Схемы, платы, компоненты

примерно описывают в AN4608 

an4608-…hyristor-scr-for-your-application-stmicroelectro.pdf

All SCRs, presenting an IGT level higher than 1 mA, embed an internal RGK resistor about = 0.6 V divided by IGT

level. An external resistor does not always improve SCR dV/dt withstanding. To get a good result, the external

RGK resistor value has to be lower than the internal RGK resistor. For 15 mA SCR, the internal RGK is about 40 Ω,

so to get an impact, an external resistor at least 40 Ω has to be added. The drawback is that a supplementary

current has to be provided by the control circuit to trigger the SCR, as from the external RGK comes a current

equals to the VGT divided by the RGK (so 32.5 mA for 1.3 V VGT and 40 Ω resistor).