Description.
There are many instances where we require 24V DC from a 12V source. I also once faced a situation like this when I decided to fit a small transistorised stereo amplifier which operated from 24V DC in my car. I found this circuit to be well enough for my application. This circuit can produce a steady 24C DC output and can deliver up to 800mA of output current. The circuit is basically a dc dc converter built around LM324 IC which is configured as an oscillator to produce the switching frequency and a transistor as the semiconductor switching element.
IC1 LM324 is the heart of this circuit. LM324 is a quad opamp and out of the four operational amplifiers inside it, only two is used here. IC1a, resistors R1, R2, R3 and C1 forms a oscillator which operates at around 500Hz. Resistor R2 and capacitor C1 is used to set the oscillator frequency. IC1b is wired as a comparator which compares the output voltage with a reference and feeds back a voltage to the oscillator stage. This is done for controlling the output voltage. Potential divider using preset R5 is connected to the non inverting pin of IC1. The output voltage is connected to the inverting input pin through a 100K resistor. Output of this comparator stage is fed to the non inverting input pin of IC1a through another 100K resistor. Output of the oscillator stage is connected to the base of transistor Q1 and resistor R7 is used for limiting the base current of Q1.
When the output of the oscillator stage is high, transistor Q1 will be switched ON and the current through the inductor L1 starts to increase. When the output of oscillator goes low, transistor Q1 will be switched OFF and now the only path for the inductor current is through diode D2, capacitor C3 and the load if any. The fly back diode D2 will be forward biased and the energy stored in the inductor during the ON state will be dumped into the capacitor. Diode D1 acts as a freewheeling diode.
An inductor will always try to oppose any variation in the current passing through it and this property of the inductor is utilized here. When charged it stores energy and when discharged it behaves like an energy source. The voltage it outputs during the discharge phase is proportional to the rate of change of current through it. As the switching frequency increases the induced emf (electro motive force) from the inductor also increases.
Circuit diagram of 12V to 24V DC DC converter.
