Accurate capacitor capacity meter

In amateur practice, there is often a requirement to measure capacitors. The described capacitive comparator allows measure capacitors from 100pF to 470nF with accuracy up to 2% (possibly even better). The connection is undemanding and cheap to build. It consists of two integrated circuits (4011 a TCA965) and several components.

Capacitor capacity

Capacitor capacity is the ability of a given component to accumulate electric charge. A capacitor consists of two conductive plates separated by a dielectric. Electric charges of opposite polarity are applied to each of the plates, which are attracted to each other by an electric force. The dielectric between the plates will not allow, for charged particles to come into contact, and thus the electric charge was discharged.

The principle of the capacitor capacity meter

The meter consists of two parts: astable flip-flop and voltage discriminator. Logic unit to zero ratio of flip-flop output signal with CMOS 4011 is determined by the ratio of the capacitances of the capacitors Cx and Cref according to the formula: d = Cx / (Cx + Cref).

This is of course true provided, that the resistance of the resistors in the flip-flop is the same. Therefore, these resistors must be selected with the greatest possible accuracy. An integrated circuit 1M and 100nF is connected to the flip-flop. A unidirectional voltage is then applied to the discriminator.

The magnitude of this DC voltage varies according to the ratio of the capacitances Cx and Cref. There will be another tension, if their capacity is the same, others if the capacity of one is greater or less.

The voltage discriminator is equipped with a TCA965 circuit. This circuit is mainly used as an indicator of FM receiver tuning, but it will also serve us. You can find the catalog sheet in the download section.

The finished meter must be calibrated. We will need two capacitors of the same capacity for this (as accurately as possible, 1% and more precisely). We will place them in the Cx and Cref positions. With the 1k trimmer we set the discriminator as follows, so that the OK LED lights up. 100k / 1k divider connected to the outlet 9 the size of the OK LED lighting range is set. If we want a more accurate meter, reduce resistor 1k. The supply voltage must be stabilized, consumption is minimal (just 9V battery and 78L06 stabilizer).

Measuring capacitor capacity with a comparator

However, this is how we make a capacitive comparator. However, determining the capacity of an unknown capacitor would be time consuming. However, if we replace the Cref capacitor with a calibrated 2x500pF rotary capacitor, we can thus quickly and accurately measure capacitances in the range of 10pF to 1nF. It is necessary to connect a small capacitor in parallel to the Cx terminals in order to balance the initial capacitance of the rotary capacitor and the parasitic capacitances of the terminals and conductors, this capacitor must be at least 100pF.

To measure larger capacities, a capacitance set with switches must be made. We will need three switches with eleven positions and one with five. Precise and stable capacitors will be connected to the eleven-position switches – at first 100, 200, 300,….1000pF; to the second 1,2,3,….10nF; on the third 10, 20, 30,…100nF. To the last capacitors 100 up to 400nF. We will create capacity decades, while in the last, eleventh (resp.piatej) no capacitor will be connected to this position. The decades will be connected in parallel to the Cref terminals.

We will set decades until we find out the capacity of the unknown capacitor, until the OK LED lights up. We start measuring by connecting the largest capacities. If the LOWER C LED is lit continuously, the unknown capacitor will have a capacitance of less than 100nF. The resulting capacity of the unknown is then calculated as the sum of the connected capacitors (e.g.. 30nF + 8nF + 200pF = 38.2nF).