Abstract: An electronic circuit for measuring the IR drop free potential of a cathodically protected structure includes a shunting transistor which is periodically pulsed on to shunt or electrically connect the output lines of a rectifier to force the voltage and current delivered to a structure being cathodically protected to zero. The reference cell potential is measured during this zero or sampling time and a minimum value detected by a detector circuit which holds the value until changed in successive sampling periods. The minimum reference cell potential is then compared to a voltage corresponding to a desired reference cell potential and the difference used to generate a firing voltage for SCRs in the rectifier and thereby control its output to achieve the desired level of cathodic protection.