Thyristor Controls

Traditionally, Electric resistance heaters have been simple to control by breaking the heater down into convenient steps and switching relays. The problem with this method is that each step can only control a fraction of the heaters temperature rise. The result is a variation in output temperature as the last step turns on & off, regular maintenance of contactor relays, increased quantity of cables and the need to isolate all of them.

 This led to the demand for a better system….

What Neatafan call Thyristor ‘pulse’ control is the switching of the heater load (in most cases the whole load) on and off every few seconds using modern power semiconductor blocks.

When used in conjunction with mineral insulated sheathed heater elements, this pulse action enables:



As the main supply alternates, the circuit switches the load on and off at the point when the current is zero, eliminating any shock and potential radio interference.

This method can respond to changes in airflow & input temperature without drifting or overshoot, holding the output temperature within + or – 1ºc.

The two-step pulse control system is a method of reducing the size of the pulsed load for larger heaters. Half of the heater load is controlled by the Thyristors and the other half by a suitable contactor or solid state relay. If the demand is for more than half the load, the contactor is on and thyristor pulses to make-up the balance. As the demand falls below 50%, the controller switches off the contactor and at the same time, increased the pulse to again balance the demand. The resulting control accuracy is the same as single step pulse control.

Heaters with built-in pulse control only need one set of power supply cables from the main fuse board and one standard isolator switch.

Neatafan’s Circular and Rectangular Duct Heaters can come with the option of built-in Thyristor Controls.



Neatafan’s Thyristor Controls have several options which can be bought separately:

TC3 – Basic Control

  • Thyristor ‘Pulse’ Control
  • Temperature set point
  • DTFS combined airflow & temperature sensor

TC8 – Full Feature Control

  • Thyristor ‘Pulse’ Control
  • Set point or 0-10v DC input from BMS
  • Fused fan outputs
  • Run on timer
  • DTFS combined airflow & temperature sensor

TH Range – Remote BMS Signal Input

Please see table below for details on the Thyristor Drives available:

240v 1Ph 400v 3Ph (47-63Hz)
TH2-4 2 x TH2-4 TH9402 TH9362 TH9402-2 TH9632-2
 kW (max at 40ºc) 4 8 27 42 54 84
Amps 17 2 x 17 (34) 40/PH 60/PH 2 step 40/PH 2 step 60/PH
Semi-conductor Fuse Rating n/a n/a 80 100
Dissapated Heat Watts 2 W/A 2W/A 2W/A 2W/A 2W/A 2W/A
Forced Vent required (Amps) <13 <2 x 13 25 35 2x 25 2x 35
Height (mm) 95 95 130 210 130 210
Width (mm) 75 75 125 125 125 125
Depth (mm) 65 65 173 173 173 173
Weight (Kg) 0.25 2x 0.25 2.9 4.4
Controller Output No No 14v DC 14v DC 14v DC 14v DC
0-10v DC Max Current mA n/a n/a n/a n/a n/a n/a
Control via 4-20mA Yes Yes Yes Yes Yes Yes
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