Tips on selecting the right SCR controller, or Zero Crossing or Phase Angle?

You need an SCR, there are so many options, now what?

If someone asks me, “what does an SCR do?” and they are not very tech savvy, I will tell them, “An SCR is just like a spigot or faucet. You adjust the valve to get the ideal flow for your need,” SCR’s are that valve, it allows you to adjust the current flow for your application.

It was not too long ago that when it came time to decide how to control your furnace or other load that you simply chose a contactor for the right current and wrote a check, a big check. With that you got a large and heavy item that was nothing more than a fancy switch with some mercury to insure a good contact. Functionally great, but your control with one was very limited and because it was mechanical you had a finite life from it.

Temperatures fluctuated greatly due to the limited resolution and your overall process control suffered. Today we have an option and it is what nearly everyone does. SCR have not only replaced the old mercury contact switch from last century but also given us a precision control with near infinite resolution. This allows us to keep temperature swings to near zero and programmable patterns for special applications using a simple off the shelf controller.



Single and three-phase are the forms of power distribution used. Three phase has 3 “hot” lines and a neutral. Three phase is used in commercial and industrial applications and is more efficient when used with motors. Three-phase has three lines sharing the load allowing higher current capabilities. Three phase is seldom if ever seen in residence and is more difficult to come by as well as being more costly.

Single Phase has one “hot” line and one neutral. This is the type of power distribution seen in residences throughout North America. It is the easiest and also is available everywhere. It is the standard two wire power system.



The current is the amperes needed to power your application at full power plus a safety margin. You do not want your SCR working at 100% output continuously. Your load will only use as much current as it needs, so you will not have to worry about overpowering your elements . We recommend going 50-100% over your highest expected need.
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Type of load Comments Recommended
Constant Resistance LoadsMedium
wave infrared, Most ceramic and Cartridge heaters. Immersion
heaters, Wire/Coil  Heaters including Nichrome, Calrod
Resistance Constant, Less than a 50% change versus temperature. PA Resistance LoadPhase Angle Fired
Variable Resistance Loads:Tungsten
(T-3) lamps and heaters,Quartz IR tubes,Some infraredpanels,       Molybdenum
Fast and precise response neededWhen the elements are cold there is a hi-inrush current upon start up. High hot to cold resistance ratio Tungsten 14:1Molybdenum 20:1Graphite: 20:1 CC Current ControlPhase Angle Fired with Current Control
Silicon-Carbide Heaters and GLO-BAR
Resistance increases as elements age. Very
Hi Temperature applications.
CC Current ControlPhase Angle Fired with Current Control
Inductive Loads:Isolation
Transformers, Motor Loads
Inductive loads can have a high initial
current inrush.
TL Inductive LoadPhase Angle Fired for Inductive loads.
Mercury Relay and SSR replacement. On/Off Control ofConstant resistanceloads. Zero Cross over




This is the voltage of the device that you will be controlling. Select the maximum voltage your application will be using. Ranges are available up to 600VAC


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