What is Joule Thief Circuit? How We Can Make Powerful LED Flashlights

In electronics, a joule thief is a discrete electronic circuit that transforms an electrical DC voltage into a higher electrical voltage. It represents a particularly simple and compact implementation of a resonance transducer.

The phrase Joule thief is a pun of the jewel thief and is intended to make it clear that the circuit squeezes (“steals”) the last remnant of electrical energy from a battery that is already unusable for other purposes, whereby the name Joule stands for the name of the unit of energy.

In the November 1999 issue of the journal Everyday Practical Electronics, Z. Kaparnik published a circuit entitled Micro-torch Circuit in the readers’ ideas section. The circuit consisted of a transistor with a 10k base resistor and a coil with a second winding as a feedback-based boost converter to supply a light-emitting diode from a 1.5V primary battery cell. Clive Mitchell built the circuit with a BC 549 NPN transistor and a resistor reduced to 1 kOhm and used it to control a white light-emitting diode. He operated this circuit on an ordinary mignon battery and called it Joule thief. The same term is also used for similar circuits.

How Joule Thief Circuit Works

The circuit works as an unregulated boost converter with the self-oscillating characteristics of a reversible oscillator. The white LED has a forward voltage of about 3 V, so it does not conduct immediately after switching on. The transistor cyclically connects the coil to the supply voltage, whereby magnetic energy is stored in it. This energy induces a higher voltage during the blocking phase, allowing current to flow through the light-emitting diode and dissipating the stored energy.

After connecting the circuit to the voltage source, a small current begins to flow through the resistor and the secondary winding of the coil into the base of the transistor, whereupon the transistor begins to conduct. Since a higher current now also flows through the primary winding of the coil and the transistor, the magnetic flux density in the core of the coil increases. This positive flux density change induces a voltage in the secondary winding that is polarized in series to the supply voltage due to the winding sense, thus increasing the base current. Due to this coupling, the transistor now steers even further and becomes saturated. As a result, the primary winding of the coil is practically directly connected to the supply voltage, whereby the current in the coil increases approximately – neglecting ohmic losses – according to the law of induction.

The most important effect for switching off the transistor is the limitation of the base current by the necessary base series resistor. If the number of turns on the primary and secondary sides of the coil is the same, at the time of the increase in the collector current, there is still twice the input voltage minus the voltage drop at the base-emitter section above the base series resistor. This results in a base current that limits the maximum current through the collector-emitter distance of the transistor and thus through the coil via the current amplification factor of the transistor.

Beyond this value, the current cannot rise further through the coil. No more voltage is induced. The base current collapses, the collector current has to follow this and the current through the coil drops, which now means a negative voltage on the secondary side. As a result, the base current drops even further until no more current flows through the transistor and the cycle begins again. The time course of the current consumption of the coil is independent of the voltage actually generated.

In addition to this controlled saturation of the collector current, there are other effects, such as omitting the base series resistor, which can cause the circuit to oscillate, and which are essentially due to the nonlinear behavior of the components. It should be pointed out once again that the limitation of the base current is actually the main cause of the oscillation of the circuit. The circuit can also be built with an air coil, i.e. without saturating the ferrite core at all.

However, if a ferrite core is used, then the storage capacity for magnetic energy of the coil is limited due to the ferrite material used. Since the current through the coil increases continuously over time, the magnetic field caused by the winding also grows steadily. However, since the flux density can no longer follow the field at a certain point – saturation – due to the material properties, the flux density increase stagnates. As a result, the voltage induced in the secondary winding is also reduced, and consequently the base current dependent on it, causing the transistor to conduct less. The flux density in the coil, which is now decreasing, in turn causes an induction voltage in the secondary winding that is opposite to the supply voltage, causing the base current to fall further and the transistor to close further. Due to this coupling, the transistor ultimately no longer conducts at all. Since the falling flux density in the core also causes an induction voltage in the primary winding and the polarity of this voltage is now in series with the supply voltage, a voltage is generated at the collector that is higher than the supply voltage. This voltage is now sufficient to allow a current to flow through the light-emitting diode connected to it, which allows the energy stored in the core of the coil to be dissipated.

Even below coil saturation, the circuit can oscillate without limited base current. In the resulting high-current range, the current gain of the transistor decreases sharply as the collector current increases, so that the increase in the collector current is slowed down. This leads to a lower flux change in the coils and consequently to a reduction in the secondary voltage. Due to the reverse tapping of the secondary side, this voltage is directed in the same direction as the collector coil current increases, i.e. it amplifies it and allows the base current to be maximized. A decrease in flux change thus causes a reduction in the base current and ultimately leads to a sudden locking of the collector-emitter section in the high-current range of the transistor through co-coupling. The current through the collector coil is maximum at this moment and, according to Lenz’s rule, a voltage is created at the collector coil that counteracts the abrupt current change. This voltage can be much higher than the battery voltage, which is the desired effect. The coil now discharges as the current slowly decreases to zero. Once the magnetic energy of the coil is zero, the cycle starts again.

In both cases, the cycle starts again after the complete discharge of the magnetic energy from the coil core, because due to the operating voltage through the resistor and the now discharged coil, a base current can flow into the transistor again. The switching frequency resulting from the usual dimensioning of the circuit is about 50 to 300 kHz, strongly dependent on the gain factor of the transistor and the choice of the base resistor. The material of the coil and the number of turns has only a minor influence.

Modification of Joule Thief Circuit

The voltage induced in the primary winding of the coil during the breakdown of the magnetic flux density in the core is limited by the circuit formed by means of the light-emitting diode. If the light-emitting diode is missing as a load, the induced voltage is limited only by parasitic capacitances and rises to values that can exceed a hundred times the input voltage, which usually exceeds the maximum collector-emitter voltage of the transistor and thus destroys the transistor.

However, the almost arbitrarily high increase in the induced voltage can also be used to obtain a stabilized high output voltage. If the light-emitting diode is replaced by a series connection of a diode and a capacitor, the induced voltage charges the capacitor. By connecting a Z-diode in parallel to the capacitor, the charging voltage at the capacitor is limited to a defined value. However, according to the data sheets of common transistors, the base voltage must not drop lower than about 5 V below the emitter voltage, otherwise a breakdown will occur. This sets limits to the tension that can be generated without further modifications.

Application

White light-emitting diodes have a forward voltage far above the nominal voltage of 1.5 V of conventional alkaline batteries. In order to be able to dispense with additional expensive batteries for inexpensive flashlights or solar lights, it is necessary to increase the supply voltage of the diode compared to the nominal voltage of a single battery. Due to the very simple design and the thus mass-cost-effective production of the circuit of the Joule thief, it is especially used in LED flashlights.