna see a digital “self-winding” watch that steals power from the kinetic motion of the user’s arm just like the old fashioned self-winding mechanical watch my father had

    Or how about a modern digital version of the clocks that derived energy from the rising and falling barometric pressure?

    Taking power from the atmosphere for free, now that’s more like “stealing”

    1. Seiko made a digital watch powered by movement called the seiko kinetic. It’s cool, but the reality is a normal digital watch only needs it’s battery changed every 5 years or so and the kinetic watch still has a rechargeable battery that needs to be replaced every 10 or 20 years, so you aren’t saving much effort or money.

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