Soft load in the power grid. Scheme, description

Soft load in the power grid. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network

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When connecting and disconnecting loads in the electrical network, interference often occurs that disrupts the normal operation of sensitive electronic devices and electrical systems.

The device, the scheme of which is shown in Fig. 1, realizes "soft" connection and disconnection of the load.

Soft load in the power grid
Ris.1

When the contacts of the switch SA1 are closed during the charging of the capacitor C1 (through the resistor R1), the transistor VT1 gradually opens and the collector current gradually increases to a value determined by the ratio of the resistances of the resistors R1 and R2. Accordingly, the current in the load also gradually increases. When turned off, the capacitor discharges through resistor R2 and the base-emitter junction of the transistor. The load current gradually decreases to zero. With the values \u200b\u0,1bof the elements and the load power of 0,5 W indicated in the diagram, the duration of the on-time process is XNUMX s, off - XNUMX s.

The voltage losses in this device are relatively small, they are determined by the sum of the forward drop on the two diodes and the collector-emitter section of the operating transistor, which is approximately:

Uce(B)=0,7+R1*In/h21e

Depending on the load current and current transfer coefficient of the base of the transistor, it is necessary to select the resistor R) in such a way that the voltage drop across the transistor and the power dissipation on it would be maintained at an acceptable level in the on state.

Soft load in the power grid
Ris.2

In the version of the device shown in Fig. 2, protection against overloads and short circuits is provided. When the current exceeds the set value, the voltage drop across the resistor R5 opens the transistor VT2 and its collector junction blocks the transistor VT1. The current at which the protection operates can be determined from the ratio

Imax=0,7/R5.

It should be borne in mind that the power dissipated by the transistor VT1 in the event of a short circuit increases significantly, and therefore additional circuit solutions are needed to prevent its failure.

In the proposed variants of the circuit solution, the transistor VT1 must have a permissible collector-emitter voltage of at least 300 V and a collector power dissipation of at least 10 W.

Literature

Franke M. Netzlasfen "weich" geschaltet. Funkamateur, 1987, No. 12, S.613

Publication: N. Bolshakov, rf.atnn.ru

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