FI63505B - KOPPLING FOER AVKAENNING AV BELASTNINGSSTROEMMENS OMKASTNINGSTIDPUNKT VID EN MEDELST TYRISTOR STYRD BELASTNINGSKRETS - Google Patents

Description

1 63505

Switching for sensing the time of reversal of a load current in a load circuit controlled by a thyristor

The present invention relates to a circuit for sensing the time of reversal of a load current in a thyristor-controlled load circuit, in particular in a resonant circuit using a isolation transformer controlled by a secondary pulse.

When controlling a resonant circuit with a thyristor, the resonant frequency of which varies, for example, due to a load, it is often necessary for the control logic to obtain information about the time at which the current is reversed. This information is especially necessary when controlling a resonant circuit in phase with two thyristors.

To measure the phase of the current, it is possible to use a measuring transformer which provides information on the phase of the load current. However, it is often necessary to galvanically separate the thyristor current circuit from the control logic, which results in an expensive transformer structure because the transformer insulation must meet the requirements for mains insulation.

The object of the invention is to provide an even simpler connection in which a separate measuring transformer is not required, but a separation transformer necessary for controlling the thyristor is also used to detect a change in the direction of the current. The invention is characterized in that the feedback for sensing the time of reversal of the load current is taken at a component in series with the primary winding of the isolating transformer, where a voltage change occurs when the direction of the load current changes and a secondary circuit voltage change is first induced.

The current of the primary winding of the isolation transformer can be controlled, for example, by a transistor connected in series with the winding, whereby the collector voltage of said transistor can be detected in order to sense the change of direction of the current.

The invention and other features thereof will be described in more detail below by way of example and with reference to the accompanying drawings, in which Figure 1 shows a circuit diagram according to the invention in part diagrammatically, Figure 2 shows a slightly modified load circuit, and Figure 3 shows voltage and current

When the control voltage of the transistor from the control logic is zero, the transistor T2 is grounded like the transistor and a constant current flows to the winding, which is determined by the resistance and the resistance of the winding.

A positive voltage pulse is applied to the base of the transistor T. ^ at time t ^ (curve ϋβτ1, Fig. 3), whereby the bottom, and T2 and T3 close. When the current in the winding is interrupted, a negative voltage is generated over it (curve UN ^), which keeps the base potential of the resistor division R ^ -R2 auxiliary - * - ^ a transistor T2 below 0.7 V even after it has closed. When the current in the winding is interrupted, a positive voltage (curve UT (,) is induced in the winding N2, which overflows the thyristor T.

3 63505

Since the load on the thyristor T has a resonant circuit, the load current (curve I_) is sinusoidal.

As the current IL changes direction at time t2, the circuit N2 ~ D2 begins to conduct, just as the gate voltage of the thyristor T goes to zero and the thyristor closes. The voltage of the winding N2 is canceled, also causing the voltage of the winding to be canceled. In this case, the transistors T2 and T3 are controlled and grounded, and the collector voltage of the transistor T1 rises to U. This change in voltage transmits information about the change in the direction of the load current to the control logic. The negative voltage induced in the winding N2 then prevents the thyristor T from igniting when the anode voltage becomes positive.

Figure 2 shows a variation of the previous connection, in which a capacitor C and a second diode are connected in series with the diode D2 between the coil N2 and the gate of the thyristor T. (There can be several diodes in a series). This connection makes the gate voltage of the thyristor more negative than before (dashed line on the curve UTG) and thus ensures that the thyristor remains in the closed state at the beginning of the next period (at time t3).

Claims (2)

4 63505 1. Switching on the load current reversal characteristic in a load circuit controlled by a thyristor (T), in particular in a resonant circuit in which a isolation transformer is used to control the thyristor, the voltage pulse of the secondary winding (N2) controls the thyristor (T) and the primary current belongs to an electronic circuit with primary winding current change components (T2-T3 ^, characterized in that the feedback for sensing the time of reversal of the load current is taken with the primary transformer (N ^) of the isolating transformer at the component (T ^) in series where the voltage change occurs when the load current The direction of (1 ^) changes and the voltage change of the secondary circuit is then induced in the primary winding. A circuit according to claim 1, wherein a transistor (T 1) is connected in series with the primary winding, characterized in that the control logic of the circuit is connected to the collector of said transistor (T 3) to sense this voltage.