AC-503 Series Delay-On-Break Timer

The AC-503 Series Delay-On-Break Timer from the ATC Diversified Electronics division of Marsh Bellofram is a family of HVAC controls provides short cycle protection by locking out the compressor for a period of time after a voltage or thermostat interruption. Subsequent interruptions will not increase the delay period. Since the delay begins when the interruption occurs, the temperature control is not affected. Under normal operating conditions, the compressor off time is longer than five (5) minutes. In this case, the use of these timers will not lengthen this off cycle.

This field adjustable model will operate on control voltages of 19 VAC through 288 VAC. Unlike most universal voltage timers, the AC-503 Series Delay-On-Break Timer is a two (2) terminal device that simply connects in series with the control voltage, thermostat and control relay making for easy installation. This timer is engineered to provide true thermostat interruption protection even when there is a continuous current flow through the thermostat’s cooling anticipator.

NOTE: When used on 120/240 VAC control circuits, the external jumper should be cut. This disables the 24 VAC bypass circuit.



  • Short Cycle Protection for Compressor
  • Adjustable delay begins when Interruption occurs
  • True Thermostat Interruption Protection