Capacitance Meter using AVR microcontroller
Digital Capacitance Meter
This is a simple capacitance meter which can measure capacitance value easy. There are some measurement methods for capacitance, at one time the capacitance was measured with a impedance bridge or a dip meter. Recently typical capacitance meters can measure capacitance and some additional characteristics from current vector by applying AC voltage to the Cx. Some simple capacitance meter use integration method that measureing transient response of the R-C network. There are some construction kits based on this method.
This project uses the integration method. There is an advantage that the resulut can be got as a digital data directly because it bases measurement of time, accurate analog circuit is not required and its calibration can be done easy by using a microcontroller. Therefor the integration method is suitable for hand built capacitance meter with high realizability.
The phenomenon appers until state of the circuit changes steady-state after state change, is called Transient. It is one of the fundamental operations of pulse circuit. When the switch in Figure 1a is opend, the capacitor C will be charged through the register R and voltage Vc will vary like shown in Figure 1b. To change state of the circuit, changing the value of EMF E instead can also be thought that equivalent. The relation between past time t and voltage VC is expressed in following formure.
Each units are: t seconds, R ohms, C farad and epsilon is a Napier’s number (approx. 2.72). When VC reaches VC1, the time t1 can be expressed in following formure.
This means that the t1 is proportional to C. Thus the capacitance can be calcurated from charge time and any other fixed parameters.
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