Joule thief

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As Joule thief ( German Joule-thief or energy thief ) is referred to in the electronics , a discrete electronic circuit , a direct electric voltage is transformed to a higher voltage. The name joule thief is a pun of the English expression jewel thief (German jewel thief illustrate) and is intended that the arrangement of an already unusable for other purposes battery even the last residue of electrical energy squeezes ( "steals"), the name Joule stands for the name of the unit of energy.


In the November 1999 issue of Everyday Practical Electronics (EPE) , Z. Kaparnik published a circuit under the title Micro-torch Circuit in the Readers' Ideas section . The circuit consisted of a on a transistor and a coil with a second winding as a feedback -based boost converter . Clive Mitchell built the circuit with an NPN transistor of the type BC 549 , a 1 kOhm resistor , a coil with two windings and a white light emitting diode . He operated this circuit on an ordinary mignon battery and called it Joule thief . The same term is used for similar circuits.


Circuit diagram of the joule thief with a light emitting diode as a load. With common dimensions, the coil consists of a toroidal ferrite with two times 20 turns and a wire diameter of 0.15 mm. An ordinary bipolar small-signal transistor, such as the BC 547B, can be used as the transistor. The base resistance should be in the range of 1 kΩ. A circuit dimensioned in this way can be operated up to a battery voltage of approximately 0.35 V.

The circuit works as an unregulated step-up converter with the self- oscillation characteristics of a blocking oscillator . The white LED has a forward voltage of around 3 V, so it does not conduct immediately after being switched on. The transistor connects the coil cyclically with the supply voltage, so that magnetic energy is stored in it. This energy induces a higher voltage during the blocking phase, which enables a current to flow through the light-emitting diode and the stored energy is dissipated.

Switch-on process

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 it begins to conduct. Since a higher current flows through the primary winding of the coil and the transistor, the magnetic flux density in the core of the coil increases. This positive change in flux density induces a voltage in the secondary winding which, due to the winding direction, is polarized in series with the supply voltage and thus increases the base current. Due to this positive feedback , the transistor now controls even further and saturates . 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 - ignoring ohmic losses - according to the law of induction:

Shutdown process

The most important effect for switching off the transistor is the limitation of the base current by the necessary base series resistor. With the same number of turns on the primary and secondary side of the coil, double the input voltage minus the voltage drop is still applied to the base-emitter path across the base series resistor at the time of the increase in the collector current. This results in a base current which, via the current amplification factor of the transistor, limits the maximum current through the collector-emitter path of the transistor and thus through the coil.

The current through the coil cannot increase further beyond this value. Voltage is no longer induced. The base current collapses, the collector current must follow this and the current through the coil drops, which now means a negative voltage on the secondary side. The base current continues to fall 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, for example when the basic series resistor is omitted, which can cause the circuit to oscillate and which are essentially due to the non-linear 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 circuit oscillating. The circuit can also be set up with an air-core coil, i.e. without allowing the ferrite core to be saturated at all.

However, if a ferrite core is used, 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 created by the winding also increases steadily. However, since the flux density can no longer follow the field from a certain point - saturation - due to the material properties , the increase in flux density stagnates. As a result, the voltage induced in the secondary winding is also reduced and, as a result, the base current dependent on it, which means that the transistor conducts less. The now decreasing flux density in the coil in turn causes an induction voltage in the secondary winding that is opposite to the supply voltage, whereby the base current decreases further and the transistor closes further. Because of this positive feedback, 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 arises at the collector that is higher than the supply voltage. This voltage is now sufficient to enable a current to flow through the connected light-emitting diode, with which the energy stored in the core of the coil can be dissipated.

Even below the coil saturation, the circuit can oscillate without a limited base current. In the resulting high current range, the current gain of the transistor decreases with increasing collector current sharply, so that the increase of the collector current is slowed. This leads to a smaller change in the flux in the coils and consequently to a lowering of the secondary voltage. Because of the reverse tapping on the secondary side, this voltage is directed in the same way as the battery voltage when the collector coil current rises, which means it amplifies it and allows the base current to reach its maximum. A decrease in the change in flux thus causes a reduction in the base current and ultimately leads to a sudden locking of the collector-emitter path in the high-current range of the transistor due to positive feedback. The current through the collector coil is at its maximum at this moment and, according to Lenz's rule , a voltage arises at the collector coil that counteracts the abrupt change in current. This voltage can be much higher than the battery voltage, which is the desired effect. The coil is now discharging as the current slowly decreases to zero. As soon as the magnetic energy of the coil is zero, the cycle starts over.


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


Joule thief with expansion circuit for a constant output voltage. The level of the voltage U out is limited and thus stabilized by the Zener voltage of the Zener diode .

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 the light-emitting diode. If the light-emitting diode is missing as a load, the induced voltage is only limited by parasitic capacitances and increases to values ​​that can exceed a hundred times the input voltage, whereby the maximum collector-emitter voltage of the transistor is usually exceeded and the transistor is destroyed.

The almost unlimited increase in the induced voltage can, however, 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 Zener diode in parallel to the capacitor, the charging voltage on this is limited to a defined value.


White light-emitting diodes have a forward voltage well above the nominal voltage of 1.5 V for conventional alkaline batteries . In order to be able to dispense with additional expensive batteries with 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 structure and the mass cost-effective production of the circuit of the Joule thief , it is used specifically in LED flashlights.

See also

Individual evidence

  1. Make a joule thief ,, accessed on 23 March 2014

Web links