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SCR Driver Board Firing Pulses

The newly introduced OZSCR-1000 SCR firing board is capable of controlling a wide range of commercially available SCRs.  SCRs operate similarly to diodes, with current flowing in only one direction through the device.  In both SCRs and diodes, current cannot pass through the device until the anode is at a higher potential than the cathode, or what is referred to as forward bias.  SCRs add additional control by delaying the flow of current until a control signal is also provided to enable a forward-biased device.  Current will continue to flow in an SCR until the device is reverse biased.  Once an SCR turns off, it will not conduct current again until it has forward bias applied and the control signal is applied.

The signal required to control an SCR is a current applied between the SCR’s gate terminal and cathode.  Since the cathode connection of an SCR is often connected to the output of a power system, the voltage at this terminal can vary by hundreds or thousands of volts with respect to other circuitry in the system.  This often necessitates high voltage isolation between the SCR signals and other circuitry.  The Oztek SCR driver board is designed for continuous operation with voltages up to 1000Vac.  Signal transformers provide a reliable and cost-effective way to isolate the control signals.

A wide variety of loads can be connected to SCR-based power stages.  These loads can vary from purely resistive to purely inductive, with unlimited combinations in between.  A resistive load will draw current directly in phase with the applied voltage. The current in an inductive load can lag the voltage by up to 90°.  In a 60Hz system, this variation causes a window of longer than 4mS when the control signal must be present to guarantee the SCR will turn on.  A large transformer would be required to sustain a control signal of this duration.

To keep the size of the SCR driver board small and to allow several SCR firing circuits to be included on a single board, smaller pulse transformers are used.  Instead of providing a single long firing pulse to turn the SCR on, several shorter pulses are output in rapid succession.  When viewed on an oscilloscope this pulse train resembles a “picket fence.”

Picket Fence Firing Pulses

A larger magnitude pulse is produced at the beginning of each firing sequence.  This larger pulse helps to ensure a complete turn-on of the SCR, minimizing stress on the device and reducing losses.  The smaller sustaining pulses are sufficient to ensure the SCR stays in full conduction under light or discontinuous loads.  Due to phase lag in systems with inductive loading, it is possible for the initial large magnitude pulse to occur while the SCR is still reverse biased.  If this occurs, the sustaining pulses will turn on the SCR immediately after the device becomes forward biased.  Under these conditions, the losses in the SCR are still minimized because the current at turn on is just beginning to ramp up.

The OZSCR-1000 SCR firing and control board provides several user-configurable parameters to control all aspects of the firing pulses.  Their duration, quantity, and frequency can all be tuned to optimize system performance.  The default configuration should be adequate in many systems as most commercially available devices have been designed to operate with similar control signals.