June, 2012 − Rev. 3

1 Publication Order Number:

AND8005/D

Automatic AC Line Voltage

Selector

INTRODUCTION

In some cases, appliances and equipment are able to

operate when supplied by two different levels of AC line

voltage to their main terminals (120 V or 240 V). This is

why, it is very common that appliances and equipment have

mechanical selectors or switches as an option for selecting

the level of voltage needed. Nevertheless, it is also very

common that these types of equipment can suffer extensive

damage caused for not putting the selector in the right

position. To prevent these kind of problems, thyristors can

be used as a solution for making automatic voltage

selectors in order to avoid possibilities of equipment

damage due to over or low voltages AC line supplied to

them. 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 until triggered. When they

are triggered, thyristors become low impedance current

paths and remain in that condition (i.e. conduction) until the

current either stops or drops below a minimum value called

the holding level. A useful application of triacs is a direct

replacement for mechanical selectors, relays or switches. In

this application, the triac furnishes on−off control and the

power regulating ability of the triac is not utilized. 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, as is

well known, there is no arcing with the triac, which can also

be very important in some applications.

The main disadvantages of the mechanical switches or

selectors appear when they are driving high current levels

that can cause arcing 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 very 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 number of turns. The mutual flux will

link the other winding, the secondary, in which it will

induce a voltage whose value will depend on the number of

secondary turns. When the numbers of primary and

secondary turns are 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. There it performs such

functions as matching the source impedance and its load for

maximum power transfer, isolating one circuit from

another, or isolating direct current while maintaining AC

continuity between two circuits.

APPLICATION NOTE

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