How long does it take a microwave capacitor to discharge

How long does it take a microwave capacitor to discharge

Short-circuiting a charged capacitor carries a high risk of burning out electronic components and other circuits and causing an electric shock or fire. The higher the capacitance and voltage of a capacitor, the greater the damage in the event of a short circuit. Before removing the component from the circuit, the capacitor must be discharged. Find out how to do this.

How do capacitors work?

A capacitor is an arrangement of two electrodes separated by a dielectric in which charges of equal and opposite potential are accumulated. There are many types of capacitors, which can be divided into several subtypes. The simplest consist of two metal plates with a dielectric between them, such as air, ceramic material or impregnated paper. These plates are the capacitors that store the current.

Applying a voltage to the capacitive cap initiates the accumulation of an electrical charge, just as in a battery. When the voltage source is removed, the charge remains on the capacitive plug due to electrostatic attraction. The accumulated charges are equal but have opposite potentials.

The safe discharge of a capacitor is a process similar to that of charging. When a constant voltage U is applied to the terminals of a capacitor with a given capacitance, a value Q, which is the product of the capacitance times the voltage, accumulates on the capacitor. The capacitance of a capacitor is measured in farads. In a capacitor with a capacitance of 1 farad, a charge of 1 coulomb produces 1 volt. Since 1 farad is a very large value, capacitors used in electronics typically have capacitances of picofarads, nanofarads, microfarads and millifarads.

Solid capacitors can be divided into two basic categories: aluminum capacitors and ceramic capacitors. The safe discharge of capacitors depends largely on their design. Polystyrene capacitors are characterized by their high stability and insulation resistance, as well as their relatively low upper operating temperature limit.

Film capacitors consist of three layers of film in an electrode sheath, which are wound and inserted into a suitable box. They are widely used in the electrical and electronic circuits of various household and electronic appliances. An example of this type of capacitor is the FKP2D021001I00HSSD type from WIMA.

Some of the most common capacitors used in integrated circuits are ceramic capacitors, which consist of ceramic plates with applied metal electrodes, such as the CC-10/100 type from SR PASSIVES. To discharge, it is necessary to use a high resistance receiver.

Capacitor parameters

To know how to discharge a capacitor, it is necessary to know the characteristic parameters of this electrical component. The basic parameters of a capacitor are its rated capacitance, capacitance tolerance, rated voltage and dielectric loss.

In addition, capacitors are characterized by their permissible AC voltage, insulation resistance, temperature capacitance factor, climatic category and size, as well as their permissible impulse load, power rating or cut-off frequency.

Capacitance is the most important parameter to consider when planning the safe discharge of a capacitor. It is the capacity of a capacitor to store charge and is directly proportional to the product of the dielectric permeability and the area of the electrodes and inversely proportional to the distance between the electrodes (dielectric thickness).

The capacitance of a capacitor indicated by the manufacturer is the nominal capacitance, which cannot be achieved in practice because various environmental factors can affect the capacitance value. For this reason, the capacitor capacitance tolerance is expressed as a percentage, which indicates the percentage deviation of the actual capacitance value from the rated capacitance.

Capacitor losses are the power losses associated with the operation of a capacitor at AC voltage and are characterized by the tangent of the loss angle. These losses are usually higher than dielectric losses, which are related to the occurrence of electrode losses, but also to the frequency and temperature acting on the capacitor system.

How are capacitors discharged?

The discharge of a capacitor depends on its type and capacitance. Capacitors with a high Faraday number must be discharged more carefully, as a short circuit can not only damage the capacitor, but also cause an explosion and electric shock.

Safe discharge of capacitors is limited to connecting any resistive load to the ends of the capacitor so that it can dissipate the energy stored in the capacitor. For example, how do you discharge a 100 V capacitor? An ordinary resistor or a 110V light bulb can be used for this purpose. By releasing its energy, the capacitor causes the bulb to glow and the light source shows the charge state of the component. Of course, another resistor receiver can also be used for this purpose.

The discharge of the capacitor must take place in a high impedance receiver. The charge accumulated in the cartridge takes longer to discharge, but we will ensure that it is completely discharged.

Discharging smaller capacitors can also be done by preparing a special discharge circuit consisting of a capacitor and a resistor connected in series. When preparing such a circuit, the discharge time of the capacitor and the required power of the resistor must be taken into account.

The capacitor discharge time is equal to the product of the value of the resistor in series with the capacitor and the capacitance. After this time, the component voltage should drop to one third of the initial voltage and the complete discharge should occur in a time equal to five times the product of the resistance and capacitance.

The smaller the resistance connected to the circuit, the faster the capacitor will discharge. For example, for a 10uF capacitor discharged with a 1kΩ resistor, the discharge time is 0.01s. When discharged with a 1mF component resistor, the discharge time increases from 1/3 of the initial charge to 1s.

It should be noted that safe discharge of a capacitor must be through a properly selected resistor. Resistors with too small a value can be damaged. Therefore, when selecting a resistor it is important to consider its power, which is equal to the quotient of its voltage and the square of its resistance. Standard resistors can withstand powers up to 0.25W. Using these types of resistors and larger capacitors with high loads and voltages can cause the resistors to burn out. Therefore, for small components it is recommended to use resistors with a power rating of 5W and a resistance of 1kΩ, such as the MOF5WS-1K type from SR PASSIVES.

Larger capacitors used for power supply operation should be equipped with resistors to discharge the components within a few minutes after disconnecting the power supply. Safe discharge of three-phase power capacitors should be performed with 4mm2 YDY cable and with a protective conductor to short-circuit the individual phases of the element.