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SCR Control Goes Digital

SCR Control circuits and products have been around for decades. An industry-standard SCR controller typically consists of an SCR firing circuit, a phase-locked loop (PLL) or line synchronization circuit, and a means of controlling the firing angle of the SCR device, typically a 0-5V or 4-20mA analog input. While there are currently several industry-standard SCR control products available that provide these basic analog control functions, Oztek’s OZSCR-1000 is described as a digitally controlled SCR firing and control board. So what can a “Digital” SCR control product possibly bring to the party?

Digital control means different things to different people. The OZSCR-1000 provides several facets of digital SCR control, including digital line synchronization, control law implementation, and product configuration. At its heart, an SCR controller must provide reliable synchronization to the input line voltage. On the OZSCR-1000, line voltage inputs are low pass filtered and immediately converted into digital signals by A/D converters. From that point forward the phase detector and PLL algorithm are implemented digitally, using a combination of microprocessor and high-speed logic resources. This approach ensures reliable, jitter-free operation when compared to similar analog circuit techniques.

Digital control law implementation implies that the closed-loop feedback control algorithms are also implemented within the microcontroller, as opposed to analog circuitry such as op-amps, etc. The OZSCR-1000 SCR control functions are implemented as Proportional plus Integral (PI) algorithms with built-in anti-windup protection. The digital nature of the controllers inherently eliminates the effects of temperature, component tolerances, and large-signal analog effects. However, since they are user-configurable via the serial interface, they also simplify and speed-up initial system design. When developing and testing their controller, the engineer no longer needs to use his or her soldering iron to change their control loop compensation, they simply reconfigure the digital loop gains.

While the OZSCR-1000 can be configured to accept standard analog voltage or 4-20mA current command inputs, the addition of the Modbus-based serial port makes it possible to implement a complete digital control approach. When properly configured, the serial interface can be used to send either open-loop phase commands or closed-loop voltage or current commands.

Digital product configuration can save the system manufacturer significant time and money. The OZSCR-1000 is 100% software configurable over a standard serial port. For example, the controller can be configured for open-loop control or closed-loop control; voltage, current, or serial interface command; phase angle or zero crossing control, all by the simple push of a button. There is no longer a need to swap components to “tune” or configure a circuit, no longer a need to stock multiple variants of the same board to handle different end product configurations, and no longer a need to mix and match different boards to realize your control configuration. All of the circuitry is provided on the OZSCR-1000, it simply requires a one-time configuration step using the Oztek supplied configuration tool.