There are many factors to consider when choosing a power control device. The role of the output device is to receive a signal from the temperature controller and switch electric current ON and OFF to the heater. When possible, it is convenient to have the output device incorporated into the temperature controller.

Let’s take a moment to explore the different types of switching devices and their advantages and disadvantages:

Electromechanical Relays – an Electromechanical Relay is a mechanical device with moving parts. When power is called for, the control output supplies current to the relay’s wire coil, creating a magnetic field, thus pulling the upper metal contact of the relay down closing the circuit and allowing the load current to flow through the contacts to the heater. Conversely, when the control output cuts off the current signal, the magnetic field is lost and the relay contact opens cutting off current to the heater. The above scenario is for a “normally open” (N.O.) relay. These types of relays are typical for switching heaters and cooling equipment. A “normally closed” (N.C.) relay would be used for alarm outputs. They operate in the opposite fashion where the contacts are closed when no current is supplied and become open once current is applied.


  • Inexpensive and rugged
  • Create positive circuit break (no current flowing when contacts are “open”)
  • Usually small enough to mount inside the controller


  • Moving mechanical parts
  • Not recommended for use with PID control
  • Shorter heater life due to acceleration of element wire oxidation
  • Electric “noise” created by electrical arcing
  • Long cycle time required (typically 10-30 seconds)
  • Eventual failure of contacts due to physical wear and burning
  • Lower cycle life (typically ~ 100,000 cycles)

Mercury Displacement Relays (MDR) – an MDR uses a metal plunger floating on top of a small pool of mercury (non-energized). In similar fashion to the Electromechanical Relay, when power is called for, the control output supplies current to the relay’s wire coil, creating a magnetic field, pulling the plunger downward, closing the circuit, and allowing current to flow. The load current then flows through the electrodes and mercury to the heater. Once the control output is cut off, the magnetic field collapses and the plunger floats to the top, breaking the circuit.


  • Less expensive than other methods
  • No contacts to wear out
  • Longer cycle life (typically ~ 10,000,000 cycles)
  • Reduced cycle time (usually ~ 4-5 seconds)


  • Moving parts which create electrical noise
  • Presence of mercury
  • Shorter heater life due to acceleration of element wire oxidation

Solid State Relays (SSR) – An SSR, unlike an Electromechanical Relay or Mercury Displacement Relay, has no moving parts. An SSR is a semiconductor “chip” made of a silicon-based material. When energized by the control output, the chip conducts load current to the heater. The SSR is essentially an electrical imitation of the physical switching performed by an electromechanical relay. Since the SSR has no moving parts, it will almost never experience mechanical breakdown and it creates no electrical noise due to the fact that the SSR only switches the heater ON and OFF when the AC voltage value reaches zero. This is called zero-cross switching. Most SSRs do require a heat sink in order to dissipate the heat generated by the flow of current through the SSR. They also have no positive circuit break meaning that even when the SSR is in the OFF state, a minimal amount of electric current still leaks through. Lastly, the SSR requires a special fuse. These I2t (pronounce “eye-squared-tee”) fuses blow very quickly (less than one millisecond) to avoid a short circuit condition in the SSR.


  • No moving parts
  • No electrical noise
  • Short cycle time (typically ~ 0.5 – 1 second)
  • Allows the use of PID control
  • Increases heater life


  • More expensive than Electromechanical Relays or MDRs
  • Require the use of a heat sink
  • Require special fuses
  • No positive circuit break

Silicon Controlled Rectifier (SCR) – An SCR is a semiconductor diode. As diodes only allow current to flow in one direction, a second SCR is placed back-to-back in parallel. This results in the full AC sine wave being sent to the heater, allowing full use of the available power. SCRs allow the heater to be switched ON and OFF very rapidly and can greatly increase the life of the heater. They also greatly reduce or eliminate overshoot and droop. SCRs will create some electrical noise, however, most if not all can be eliminated by Zero Cross Switching as discussed with SSRs. By switching the SCR ON and OFF at the lowest possible voltage, you can greatly reduce or eliminate the electrical noise. SCRs can be fired in a variety of ways including contactor firing, burst firing (either fixed or variable time based), and phase-angle firing. Each method has its advantages and since each application is unique, the right choice must be made for SCR control.


  • Allows for PID Control
  • Little to no electrical noise (zero-cross)
  • Extremely fast switching (increases heater life)
  • Minimizes or eliminates overshoot and droop


  • Higher initial cost
  • Requires the use of a heat sink and special fuses
  • No positive circuit break