Introduction to SCR Power Controllers

   Power Delivered to a load may be regulated using  silicon controlled rectifiers in a phase or integral cycle control configuration.  With the former, a transport lag not exceeding 8.3 msec occurs.  With integral cycle control, transport lag, a function of base frequency, is never less than 8.3 msec.

    The significance of the transport lag depends on the load and its thermal time constants.  A lamp could not tolerate a long lag controller, because it has a short thermal time constant and would have objectionable flicker.  A furnace, with its relatively long thermal time constant, could accept either system and respond with similar performance.

    Phase Control provides true continuous regulation of power to a load.  the power is supplied every half cycle in a typical single-phase system, and three times/cycle in three-phase operation.  Because of the frequency at which power is supplied, this control is applicable to loads with short thermal time constants where precise temperature regulation is a prime factor.

    RFI is a function of load parameters.  High inductance gives a low rate of current rise and decreases the amount of RFI generated.

    In Phase-Angle Control, the SCR unit is turned on at a certain phase angle of the AC power supply that provides the correct percentage of power.  Power is regulated by advancing or delaying the point at which the SCR is turned on within each half cycle.  Below is an example of this for 50% power output.

scrpic.bmp (131954 bytes)

Phase angle control provides a very fine resolution of power and is used to control fast responding loads such as tungsten-filament lamps or loads in which the resistance changes as a function of temperature.  Phase-angle control is required if the load is transformer-coupled, inductive, or capacitive.

    Phase-angle controllers are typically more expensive than zero-cross controllers because the phase-angle circuit requires more sophistication than a zero-cross circuit.

Features for Phase Angle Control


Over Current Trip

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Current Limiting


Soft-start ramp


Phase loss, low voltage detection


Time delay for power outage conditions in conjunction w/soft start prevents nuisance fuse blowing


Control Signals 4-20 ma, 0-5VDC, potentiometer
Control Modes Zero Crossover, phase angle
Load Configuration Zero Crossover 2 leg, zero crossover three leg
Phase Angle 3 SCR/3 Diode, 6 SCR (all three leg control) single
 phase zero crossover or phase angle
Line Voltage 120, 208, 240, 380, 415, 440/480, 575/600 single phase or three phase
Load Current 10 to 1000 amps single or 3 phase
Ambient temperature 32 deg. F - 120 deg. F
Cooling Convection cooled (no fans)

The integral cycle (zero crossover) type of control is used for either single phase or three phase resistive loads where switching load voltage in full ON and OFF integral cycles is not objectionable.   The integral cycle zero crossover control switch ON/OFF at zero crossover of the voltage/current waveform thus practically limiting the generation of RFI.  A further disadvantage is that voltage and current meters pulse making load monitoring difficult.

   Since phase control provides continuous voltage to the load (not ON/OFF control), it can also provide a continuous current feedback which can be used for current limiting or current control of the load current.

Adjustable Current Limiter-phase angle control

   Restricts output current to a pre-set adjustable value.  A doughnut type of current transformer is provided on the current limiting circuit.  This is factory set for maximum rated current of the SCR Controller but can be adjusted to limit the current to the load circuit.  This feature is often required for heating applications, D.C. Power Supplies and motor controls.

Soft-start ramp-phase angle control

   Provides a gradual increase of load voltage/current for heating elements which have a low cold resistance or to prevent load inrush (saturation).

The TEMP, INC.  Series of  High Performance Three Phase and Single Phase SCR Power Controllers are completely solid state, utilizing silicon Controlled Rectifiers (Thyristors) and magnetic amplifier firing circuits.  Designed and built to cope  with rugged industrial environments and for precision control of power in a variety of applications including electric furnaces, kilns, temperature chambers, lamp control, DC power supplies, high performance replacements for saturable core reactors, etc. 

Single Phase Connections

Three Phase Connections



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