Capacitance measurement is of immense importance in the field of electronics. An instrument that measures capacitance to accuracy better than 0.5% has been improvised and rendered auto-ranging by means of an 89c51 circuit. The principle is that of a “direct” or fundamental principle of the time constant of the component. For e.g., the time taken to charge a capacitor to 50% of the applied voltage is given by 0.5 V = V (1- exp (-t/τ) )
The value of t is determined as 0.69314718τ. Knowing t by an uninterrupted time measurement on the processor, we can find the value of capacitance.
* It is used for measuring capacitance which are used in almost all circuits. * This is very useful in laboratories.
Capacitance meter Chapter 1
INTRODUCTION TO CAPACITANCE METER
An instrument that measures capacitance to accuracy better than 0.5% has been improvised and rendered auto-ranging by means of an 89c51 circuit. The principle is that of a “direct” or fundamental principle of the time constant of the component. Capacitance measurement finds importance for various applications. The present technique is a non-bridge fundamental time based measurement which is useful for capacitance measurement.
1.2 CIRCUIT DESCRIPTION
The Capacitance meter is based on the basic relation of charging time through a resistor and uses a simple dedicated micro controller 89C51, with built in program for the Job. The “transducer” which measures the capacitance is just the sensing of a bit of information relating to charging the capacitor to half the applied voltage, through a CMOS Gate. The property of a CMOS AND Gate that its threshold voltage is 50% of the supply voltage is applied here. This is a fact which is true for CMOS gates only, and hence any variation of the supply voltage (3 to 15V) for the CMOS gate does not affect the capacitance measurement ,The gate threshold works like a Schmitt trigger whose level is at 0.5Vcc and varies only by a very small percent, thereby providing a measurement accuracy of 0.5%. Also, since the CMOS gate has virtually infinite input impedance, it hardly loads the charging capacitor. This requires only a single bit to be sensed from the charging resistor and capacitor circuit through a CMOS gate IC. There are three charging resistors used for charging the unknown capacitor. First, we charge with the lowest resistor value and if the time taken to reach 50% of the charging supply is too small, then we use the next higher charging resistor. 10K, 100K and 1megohm are the three choices. The use of CMOS switch IC 4066 is useful to select the resistor by switching one or the other resistor to the capacitor. This IC is having four switches. The control signals for closing these switches come from port bits of port 1 on the 89c51 board. So, four control bits are used. Three switches are for range selection, while the fourth one is used for shorting the unknown capacitor to bring it to zero charge to start with. So, this fourth switch is wired across the Capacitor terminals. Then, the voltage of the capacitor is sensed by the high input resistance gate CD4081, which is an AND gate. Here, the AND property is not used, so both pins of the gate are tied together, The property that is used is that a CMOS gate of this type changes output exactly at 50% of the voltage applied to the gate power supply pins 14, 7. Thus, when we charge the capacitor, we get the indication of 50% charge when the gate output goes high. So, another port 1 bit is used to read the logic level there. When that bit gets high logic, at once, we know that the capacitor has got charged to 50% of Vcc. The fundamental relation of charging a C...
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