Simple reverse-polarity-protection circuit has no voltage drop
Aruna Rubasinghe -October 20, 2011
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Common methods of reverse-voltage protection employ diodes to prevent damage to a circuit. In one approach, a series diode allows current to flow only if the correct polarity is applied (Figure 1). You can also use a diode bridge to rectify the input so that your circuit always receives the correct polarity (Figure 2). The drawback of these approaches is that they waste power in the voltage drop across the diodes. With an input current of 1A, the circuit in Figure 1 wastes 0.7W, and the circuit in Figure 2 wastes 1.4W. This Design Idea suggests a simple method that has no voltage drop or wasted power (Figure 3).
Select a relay to operate with the reverse-polarity voltage. For example, use a 12V relay for a 12V supply system. When you apply correct polarity to the circuit, D1 becomes reverse-biased, and the S1 relay remains off. Then connect the input- and output-power lines to the normally connected pins of the relay, so current flows to the end circuit. Diode D1 blocks power to the relay, and the protection circuit dissipates no power.
When you apply incorrect reversed polarity, diode D1 becomes forward-biased, turning on the relay (Figure 4). Turning on the relay cuts the power supply to the end circuit, and red LED D3 turns on, indicating a reverse voltage. The circuit consumes power only if reverse polarity is applied. Unlike FETs or semiconductor switches, relay contact switches have low on-resistance, meaning that they cause no voltage drop between the input supply and the circuit requiring protection. Thus, the design is suitable for systems with tight voltage margins.