Ballast capacitor selection. Reference data

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Ballast capacitor selection. Reference data

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Among radio amateurs, network power supplies are now very popular, in which a capacitor plays the role of a ballast element. The choice of a capacitor for this purpose has some peculiarities, and they must be taken into account. The main parameter here is the permissible value of the voltage amplitude with a frequency of 50 Hz.

As you know, the rated voltage of a capacitor intended for use in electronic equipment is the value of the highest constant voltage at which the capacitor operates reliably for a specified service life. When a capacitor operates in AC circuits, the amplitude of the AC voltage on it must always be less (in the extreme case, equal to) the rated voltage, and how much less depends entirely on the type of capacitor. So, for example, for a MBGO capacitor for a rated voltage of 630 V, the allowable amplitude of an alternating voltage with a frequency of 50 Hz is 126 V [1], and for an MBGO capacitor for a voltage of 1000 V - 250 V [2].

The reliability of the ballast capacitor of a transformerless power supply can be ensured provided that the value of the AC voltage amplitude acceptable for the capacitor is greater than the mains voltage amplitude. If the effective voltage value is 220 V, then the amplitude will be

The choice of a capacitor begins with the calculation of its capacitance. To do this, use the technique described in [3]. Then the required capacitor rating is determined from reference books. References [1, 2] contain the characteristics of more than three hundred types of capacitors with thousands of capacitance and voltage combinations.

The vastness of the initial information makes it difficult to choose a suitable rating of the ballast capacitor. The table presented here will help to eliminate probable errors and speed up the choice of a ballast capacitor, its rated voltage, as well as the capacitance value closest to the calculated value.

(click to enlarge)

A number of restrictions are imposed on the characteristics indicated in the table.

Firstly, the upper limit of the permissible amplitude of the alternating voltage is chosen to be 630 V. A larger amplitude margin will not add reliability to the block, but will significantly increase the dimensions. Secondly, the lower limit is set for the capacitance - 0,22 uF. The selected capacitive interval satisfies most practical cases. The third limitation is the largest mass of 60 g; it requires no comment.

And finally, fourthly, at least three values of the same type of capacitors are included in the table, corresponding to the first three restrictions.

The table provides a choice of capacitance rating, rated voltage and type of ballast capacitor for a transformerless device, designed for connection to a 200 V, 50 Hz network and for a current consumption of 12 ... 100 mA. The current can be increased by connecting two or three capacitors in parallel.

According to the table, it is possible to compare several options for choosing from capacitors of various types of capacitance close to the calculated one. The comparison is carried out according to the sign and value of the deviation of the capacitance from the calculated value, the margin of electrical strength of the insulation, as well as the mass and volume of capacitors. The volume is calculated from the linear dimensions of the capacitors. The safety factor for electrical strength is equal to the ratio of the voltage amplitude allowed for the capacitor with a frequency of 50 Hz to the amplitude of the mains voltage.

For the types of capacitors listed in the table, some capacitance values were left outside the table. The direction of the search in the directory of missing capacity values in the table is shown with dots.

Due to imposed restrictions, some types of capacitors are not included in the table. Among them are MBGCH, K42-19, K75-10 and K78-2, whose reference characteristics do not correspond to the third and fourth restrictions. You can find a suitable one among these capacitors in the directory, taking into account the first limitation.

Literature

Electrical capacitors and capacitor units. - M.: Energoatomizdat, 1987, p. 495.
Handbook of electrical capacitors. - M.: Radio and communication, 1983, p. 168.
Biryukov S. Calculation of a network power supply with a quenching capacitor. - Radio, 1997, No. 5, p. 48 - 50.

Author: A. Trifonov, St. Petersburg

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