Capacitance (Timing Circuits) - Essay Example

Introduction  The tendency of a capacitor to store energy in an electric field is known as Capacitance. We can also say that it is the amount of electric potential stored for a given potential energy store in a parallel plate capacitor. Capacitance is directly proportional to the surface area of the plates and inversely proportional to the separation distance between the plates in parallel plate capacitor ("Capacitance"). The formula for calculating capacitance is C=Q/VTheory CapacitanceCertain amount of energy is needed to transfer charge to the object this energy per unit of charge is called as voltage.

By providing certain voltage charge can be transferred to an object. For capacitance derived unit is useful for expressing the charge (unit of charge is Coulombs)that can be transferred to an object per unit of voltage ( unit of voltage is in Volts) and thus a unit which we got from this ratio is known as Farad. Each capacitor having two parallel plates and will have capacitance equal to 8.85 ρF/m, times the area of one of the plates, divided by the distance between them. When sharing the charge applied to one capacitor with a second capacitor, charge is conserved, therefore Vf * (C1 + C2) = Vi * C1.

When discharging a capacitor through a resistor, V(t) = V0 * e-t/RC. When charging a capacitor through a resistor, V(t) = Vf – Vf * e-t/RC (“Courseworkbank”).Timing CircuitIn timing circuits there are two possibilities either we are controlling the rate of response of the capacitor or controlling the rate of voltage across the capacitor. If a series combination of a resistor and capacitor are present such type of circuit is simplest timing circuit. The amount of time to charge the capacitor to 63% of the input voltage as well as it is equal to the resistance times the capacitance is known as time constant T=RC.

Varying the values of resistance and capacitance ultimately results in time ranging from microseconds to weeks. The important characteristics of capacitor are:ESR Insulation resistance Capacitance change vs. time Capacitance change vs. temperature DiscussionIn this section I would prefer to discuss a special diagram which will further clarify our idea about capacitance and timing circuit.A switched timer with equal make and equal space periods timing adjustable from over 6 minutes to 38 minutes.

This timer circuit is similar to the 5 to 30 minute timer except that when switch S1 is closed, the on/off action of the circuit will continue indefinitely until S1 is opened again. A 7555 time and low leakage type capacitor for C1 must be used. The 6 way rotary switch S3 adds extra resistance in series to the timing chain with each rotation, minimum resistance point "a" maximum point "f". The 7555 is wired as an equal mark/space ratio oscillator, the timing resistor chain R1 to R6, being connected back to the output of the timer at pin 3.

The output pulse duration is defined as:-T = 1.4 R1 C1This gives on and off times of about 379 seconds for position "a" of S3 (just over 6 minutes), to about 38 minutes at point "f". The times may of course be varied by altering R1 to R6 or C1.ConclusionCapacitance can be defined as the tendency of a capacitor to store energy in an electric field. The formula for calculating capacitance is C=Q/VThe unit of capacitance can be derived from the above formulaCapacitance= Coulombs(Unit of charge) Volts(unit of voltage)Therefore, the derived unit of capacitance is Farad.

In timing circuits when a capacitor is charged through a resistor then the voltage equation is given byV(t) = V0 * e-t/RC.Where V0 is initial voltage Similarly, when a capacitor is charged through a resistor then the voltage equation is given by V(t) = Vf – Vf * e-t/RC.The timing constant of a timing circuit is given by the equationT=RCWhen the value of resistance of capacitance is changed then the time can change from microseconds to weeks. Therefore, this timing constant plays an important role in the timing circuit.

References1. "Capacitance." Capacitance. N.p., 6 Sep 2011. Web. 12 Sep 2011. .2. "Capacitance." Courseworkbank. N.p., n.d. Web. 12 Sep 2011. .