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SCR And Triac Triggering Circuit (LP2997MR)

2017-07-08 04:44  
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This article describes the SCR And Triac Triggering Circuit (LP2997MR). The principle is very simple, very practical. The circuit components can help you understand better grasp this principle. For example, in this circuit, you can go to find and buy these components:LP2997MR.

The control-circuit output current has to be amplified when the gate current required to trigger the device is higher than the control-circuit output current capability. For example, A lot of MCUs feature output pins with a current capability around 30 mA today. With IGT, they can switch Triacs safely up to 15 to 20 mA. Below are two solution if a Triac with a 35 or 50 mA IGT has to be controlled by such an MCU. First, Use several MCU output pins in parallel (the best is to use a separate gate resistor between each output pin and the Triac gate to ensure a good current repartition between each pin). And second, use a bipolar transistor shown in the schematic diagram below.

SCR and Triac triggering circuit with a positive power supply circuit schematic

With the bipolar solution, the only way is use a PNP transistor to keep the current sourced to the gate. To set its drive reference to a stable bias, a PNP transistor has to be used, which is the power supply (Vdd) in this case.

This is another drawback of the positive power supply topology. A PNP transistor has to be used instead of an NPN transistor to amplify the control circuit output current. Than an NPN, a PNP transistor has a lower current gain and higher price.


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