Abstract: A self-limiting electro-thermal actuator, capable of improved response time and continuous energization operation to produce a mechanical output uses plural heaters, e.g. a resistance heater and a positive temperature coefficient (PTC) heater, connected in series electrically. The actuator includes a boiler chamber, a thermally expansible working medium in the boiler chamber, a movable member responsive to expansion of the working medium to provide a mechanical output from the actuator, and a heater assembly for heating the working medium. The heater assembly includes the series-connected resistance electrical heater and PTC material heater. The electrical resistance of the PTC material increases with increasing temperature to limit current flow to the resistance heater. The heater arrangement produces improved response time over use of a PTC heater alone permitting actuators to be employed in applications for which known actuators are too slow.