Introduction to SCR Power Controls, as used in electrical heater applications

A SSR (solid state relay) or SCR (silicon controlled rectifier) is a solid state switching device that can provide fast, infinitely variable proportional control of electric power. This provides the best control of your heat process, and it can extend heater life many times compared to other control methods. Since the SCR is a solid state device, it can be cycled on and off over without any wear and tear resulting in virtually an indefinite life IF the consumer addresses the 3 "over" conditions. That is: avoid over-temperature, over-amperage, and over-voltage conditions. If you fail to plan for the three "over" conditions, the SSR or SCR is guaranteed to fail. Since the damage from the 3 "over" conditions is an irreversible cummulative effect, it is imperative that the installation be designed properly from the first day.
  • Over Temperature occurs since the SSR/SCR is generating internal heat at the rate of approximately 1.1 - 2.2 watts of heat per amp switched (depending upon the vendor's specification). If that heat is not removed, the internal device will achieve a temperature of 125C and will fail. It is critical that the internal heat rise is kept below this failure point. ControlOnLine specializes in SSR / SCR / IGBT die design + ceramic copper bonding techniques + heat sink transfer. By doing this, we minimize the initial thermal rise rate and then effectively transfer that heat in a highly efficient manner to the appropriate heat sink.
  • Over amperage occurs due to exceeding the SSR's amperage rating, when at operating temperature. In general, you should design an application to use no more than 80% of a SSR's rating, when at 40C, in order to anticipate short term fluctuations in the load, supply, or application. As the ambient temperature rises, or if air flow is "restricted or blocked", then the SSR must have the amperage capability derated according to the published graph for that particular model.
  • Over voltage occurs due to voltage surges in an installation. These are typically the result of turning off a nearby inductive load (a motor, pump, coil, or any other inductive source). The resulting surge on the power line is damaging to other products that share that same power line. Over voltage damage is the most difficult problem to troubleshoot so it is to the customer's advantage to anticipate it in advance.

ZERO Crossing Switched controls proportionally turn on and off each full sine wave of the power line. By varying the number of AC power line cycles, the SCR provides power to the heaters. For example: if 33% power is required, when using on a one second cycle time controller (PID controller, or PLC, or similar controller), then 33% = 20 sine waves "on" and 40 sine waves "off", 20 sine waves "on" and 40 sine waves "off" etc. Since modern PID temperature controllers can create an overall cycle time of as little as 0.2 seconds, in this same example, the same SCR would now operate at 3 sine waves "on", 9 sine waves "off", 3 sine waves "on", etc. On small, low mass IR heaters, you might see the elements slightly flickering. On larger heaters or resistive heaters, then you would not see any flickering. By using a short on/off time, the thermal expansion and contraction the heater is reduced and, therefore, the heater life is increased.

PHASE Angle fired turns on a percentage of each power line half sine wave. By delaying the "start" until the sine wave has already crossed the initial zero mark, this method can provide infinitely variable application of power to the load. This method has the draw back of generating electric noise so it is usually used only with loads that require a soft start such as variable resistance heater loads, transformer coupled heater loads, and some IR heater loads.

ON/OFF Control is usually used with mechanical or mercury relays. The load stays on or off for an extended period of time such as 20-60 seconds minimum. This extended time is required to prevent the mechanical or mercury contactor from failing too soon since they have a projected life time from 250,000 - 6 million cycles, when under ideal conditions. Unfortunately to extend the life of the relay, you may decrease the life of the heater. The longer on vs. off cycles cause a dramatic increase in thermal fatigue of the heater so it will fail sooner.

IGBT Control, as used in the customized Power-io designs, is similar to phase angle control. An IGBT turns on at the zero cross mark and turns off later in the same half sine wave. By delaying the "end" until later in the sine wave, this method can provide infinitely variable application of power to the load. The advantage in IGBT technology is that by activating the load at the zero crossing mark, electronic noise is minimized, the load receives the gradually increasing power reducing shock to the load (such as eliminating element "sing" in lamp circuits), and shutdown strategies can by developed to protect the IGBT from failure.

More definitions of relay terminology is found at: www.power-io.com/library/appnotes