Capacitance is a measure of a capacitor's ability to store charges. Capacitance is measured in farads, after English physicist Michael Faraday, an honorary member of St. Petersburg University.
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What is capacitance?
If you remove a single electric conductor infinitely far away and exclude the influence of charged bodies on each other, the potential of the removed conductor becomes proportional to the charge. But conductors that differ in size do not have the same potential.
The unit of capacitor capacity in the SI is the farad. The coefficient of proportionality is denoted by the letter C, which is the capacitance affected by the size and external structure of the conductor. The material, the phase state of the substance of the electrode do not play a role - the charges are distributed on the surface. Therefore, in the international rules of the GHS, capacitance is not measured in farads, but in centimeters.
A solitary ball with a radius of 9 million km (1400 radius of the Earth) contains 1 farad. A single conductive element holds charges in insufficient quantities for technical applications. According to the technology of the 21st century. capacitors with units higher than 1 farad are created.
A structure of at least 2 electrodes and a separating dielectric is capable of storing the amount of electricity required for electronic circuits. In such a structure positive and negative particles are mutually attracted and hold themselves. The dielectric between the electron-positron pair prevents annihilation. Such a state of charges is called bound.
Previously, to measure electrical quantities, cumbersome equipment was used that was not known for its accuracy. Now, even a novice radio amateur knows how to measure capacitance with a tester.
Capacitor markings
Knowing the characteristics of electronic devices is required for accurate and safe operation.
Determining the capacitor capacity involves measuring the value with an instrument and reading the markings on the case. The labeled values and the measured values are different. This is caused by imperfect manufacturing techniques and operational variation (wear and tear, temperature effects).
The rated capacity and tolerance parameters are indicated on the housing. Household appliances use devices with a deviation of up to 20%. In the space industry, military equipment and in the automation of hazardous objects, a variation of 5-10% in the characteristics is allowed. Operating circuits do not contain tolerance values.
Rated capacitance is coded according to IEC standards, the International Electrotechnical Commission, which unites national standards organizations of 60 countries.
The IEC standard uses notations:
- 3-digit coding. The 2 digits at the beginning is the number of pF, the third is the number of zeros, the 9 at the end is the rating less than 10 pF, the 0 at the front is no more than 1 pF. Code 689 - 6.8 pF, 152 - 1500 pF, 333 - 33000 pF or 33 nF or 0.033 µF. The decimal point in the code is replaced by the letter "R" to make it easier to read. R8=0.8 pF, 2R5 is 2.5 pF.
- 4 digits in the marking. The last one is the number of zeros. The first 3 are the value in pF. 3353 - 335000 pF, 335 nF or 0.335 µF.
- Use of letters in the code. The letter µ is µF, n is nanofarads, p is pF. 34p5 is 34.5 pF, 1µ5 is 1.5 µF.
- Planer ceramic products are coded with the letters A-Z in 2 registers and a number denoting the degree of the number 10. K3 is 2400 pF.
- Electrolytic SMD products are marked in 2 ways: digits - the rated capacitance in pF and next to or in the 2nd line if space - the value of rated voltage; the letter coding voltage and next to 3 digits, 2 define the capacitance, and the last - the number of zeros. A205 means 10 V and 2 μF.
- Surface mount products are marked with a code of letters and numbers: CA7 - 10 μF and 16 V.
- Coding is by color of enclosure.
IEC marking, national designations and brand coding make memorizing codes meaningless. Hardware designers and repair technicians need reference sources.
Calculation with formulas
Calculation of the rated capacitance of a cell is required in 2 cases:
- Designers of electronic equipment calculate the parameter when creating circuits.
- Masters, in the absence of capacitors of suitable capacity and capacity, use a cell calculation to select from available parts.
RC circuits are calculated using the value of impedance - complex resistance (Z). Ra - current loss on heating of circuit members. Ri and Re - take into account the influence of inductance and capacitance of the elements. At the terminals of the resistor in the RC circuit the voltage Up is inversely proportional to Z.
The thermal resistance increases the potential at the load and the reactive resistance decreases it. Capacitor operation at frequencies above resonance, when the reactive component of the complex resistance increases, leads to voltage losses.
The frequency of resonance is inversely proportional to the ability to accumulate charge. From the formula for determining Fp calculate what values of Cc (capacitor capacity) are required for the circuit.
To calculate pulse circuits, the time constant of the circuit is used, which determines the effect of RC on the structure of the pulse. If you know the resistance of the circuit and the charging time of the capacitor, use the time constant formula to calculate the capacitance. The truth of the result is affected by the human factor.
Masters use parallel and series connections of capacitors. The formulas for calculating are the inverse of those for resistors.
A series connection makes the capacitance smaller in the connection of the elements, a parallel circuit adds up the values.
How to measure the capacitance of a capacitor with a multimeter?
When measuring the parameters, the capacitor is first discharged by closing the terminals between them with an insulated screwdriver on the handle. If this is not done, the low-power multimeter will fail.
The answer to the question of how to check the capacitance of a capacitor with a multimeter with the "Cx" mode is as follows:
- Turn on the "Cx" mode and select the measuring limit - 2000 pF to 20 μF in a standard meter;
- Insert the capacitor into the sockets in the instrument or put the probes to the pins of the capacitor and look at the value on the scale of the instrument.
Use an ammeter or multimeter to determine if there is a short circuit or an open circuit inside the case.
The polar capacitor is included in the circuit of the device, taking into account the direction of the current. The electrodes of the product are labeled by the manufacturers. A capacitor designed for voltages of 1-3 V will fail if the reverse current is higher than normal.
Before you measure the characteristics, the polar electrolytic capacitor is unsoldered from the board. Turn on the multimeter in resistance measurement or semiconductor test mode. Place the probes to the electrodes of the polar capacitor - plus to plus, minus to minus. A faulty capacitor will show a smooth increase in resistance. As you charge, the current decreases, the EMF increases and reaches the power supply voltage.
A break in the capacitor will look like an infinite resistance on the multimeter. The device will not respond or the arrow on the analog copy will barely move.
If the element fails, the measured parameter does not correspond to the nominal value in a smaller way, proportional to the magnitude of the breakdown.
If you wonder how to measure the complex or equivalent series resistance (ESR of a capacitor) with a multimeter, it is problematic to do so without a scope. A capacitor exhibits reactive properties at high-frequency current.
Other ways to measure
Capacitor capacitance meter with your own hands are assembled according to the schemes of pulse devices. Sequences of RC circuits with variable resistors create a series of signals at the output of the product with a step change in frequency. To adjust the device use a multimeter, with which the device will be used.
A set of tested capacitors are connected in turn to the design and adjust the accuracy of operation in each subrange.
The capacity meter of polar electrolytic elements with your own hands is schematically implemented and tuned as part of the set-top box without an oscillating circuit. At the output instead of a pulsed voltage is a constant voltage.
In digital capacitance meters the power supply is highly stable. "Floating" parameters of the elements from which the circuit is assembled will give an unacceptable error for the accuracy of measurements.
On logical elements, pulsed alternating current sources are created for ESR measurements.
Inexpensive capacitor capacitance measuring devices, RLC bridge type devices with additional function of checking SMD resistors, network charging and liquid crystal display, themselves the size of a finger. Act as a professional metrology complex. Able to act as a capacitance meter for electrolytic capacitors, both polar and variable.
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