Introduction

In some cases appliances and equipment are able to operate when supplied by two different levels of ac line voltage (120 V or 240 V.) This is why it is very common for appliances and equipment to have mechanical selectors or switches as an option for selecting the voltage available. Nevertheless, it is also common that such equipment can suffer extensive damage caused by having the selector in the wrong position. To prevent this kind of problem thyristors can be used to make automatic voltage selectors, avoiding equipment damage due to higher or low ac voltages on the supply.

Thyristors can take many forms, but they have certain things in common. All of them are solid-state switches, which act as open circuits, capable of withstanding the rated voltage. But when they are triggered, thyristors become low-impedance current paths and remain in that condition until the current either stops, or drops below a minimum value called the holding level. A useful application of triacs is as a direct replacement for mechanical selectors, relays or switches. In these roles the triac furnishes on-off control and the power regulating ability of the triac is not used. The control circuitry for these applications is usually very simple and these circuits are useful in applications where simplicity and reliability are important. In addition there is no arcing within a triac, which can be very important in some applications.

The main disadvantages of mechanical switches or selectors appear when they are driving high current levels that can cause arcs and sparks on their contacts each time they are activated or de-activated. Because of these kind of effects the contacts of the switches get significantly damaged causing problems in the functionality of the equipment or appliances.

Definitions

Control Transformers. This transformer consists of two or more windings coupled by a common, or mutual, magnetic field. One of these windings, the primary, is connected to an alternating voltage source. An alternating flux will be produced whose amplitude will depend on the primary voltage and the number of turns in the winding. The mutual flux will link the other winding, the secondary, across which it will induce a voltage whose value will depend on the number of secondary turns. When the primary and secondary turns ratio is properly proportioned almost any desired voltage ratio, or ratio of transformation, can be obtained. This transformer is also widely used in low power electronic and control circuits where it performs such functions as matching the source impedance and its load for maximum power transfer, isolating one circuit from another, or isolating dc current while maintaining ac continuity between two circuits.

The following schematic diagram shows an automatic voltage selector for an ac voltage supply of 110 V/220 V and a load of 10 A rms max. Loads can be equipment or appliances.

click for larger image

When the mains terminals of the equipment are connected to the ac line voltage, one of the comparators (LM339) keeps its output at LOW and the other one at HIGH because of the voltage references connected to their inverting and non-inverting input pins. So, one of the transistors (2N2222) is activated allowing current through the LED of the optocoupler, which triggers one of the triacs (MAC15A8.) That then provides the right level of ac line voltage to the main transformer of the equipment by connecting one of the primary windings through the triggered triac.

The operational range in this circuit, in the lower ac line voltage condition, is from 100 Vrms to 150 Vrms. This means that the triac driving the winding of the main transformer for 110 V would keep itself triggered whenever the input voltage across the control transformer is within that 100 to 150 Vrms range. The operational range in the higher ac line voltage condition is from 180 Vrms to 250 Vrms, with the corresponding triac-trigger range. Another important item to take into consideration is the operational temperature range from 0ýC to 65ýC. If the circuit is working outside of these limits it will probably be unreliable.

In conclusion, this automatic voltage selector provides an important protection for any kind of voltage-sensitive equipment or appliance against incorrect ac line voltages. It eliminates the possibility of circuit damage in the equipment caused by lower or higher mains voltages. The total price of the circuitry is inexpensive compared to equipment damage costs.

Note: A print version of this application note is available from ON Semiconductor, http://www.onsemi.com as publication AND8005.

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