Testing appartus and method for testing a regulator for switching current to an external load device in response to applied voltage, e.g., field current of an alternator. This tester is arranged to enable accurate determination of regulated voltages and operates to apply load circuitry to simulate in a closed loop mode of operation the operation of the regulator in its operating environment. A power delay circuitry is connected across the terminals of the regulator and supplies load current to a third terminal when the regulator is turned on by charging a capacitor in the delay circuitry. When the charge on the capacitor reaches a high limit, the regulator switches off and load circuitry is switched on to ramp the capacitor voltage downwardly until the regulator again switches on. The load circuitry at that time switches off. An auxiliary power supply is effective when the regulator is switched on to supply additional load current.