A simple current regulator for a welding transformer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / welding equipment

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An important design feature of any welding machine is the ability to adjust the operating current. In industrial devices, different methods of current regulation are used: shunting with the help of various types of chokes, changing the magnetic flux due to the mobility of the windings or magnetic shunting, the use of active ballast resistance stores and rheostats. The disadvantages of such an adjustment include the complexity of the design, the bulkiness of the resistances, their strong heating during operation, and inconvenience when switching.

The most optimal option is to make it with taps even when winding the secondary winding and, by switching the number of turns, change the current. However, this method can be used to adjust the current, but not to adjust it over a wide range. In addition, adjusting the current in the secondary circuit of the welding transformer is associated with certain problems. Thus, significant currents pass through the control device, which leads to its bulkiness, and for the secondary circuit it is almost impossible to select such powerful standard switches that they can withstand currents up to 200 A. Another thing is the primary winding circuit, where the currents are five times less.

After a long search through trial and error, the best solution to the problem was found - a well-known thyristor regulator, the circuit of which is shown in Fig. 1.

With the utmost simplicity and availability of the element base, it is easy to manage, does not require settings and has proven itself in work - it works just like a "watch".

Power control occurs with periodic disconnection for a fixed period of time of the primary winding of the welding transformer at each half-cycle of current (Fig. 2). In this case, the average value of the current decreases.

The main elements of the regulator (thyristors) are connected opposite and parallel to each other. They are alternately opened by current pulses generated by transistors VT1, VT2.

When the regulator is connected to the network, both thyristors are closed, capacitors C1 and C2 begin to charge through the variable resistor R7. As soon as the voltage on one of the capacitors reaches the avalanche breakdown voltage of the transistor, the latter opens, and the discharge current of the capacitor connected to it flows through it. Following the transistor, the corresponding thyristor opens, which connects the load to the network. After the beginning of the next, opposite in sign, half-cycle of the alternating current, the thyristor closes, and a new capacitor charging cycle begins, but in reverse polarity. Now the second transistor opens, and the second thyristor reconnects the load to the network.

By changing the resistance of the variable resistor R7, you can control the moment the thyristors are turned on from the beginning to the end of the half-cycle, which in turn leads to a change in the total current in the primary winding of the welding transformer T1. To increase or decrease the adjustment range, you can change the resistance of the variable resistor R7 up or down, respectively.

Transistors VT1, VT2, operating in avalanche mode, and resistors R5, R6 included in their base circuits, can be replaced with dinistors (Fig. 3).

The anodes of the dinistors should be connected to the extreme terminals of the resistor R7, and the cathodes should be connected to the resistors R3 and R4. If the regulator is assembled on dinistors, then it is better to use devices such as KN102A.

As VT1, VT2, old-style transistors such as P416, GT308 have proven themselves well. It is quite possible to replace them with more modern low-power high-frequency ones with similar parameters.

Variable resistor type SP-2, the rest type MLT. Capacitors of the MBM or MVT type for an operating voltage of at least 400 V.

A properly assembled regulator does not require adjustment. It is only necessary to make sure that the transistors are stable in the avalanche mode (or that the dinistors are turned on in a stable way).

Attention! The device has a galvanic connection to the network. All elements, including thyristor heat sinks, must be insulated from the case.

Literature

1. Medvedev A. Yut. From regulator to antenna.
2. Zubal I. Do-it-yourself welding transformer // Radioamator.-2000.-№5.

Authors: S.V.Prus, R.P.Kopchak, Starokonstantinov, Khmelnitsky region.

See other articles Section welding equipment.

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