Diode A diode allows current to flow in onedirection. It’s on or current is flowing,when the anode is positive with respect to the cathode. The SCR, like a diode, can only passcurrent in one direction. The voltagepolarity (anode-to-cathode) must be positive when applying a signal to thegate. Onceon,it latches and will onlyturn off when the cathode becomes more positive then the anode. This happens after passing through zero into the opposite polarity on thealternating current (ac) sine wave.

Application Guide

Alternating Current

Power Controllers

Ref. 94 Ref. 95 Ref. 96Ref. 97 Ref. 98 Ref. 93

Theory of SCR PowerControllers

- Cathode+ AnodeCurrent Flow AC voltage changes polarity accord- ing to the frequency of the current. In North America this is usually 60 times a second. In Europe and many other parts of world, this is 50 times a sec- ond. Polarity changes at zero voltage potential. Circuits which detect this zero point are called “zero cross detectors.” SCR
- Cathode+ AnodeCurrent Flow Back-to-Back SCRs Energizing the gate will turn the SCR on for one half ac cycle. Going through zero will turn it off; correspondingly, the second SCR (facing the opposite direction) must be turned on. Three-phase applications require twoor three pair of back-to-back SCRs,ormay utilize SCRs and diodes. For some three-phase applications, thereis an economy, as well as simplicity, in using three pair of hybrid thyristors.The diodes will conduct only if there isa return path. This return path existswhen SCRs are gated on. Forward Voltage Drop Hybrid (SCR & Diode) An SCR requires a small amount ofvoltage to turn on. Without a load connected, it will never turn on. Each time the SCR is turned fully on, there is a 1.2 volt forward drop on the ac voltage sine wave. This generates heat and produces some electrical noise. 108