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How to safely discharge a capacitor?


Short circuit of a charged capacitor entails a great risk of burning out the electronic component and other circuit elements. It also poses a danger of electrocution and fire. The greater the capacitance and voltage of the capacitor, the greater the damage caused in the event of a short circuit. Always remember to discharge the capacitor before removing it from the circuit. See how to do it.

In this article you will find out:

How does a capacitor work?

Capacitors are a system of two electrodes separated by dielectric material, in which electric charges of the same value and opposite potentials are accumulated. There are many types of capacitors that can be divided into several subtypes. The simplest of them are made of two metal elements, between which dielectric material is placed – e.g. air, ceramic material or impregnated paper. These metal elements are called plates and are used to store electrical energy.

Voltage supply to the capacitor plates begins the process of electricity accumulation – just like in the case of battery cells. When the voltage source is disconnected due to electrostatic attraction, the electrical charge remains on the plates of the capacitor. The accumulated charges are of equal value but have opposite potentials.

Safe discharge of the capacitor is a process that is similar to charging the capacitor. When DC voltage (U) is applied to capacitor terminals with a specific capacity, charge (Q) is stored in the capacitor, which is the product of the capacitance and the voltage. The capacitance is measured in farads. In a capacitor with a capacitance of 1 farad, a charge of 1 coulomb generates 1 volt. Due to the fact that 1 farad is a very high value, capacitors used in electronics and electrical engineering are usually characterized by capacitances measured in picofarads, nanofarads, microfarads and millifarads.

Solid capacitors can be divided into two basic subcategories: film and ceramic capacitors. Safe discharge of a capacitor largely depends on its design. Polystyrene capacitors are characterized by high stability and insulation resistance, as well as a relatively low upper operating temperature limit.

Foil capacitors are made of three-layer foil in an electrode-dielectric-electrode arrangement, which is then rolled up and placed in a suitable housing. They are quite often used in electrical and electronic circuits in various types of household appliances and audio/video devices. An example of such capacitors is the WIMA FKP2D021001I00HSSD model.

One of the most common types of capacitors in integrated circuits are ceramic capacitors made of ceramic plates with metal electrodes, such as the SR PASSIVES CC-10/100 model. It is recommended to use a high resistance receiver to discharge them.

Capacitor parameters

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

In addition, the capacitor is characterised by: permissible AC voltage, insulation resistance, temperature coefficient of capacitance, climate class and dimensions, as well as pulse load capability, rated power and cut-off frequency.

Capacitance is the most important parameter to consider when planning safe discharge of a capacitor. It is the ability of a capacitor to accumulate a charge and it is proportional to the product of the dielectric permeability and the surface of the electrodes and inversely proportional to the distance between the electrodes (dielectric thickness).

The capacitance of the capacitor specified by the manufacturer is a nominal capacitance that is practically impossible to achieve – the value of the capacitance may be affected by many environmental factors. For this reason, a percentage tolerance of the capacitance is given, i.e. the percentage deviation of the actual capacitance from the rated value.

The lossiness of a capacitor determines the loss of energy associated with the operation of the capacitor under alternating voltage, which is characterized by a loss tangent. These losses are usually greater than dielectric losses, which is related to the occurrence of losses on the electrodes, as well as to the frequency and temperature that affects the capacitor circuit.

How to discharge a capacitor?

Capacitor discharge depends on the type and capacitance of the capacitor. Capacitors with more than one farad should be discharged with greater care as their short circuit may cause not only damage to the capacitor but also explosion and electric shock.

Safe discharge of a capacitor boils down to connecting to its terminals of any resistance load that will be able to dissipate the energy stored in the capacitor. For example: how do I discharge a 100 V capacitor? A standard resistor or a 110 V light bulb can be used for this purpose. The capacitor will illuminate the bulb by transferring its energy and the light source will also indicate the level of charge in the component. Of course, you can also use a different resistive receiver.

For discharging the capacitor, a high resistance receiver should be used. It will take longer to discharge the charge stored in the plates, but the plates will surely be fully discharged.

A capacitor with a smaller capacitance can also be discharged by preparing a special discharging system consisting of a serially connected capacitor and a resistor. When designing such a system, pay attention to the discharge time of the capacitor and the required power of the resistor.

The capacitor discharge time is equal to the product of the resistance which is serially connected to the capacitor and of the capacitance. After this time the voltage of the element should drop to one third of the initial voltage, and its complete discharge should take place in a time equal to five times the product of the resistance and the capacitance.

The smaller the resistor is, the faster the capacitor will discharge. For example: in the case of discharging a 10 uF capacitor with the use of a 1 kΩ resistor, the discharge time will be 0.01 seconds. In the case of discharging of a 1 mF component using the same resistor, the discharge time of 1/3 of the initial value of the charge will be extended to 1 s.

Remember that the safe discharge of the capacitor must be carried out by means of suitable resistance. Using an underrated resistor may lead to its damage. Therefore, when selecting a resistor, take into account the power emitted by the resistor, which is equal to the quotient of the square root of its voltage and resistance. Standard resistors can transmit power of up to 0.25 W. The use of such a resistor with a larger capacitor with a large charge and voltage will result in burning it out. Therefore, in the case of small components, it is worth using a resistor with a power of 5 W and a resistance of e.g. 1 kΩ, such as SR PASSIVES MOF5WS-1K.

Larger capacitors for electrical power applications should be equipped with discharge resistors, which after disconnecting the power supply discharge this element within a few minutes.Safe discharge of a three-phase power capacitor should be carried out using a 4 mm2 YDY cable and consist in short-circuiting the individual phases of the element with a PE wire.

Symbol: Description:
FKP2-10N/100 Capacitor: polypropylene; 10nF; 5mm; ±10%; 6.5x8x7.2mm; 1kV/μs
CC-10/100 Capacitor: ceramic; 10pF; 100V; C0G; THT; 5mm
MOF5WS-1K Resistor: metal oxide; THT; 1kΩ; 5W; ±5%; Ø6x17mm; axial

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