The A1Z (single Phase 1-leg), A3Z (3-phase, 2-leg) and A6Z (3-phase 3-leg) are proportional, zero voltage switched, single phase power controller capable of delivering 0-99% of the applied line voltage to directly connected electric heaters. 100% power factor. Almost zero harmonics. Can control every half cycle of power to heaters. 50/60 Hz. There are four field replaceable items:
  • Printed circuit board
  • Fuses
  • SCR packages
  • MOV's (Metal Oxide Varistors)
Unit of Measure


Voltage Designations

N/A 380 VAC 50/60 Hz


N/A 250 AMPS

Output Voltage

N/A 0 to 99% of input voltage

Control Signal

N/A 4 to 20 mA at 10 volts DC

Input Impedance

N/A 500 ohms

Power Dissipation (Watts)

N/A 3.6 x maximum current


N/A Convection: 30-80 amps
Fan (120 VAC): 100-1200 amps


N/A 190 ºF (fan cooled units only)

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

N/A 4/0 AWG

Net Weight

N/A 50 Pound

Shipping Weight

N/A 65 Pound

Wire Lug Torque (90 ºC Temp Wire)

N/A 250 lb/in

Lug Size (90 ºC Temp Wire)

N/A 250MCM - 6


N/A No


R Options

N/A -R120 - 120 VAC on-off command signal -R32 - 10 to 32 VDC on-off command signal



  • UL 508 listed, CE pending
  • 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
  • Variable time base provides high resolution control
  • Electrically isolated chassis and heat sinks to 600 Amps
  • 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

    N/A Zero voltage switching controls proportionally turn on and off each full cycle of the power line. By varying the number of AC power line cycles, the SCR provides power to the heaters. With a variable time base, the optimum number of cycles turned on/off is achieved. This method produces less RFI line noise than phase angle fired SCRs. Power factor is 100%, while harmonic distortion is limited. Charts below, left and center show 25% power output using variable time based zero voltage switching. The chart on the right shows 50% output.

    Avatar Instruments variable time base SCR firing. Note how half cycles can be turned on and off providing 0.08 second resolution on 60 Hz power lines.
    The standard input signal (4-20 mA) is applied to the blue input terminal block, processed and fed to optical couplers. The output of each optocoupler controls its respective SCR. An I2T fuse connected in series with the input provides over current protection in the event of external wiring shorts, or too large of a heater load. Additionally, a MOV (metal oxide varistor) is connected in parallel with each SCR, providing voltage spike protection to the controller.