Joule thief
Encyclopedia
"Joule thief" is a nickname for a minimalist self-oscillating voltage booster
that is small, low-cost, and easy-to-build; typically used for driving light loads. It can use nearly all of the energy in a single-cell electric battery
, even far below the voltage where other circuits consider the battery fully discharged (or "dead"). Hence the name suggests the notion that the circuit is stealing energy or "Joule
s" from the source. The term is a pun
on the expression "jewel thief", one who steals jewelry or gemstone
s.
The circuit uses the self-oscillating properties of the blocking oscillator
, to form an unregulated voltage boost converter
. As with all power conversion technology, no energy is actually created by the circuit. Instead, the output voltage is increased at the expense of higher current draw on the input. As a result, the amount of power entering the circuit is the same as the amount leaving, minus the losses in the conversion process
.
. The Joule Thief circuit is based on the blocking oscillator, which uses a vacuum tube / thermionic valve and dates to prior to World War II
.
) which turns the transistor on harder. This self-stoking/positive-feedback process almost instantly turns the transistor on as hard as possible (putting it in the saturation region), making the collector-emitter path look like essentially a closed switch (since VCE will be only about 0.1 volts, assuming that the base current is high enough). With the primary winding effectively across the battery, the current increases at a rate proportional to the supply voltage divided by the inductance. Switch-off of the transistor takes place by different mechanisms dependent upon supply voltage.
The predominant mode of operation relies on the non-linearity of the inductor (this does not apply to air core coils).
As the current ramps up it reaches a point, dependent upon the material and geometry of the core, where the ferrite saturates (the core may be made of material other than ferrite). The resulting magnetic field stops increasing and the current in the secondary winding is lost, depriving the transistor of base drive and the transistor starts to turn off. The magnetic field starts to collapse, driving current in the coil into the light emitting diode (raising the voltage until conduction occurs) and the reducing magnetic field induces a reverse current in the secondary, turning the transistor hard off.
At lower supply voltages a different mode of operation takes over:
The gain of a transistor is not linear with VCE. At low supply voltages (typically 0.75v and below) the transistor requires a larger base current to maintain saturation as the collector current increases. Hence, when it reaches a critical collector current, the base drive available becomes insufficient and the transistor starts to pinch off and the previously described positive feedback action occurs turning it hard off.
To summarize, once the current in the coils stops increasing for any reason, the transistor goes into the cutoff region (and opens the collector-emitter "switch"). The magnetic field collapses, inducing however much voltage is necessary to make the load conduct, or for the secondary-winding current to find some other path.
When the field is back to zero, the whole sequence repeats; with the battery ramping-up the primary-winding current until the transistor switches on.
If the load on the circuit is very small the rate of rise and ultimate voltage at the collector is limited only by stray capacitances, and may rise to more than 100 times the supply voltage. For this reason, it is imperative that a load is always connected so that the transistor is not damaged. Note that, because VCE is mirrored back to the secondary, failure of the transistor due to a small load will occur through the reverse VBE limit for the transistor being exceeded (this occurs at a much lower value than VCEmax).
The transistor dissipates very little energy, even at high oscillating frequencies, because it spends most of its time in the fully on or fully off state, thus minimizing the switching losses.
The switching frequency in the example circuit opposite is about . The light-emitting diode will blink at this rate, but the persistence
of the human eye means that this will not be noticed.
When a more constant output voltage is desired, a voltage regulator
can be added to the output of the first schematic. In this example of a simple shunt-regulator, a blocking diode ("D_rect") allows the secondary winding to charge a filter capacitor ("C_filter") but prevents the transistor from discharging the capacitor. A Zener diode
("Z1") is used to limit the maximum output voltage.
Boost converter
A boost converter is a power converter with an output DC voltage greater than its input DC voltage. It is a class of switching-mode power supply containing at least two semiconductor switches and at least one energy storage element...
that is small, low-cost, and easy-to-build; typically used for driving light loads. It can use nearly all of the energy in a single-cell electric battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
, even far below the voltage where other circuits consider the battery fully discharged (or "dead"). Hence the name suggests the notion that the circuit is stealing energy or "Joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
s" from the source. The term is a pun
Pun
The pun, also called paronomasia, is a form of word play which suggests two or more meanings, by exploiting multiple meanings of words, or of similar-sounding words, for an intended humorous or rhetorical effect. These ambiguities can arise from the intentional use and abuse of homophonic,...
on the expression "jewel thief", one who steals jewelry or gemstone
Gemstone
A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...
s.
