March 2010

Doc ID 17193 Rev 1

1/20

AN3168

Application note

Non-insulated SCR / Triac control circuits

Introduction

In alternating current applications the direct current power supply for low-voltage electronic

devices (MCU, LEDs, optocouplers, Triacs and so on) can be provided using one of several

different circuits. There are traditionally two major types of power supplies used in

appliances, capacitive power supply and linear power supply using a step-down transformer.

Today, designers are using more and more switches mode power supplies (SMPS) to

achieve higher output current levels and especially lower standby power consumption. The

power supply choice is a trade-off between several parameters. These are the cost, the

required power, the output voltage level and polarity, the standby power consumption and

the necessity or not of an electrical insulation between the mains and the low output DC

voltage.

This application note considers only non-insulated power supplies. After a brief description

of the triggering quadrants and key parameters for SCR, Triac, ACS and ACST, the usual

control circuits are described according to the output voltage polarity of the power supply.

Finally, some examples of negative power supply circuits are introduced.

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