Get tips on when to select phase-angle vs. zero-cross, understand the advantages of SCR controllers vs. mechanical contactors, and more. Here are 10 questions and answers about power controls.

  1. How does an SCR power controller work?
    Phase-angle controllers work like a light dimmer. The load voltage is varied by varying the time within each electrical half-cycle during which voltage is applied to the load. By contrast, zero-cross is analogous to a very fast contactor that “makes” only at the beginning of each electrical half-cycle and always “breaks” at the end of the electrical half-cycle.

  2. When should phase-angle be used and when should zero-cross be used?
    Phase-angle can be used to control all loads. Phase-angle must be used to power transformer-coupled loads and fast heating loads like lamps. By comparison, zero-cross has the advantages of being lower in price, slightly more reliable and generating substantially less harmonics than phase-angle controllers.

  3. What are the advantages of SCR controllers vs. mechanical contactors?
    • Infinite resolution.
    • No inherent wear out modes.
    • Often less expensive.
    • Voltage compensation, current limiting and overcurrent trip features often are designed into the SCR controller.

  4. How should a controller be fused?
    It is recommended that a class “T” fuse be used to protect the SCR; it provides a balanced compromise between controller protection and premature fuse failure.

  5. How much heat does the SCR controller generate?
    Approximately 1.2 W per amp switched per power line controlled by an SCR.

  6. What command signals will the SCR controllers operate on?
    SCR controllers operate on all of the commonly available command signals of the various temperature controllers and PLC outputs. These include analog signals, 4 to 20 mA, 0 to 5 VDC, 0 to 10 VDC, potentiometer and on/off, 3 to 32 VDC, 0 to 24 VDC, 0 to 120 VAC or 0 to 240 VAC. Other command signals can be ordered to match with older control signals if needed.

  7. Can I put parallel loads on one controller?
    In most cases, parallel loads can be placed on a single controller, provided that the total load current is not more than the SCR controller is rated to handle.

  8. What size of fuse should I use on a 30 A single-phase load?
    Size the fuses to be 25 percent over the full load current. For the example cited (a 30 A single-phase load), fuse the SCR controller at 40 A with a very fast fuse, “Type T” or faster.

  9. Do controllers generate harmonics on the electrical lines?
    The controllers do generate harmonics on the power lines. A zero-cross controller generates most of its harmonics below 50/60 cycles. Phase-angle controllers generate harmonics above 50/60 cycles, with the largest distortions at third, fifth and seventh harmonics for single-phase controllers. For three-phase, phase-angle controllers, the third, and multiples of the third, are not created.

  10. What are the effects of harmonics and should I be concerned?
    Harmonics have the potential to cause inductive equipment such as transformers and motors to overheat. However, actual occurrences of this problem from harmonics are essentially non-existent.

For more information, contact Control Concepts Inc., Chanhassen, Minn., a manufacturer of SCR power controllers, by calling (952) 474-6200 or visiting