Relay contact switches prevent voltage drop

Designers commonly prevent damage to a circuit by applying reverse-voltage protection that employs diodes. The problem is that most common methods waste power in the voltage drop across the diodes. One approach, where a series diode allows current to flow only if the correct polarity is applied (Figure 1), wastes 0.7W with an input current of 1A. Another method that uses a diode bridge to rectify the input so that the circuit always receives the correct polarity (Figure 2) wastes 1.4W also with an input current of 1A.



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). This 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, which means 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.