Thermolec Application &Technical Notes - Manual (Page 29)

THREE EASY STEPS
STEP 3 - (continued)
Thermolec strongly recommends the use of SCR proportioning controls for the best results in energy
efficiency and maximum comfort.
When the output of the entire heater is being modulated in this manner, the control system is defined as
"FULL SCR".
When one control stage only is controlled by a SCR while the balance of the heater stages are handled
by ON - OFF control, the system is defined as
"HYBRID or VERNIER".
FULL SCR
SCR is the abbreviation of Silicon Controlled Rectifier, also called current valve because of its ability to
modulate the current supplied to, and consequently the capacity of, the heater. The example below
sketches how a typical SCR modulates the heat output of a 15 KW / 600 V / 1 PH, electric heater.
As long as the temperature controller demands less than full heat, the SCR will switch the heater on and
off repetitively on a very short cycle or period, typically 2 seconds. The net heat output is the product of
the KW capacity of the heater multiplied by the ratio of the "ON" to "ON+OFF" or "period" time.
This ratio is determined by an electronic logic in the SCR according to a signal from a proportioning
thermostat. Enough heat is stored in the electric elements so that there is an almost constant operating
temperature corresponding to the demand.
FULL SCR control is recommended where FINE temperature control is required.
Note: All Thermolec controllers are equipped with a zero-cross thyristor switching system that virtually
eliminates all Electro-magnetic interference (EMI) et radio frequency interference (RFI).
The Light Emitting Diode (LED) on the controller indicates when the heating element is on.
0 V 10 V
Total KW
Heat demand or
Control Input Voltage
Typical control
up to 80 Amps
Full SCR Control Electric Diagram
Full SCR Load Diagram