This circuit was found in the December 1992 Radio Electronis magazine. R2 is an NTC, there is a MOV protector or NTC thermistor. The current depends on the thermistor used.
This circuit, found in a 1994 documentation, uses 20 A 2N5448 triacs or equivalent and should be...
This traditional amplifier configuration is from the National Semiconductor's Linear Applications Handbook...
This circuit is from GE technical documentation from the 1970s. The circuit can be implemented with SCRs...