The circuit uses the self-oscillating properties of the blocking oscillator
Blocking oscillator
A blocking oscillator is a simple configuration of discrete electronic components which can produce a free-running signal, requiring only a resistor, a transformer, and one amplifying element. The name is derived from the fact that the transistor is cut-off or "blocked" for most of the...
, to form an unregulated voltage boost converter
Boost converter
A boost converter is a power converter with an output DC voltage greater than its input DC voltage. It is a class of switching-mode power supply containing at least two semiconductor switches and at least one energy storage element...
. As with all power conversion technology, no energy is actually created by the circuit. Instead, the output voltage is increased at the expense of higher current draw on the input. As a result, the amount of power entering the circuit is the same as the amount leaving, minus the losses in the conversion process
Energy conversion efficiency
Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.-Overview:...
.
History
In the November 1999 issue of Everyday Practical Electronics (EPE) a simple circuit was published by Z. Kaparnik that consisted of a transformer-feedback single-transistor voltage converterBoost converter
A boost converter is a power converter with an output DC voltage greater than its input DC voltage. It is a class of switching-mode power supply containing at least two semiconductor switches and at least one energy storage element...
. The Joule Thief circuit is based on the blocking oscillator, which uses a vacuum tube / thermionic valve and dates to prior to World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
.
Description of operation
The circuit works by rapidly switching the transistor. Initially, current enters the transistor base terminal (through the resistor and secondary winding), causing it to begin conducting collector current through the primary winding. This induces a voltage in the secondary winding (positive, because of the winding polarity, see dot conventionDot convention
In circuit analysis, the dot convention is a convention used to denote the voltage polarity of two mutually inductive components, such as winding on a transformer....
) which turns the transistor on harder. This self-stoking/positive-feedback process almost instantly turns the transistor on as hard as possible (putting it in the saturation region), making the collector-emitter path look like essentially a closed switch (since VCE will be only about 0.1 volts, assuming that the base current is high enough). With the primary winding effectively across the battery, the current increases at a rate proportional to the supply voltage divided by the inductance. Switch-off of the transistor takes place by different mechanisms dependent upon supply voltage.
The predominant mode of operation relies on the non-linearity of the inductor (this does not apply to air core coils).
As the current ramps up it reaches a point, dependent upon the material and geometry of the core, where the ferrite saturates (the core may be made of material other than ferrite). The resulting magnetic field stops increasing and the current in the secondary winding is lost, depriving the transistor of base drive and the transistor starts to turn off. The magnetic field starts to collapse, driving current in the coil into the light emitting diode (raising the voltage until conduction occurs) and the reducing magnetic field induces a reverse current in the secondary, turning the transistor hard off.
At lower supply voltages a different mode of operation takes over:
The gain of a transistor is not linear with VCE. At low supply voltages (typically 0.75v and below) the transistor requires a larger base current to maintain saturation as the collector current increases. Hence, when it reaches a critical collector current, the base drive available becomes insufficient and the transistor starts to pinch off and the previously described positive feedback action occurs turning it hard off.
To summarize, once the current in the coils stops increasing for any reason, the transistor goes into the cutoff region (and opens the collector-emitter "switch"). The magnetic field collapses, inducing however much voltage is necessary to make the load conduct, or for the secondary-winding current to find some other path.
When the field is back to zero, the whole sequence repeats; with the battery ramping-up the primary-winding current until the transistor switches on.
If the load on the circuit is very small the rate of rise and ultimate voltage at the collector is limited only by stray capacitances, and may rise to more than 100 times the supply voltage. For this reason, it is imperative that a load is always connected so that the transistor is not damaged. Note that, because VCE is mirrored back to the secondary, failure of the transistor due to a small load will occur through the reverse VBE limit for the transistor being exceeded (this occurs at a much lower value than VCEmax).
The transistor dissipates very little energy, even at high oscillating frequencies, because it spends most of its time in the fully on or fully off state, thus minimizing the switching losses.
The switching frequency in the example circuit opposite is about . The light-emitting diode will blink at this rate, but the persistence
Persistence of vision
Persistence of vision is the phenomenon of the eye by which an afterimage is thought to persist for approximately one twenty-fifth of a second on the retina....
of the human eye means that this will not be noticed.
When a more constant output voltage is desired, a voltage regulator
Voltage regulator
A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components...
can be added to the output of the first schematic. In this example of a simple shunt-regulator, a blocking diode ("D_rect") allows the secondary winding to charge a filter capacitor ("C_filter") but prevents the transistor from discharging the capacitor. A Zener diode
Zener diode
A Zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "Zener knee voltage" or "Zener...
("Z1") is used to limit the maximum output voltage.