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Part of Made in India,Design Innovation
Workshop,2015
A Fresh New Experience in Project-‐Making & Learning!
PHYSICS INVESTIGATORY PROJECT ON
JOULE THIEF
If you have dead or discharged baCeries in your home, don’t throw them out. Instead use them to make a Joule Thief circuit, which brings back the dead baCery by boosKng its energy to light up an LED. The name ‘joule thief’ suggests the noKon that the circuit is stealing every drop of energy or ‘joules’ from the source.
In this project, you will need a breadboard, a resistor, an inductor, a 1.2 V cell or baCery, a capacitor, two transistors, an LED and a few connecKng wires. The connecKons are made such that the inductor (L), the capacitor (C) and the resistor (R) are connected in series. The intersecKon point of the capacitor and the resistor is further connected to the base of a PNP transistor that drives another NPN transistor. The supply is given aUer the compleKon of the circuit. On giving the supply, the capacitor starts charging and the LED glows.
We know that one cannot glow an LED using a 1.2 V cell but sKll the LED glows. The concept of the decreasing current with Kme through an inductor will help you understand this phenomenon. This decrease in current leads to a change in flux causing an induced voltage (L di/dt), which adds up to 1.2 V (cell voltage), resulKng in a sum of more than 1.8 V due to which the LED glows. The net voltage across the LED = voltage of Cell (1.2 V) – L di/dt. Since the current is decreasing with Kme, so di/dt is negaKve and hence, -‐ Ldi/dt is posiKve. Note that during the charging process, the transistors remain OFF.
When the capacitor becomes almost charged, the LCR circuit becomes open and the PNP transistor becomes ON since the base voltage is low. This transistor reaches its saturaKon state due to which its emiCer and collector get shorted and its collector receives Vcc. Since the collector of the PNP transistor is connected to the base of the NPN transistor, therefore, its base also receives Vcc. This makes the NPN transistor ON. Like wise, the transistor is in the saturaKon region and its emiCer and collector also get shorted. The potenKal of the collector is nearly zero volt as the emiCer of the NPN transistor is grounded. Since the posiKve terminal of the LED is connected to the collector of this transistor, it stops glowing due to insufficient voltage. Hence, the LED is OFF.
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