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03-09-2011

Voltage Limiter Generates Supplies That Improve Standard Diode Technique

Electronic Design Europe 10.02.11

Yossi Barkan

Some systems require a voltage limiter to protect the input circuit. Designers can connect diodes to VCC and Gnd to create a simple solution (Fig. 1). This simplicity, however, comes at a disadvantage: the input circuit can see voltages within a range of Gnd – VD up to VCC + VD.

A very simple overvoltage protection technique uses diodes connected to VCC and Gnd
Figure 1. A very simple overvoltage protection technique uses diodes connected to VCC and Gnd.

If the circuit is sensitive and can handle only voltages between Gnd and VCC, the circuit in Figure 2 is a better alternative. The improved limiter includes circuitry to generate two voltage power supplies:

VP = VCC – VD

which can sink current, and:

VN = Gnd + VD

which can source current.

The circuit assumes that VD = VBE. In this case, the protection diodes at the input will clamp the input voltage at no more than VCC and no less than Gnd.

 A better limiter generates VP and VN, which can be common to all the protection diodes on the same printed-circuit board
Figure 2. A better limiter generates VP and VN, which can be common to all the protection diodes on the same printed-circuit board.

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