A general rule of thumb when driving SCRs is to use as short a cable as possible in order to provide the most efficient turn-on characteristics. As cable length increases, two important parameters are degraded: inductance and series resistance.
The newly introduced OZSCR1000 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.
When controlling power with SCR based designs, engineers are faced with the decision of what type of control mode to use; zero-crossing or phase angle control. In order to make the right decision the design engineer must understand the advantages and disadvantages of both control modes as well as the attributes of his or her load.
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 OZSCR1000 is described as a digitally controlled SCR firing and control board. So what can a "Digital" SCR control product possibly bring to the party?
For our SCR1000 board, we needed to implement a Modbus stack for communicating with the board over RS-485. We have previously ported the FreeMODBUS stack to the Texas Instruments 28335 for use on the OZDSP3000 board, so again we selected FreeMODBUS for use on the Stellaris part that controls the SCR1000.