NEGATIVE VOLTAGE GENERATOR

Every circuit needs perfect bias supply to perform well and some times. Integrated circuits or special components or output actuators need negative voltage.

Everyone needs a negative voltage rail eventually, but most only have one rail supply. This project will show you how to build a negative voltage generator that runs off a single rail supply voltage!

 

CONCEPT OF GENERATING NEGATIVE VOLTAGE

But using an oscillator, some diodes, and an ingenious arrangement of capacitors, we will exploit an impact called capacitive coupling whereby a negative voltage near adequate to the availability voltage can be generated... from a single rail supply!

CAPACITIVE COUPLING

Capacitive coupling is an impact where a capacitor “attempts” to stay the voltage across its terminals constant. For example, if each side of a capacitor are at 0V and plate 1 is suddenly delivered to 10V, the voltage on the opposite plate will immediately begin to rise to 10V. The same goes for the opposite: If the first plate is brought from 10V to 0V, then the other plate will attempt to drop by 10V to keep the voltage across the capacitor constant.

REMOVING POSITIVE VOLTAGE

As the input steps up from 0V to 10V, the negative plate on C1 also will attempt to match thanks to capacitive coupling. However, once the plate reaches 0.7V, D1 conducts and this prevents the plate voltage from getting above 0.7V. Now with the positive plate at 10V and the negative plate at 0.7V, the potential difference across the plate is 10 – 0.7 = 9.3V. When the positive plate drops rapidly from 10V to 0V, this 9.3V potential difference must be maintained due to coupling. Therefore, the negative plate drops to -9.3V. D1 cannot conduct as it is reversed biased, so this negative voltage remains on the capacitor.
D2, however, is now conducting because the anode (connected to C2 negative plate), is at a higher potential than the cathode (connected to the -9.3V). Therefore, the voltage at the negative plate of C2 will reduce until D2 stops conducting. D2 will stop conducting once the voltage drop across it becomes less than 0.7V. This will happen once the negative plate of C2 becomes -9.3 - -0.7 = -8.6V.
But if an external device attempts to use the negative voltage, it might very quickly be used up! This is because of the relatively small sizes of C1 and C2 as well as the current limits of the circuit driver. The solution is to attach an oscillator to the input in order that a negative voltage is consistently being generated at C1, which keeps C2 topped up.
C2 is used as a de coupler for external devices using the negative voltage to try and keep the value as constant as possible. For our project, C1 will be a 10uF capacitor and C2 will be a 100uF capacitor.


COMPONENTS
• 555 timer Ic
• Diode(D1,D2)
• 10uF Capacitor (C1)
• 100uF Capacitor (C2)
• 5.6kΩ (R1)
• 47kΩ (R2)
• 100nF (C4)
• 10nF (C3)
• Assorted Wires
• Breadboard
• Supply voltage




COMPLETE CIRCUIT