The A3P is a proportional, infinitely variable power controller capable of delivering 0-97% of the applied line voltage to directly connected electric heaters. It will not control transformers or motors. This is accomplished by phase angle firing three SCR's (one per phase). Modular construction allows for simple and solderless field serviceability. There are four field replaceable items:
  • Printed circuit board
  • Fuses
  • SCR/ diode packages
  • MOV's (Metal Oxide Varistors)
Unit of Measure


Voltage Designations

N/A 380 VAC 50/60 Hz



Output Voltage

N/A 0 to 97% of input voltage

Control Signal

N/A 4 to 20 mA at 6 volts DC

Input Impedance

N/A 300 ohms


N/A < 5% deviation

Tracking (phase to phase)

N/A < 5% deviation (balanced loads)

Response Time

N/A <12 mS (std unit, non soft start)


N/A Convection: 30-80 amps
Fan (120 VAC): 100-500 amps


N/A 190 ºF (fan cooled units only)

Power Dissipation (Watts)

N/A 1.2 x maximum current

Minimum Wire Gauge (Copper Wire Only- 90 ºC Temp Wire)


Net Weight

N/A 14 Pound

Shipping Weight

N/A 15 Pound

Wire Lug Torque (90 ºC Temp Wire)

N/A 45 lb/in

Lug Size (90 ºC Temp Wire)

N/A 1/0 - 14


N/A No


Soft Start1 N/A S05-5 Seconds S10-10 Seconds S20-20 Seconds S02-2 Seconds

Current Limit

N/A CL- Current Limit CLP-Current Limit with Panel Mount Potentiometer

Voltage Limit

N/A VL-Standard 1 Turn PCB VL25-25 Turn PCB



  • UL 508 listed
  • Compact and light weight
  • 100 % solid state circuitry
  • Over-rated SCR power modules
  • Over-sized heat sink
  • Full power ratings up to 50 ºC (122 ºF) ambient temperature
  • Optically isolated 4-20 mA input standard
  • Thermostats standard on fan-cooled units
  • Electrically isolated chassis and heat sinks
  • Gold alodined chassis and heat sink
  • I2T fusing and MOV protection on all power SCR's
  • SanRex, SemiKron, International Rectifier and Fairchild semiconductors
  • Double sided FR4 VO-94 rated glass epoxy PCB's
  • Theory of Operation

    Phase Angle Fired

    These controls proportionally turn on a percentage of each power line half cycle. This gives smooth, infinitely variable application of power to the heaters. Imagine a light dimmer and how it provides power to a light bulb. This method will provide the most precise control of heaters. Phase angle fired SCR’s will respond the fastest to load change and provide maximum heater life. It is the most precise method of control. Phase angle firing can increase heater life up to seven times depending on heater type. Phase angle firing also allows options such as soft start, voltage limit and current limit. These options are not available with any other means of control. Graphs below shows phase angle fired voltage output on a 240 V power line.
    The standard input signal (4-20 mA) is applied to the blue input terminal block, processed and series fed to three optical couplers. The output of each optocoupler controls the drive circuit for its associated phase which is then fed to its respective SCR. I2T fuses connected in series with each phase input provides over current protection in the event of external wiring shorts, and or too large of a heater load. Additionally, a MOV (metal oxide varistor) is connected in parallel with each SCR/ diode, providing voltage spike protection to the controller. Diodes are used in series with the SCR for sound engineering factors: Diodes are stronger than SCRs. Diodes can take higher temperatures than SCRs. Diodes will not misfire.
    • 1 Soft start gets voltage limit option.