Abstract: An interface circuit for converting high current output signals provided by a test equipment to corresponding low voltage signals for testing digital relays used in power transmission systems. The high current signal outputs range in value between 0.001 and 26 amperes. The interface circuit includes a current sensing resistor for converting the high current output signal provided by the test equipment to a low voltage signal. The low voltage signal is amplified to the appropriate voltage level to correspond to the test equipment current signal output. The gain of the amplifier is adjustable using a potentiometer to allow the interface circuit to be used with different digital relays having different input signal requirements.