ON/OFF INDICATION AND EN RELIEF SWITCH ELEMENT
A Weller soldering station is in my opinion one of the best purchases for a DIY radio amateur. At government repair facilities, these tools are switched on at the start of the working day at 0800 hours and switched off at 1700 hours at the end of the day and that year in, year out. A solid product, but too much money for what it contains.
RELIEF SWITCH ELEMENT
In 1996 PA1AIS devised a circuit to relieve the switching element. Previously every few years there were problems and he finally got enough of it.
With his modification, a maximum of 11 mA passes through the switching element and the heating element is switched on during the zero crossing of the 24 VAC. It works with a 'power switch zero-cross power-on triac' or 'zero-crossing optotriac' of SHARP type S216S02. The latter has more or less become obsolete, but a replacement can be type CPC1998J of IXYS.
When the switch closes, the 7805 stabilizer is supplied by the rectifier with paralleleled elco. The 7805 is connected as a constant 11 mA current source by the 470 ohm resistor. The current causes the LED to light up so that the triac starts to conduct. Simplicity itself, but to make the circuit work, a three-wire cable is required. That is missing in a WTCP, but is standard with a WTCP-S!
Thanks to its design, PA1AIS has been enjoying his soldering station for more than 22 years and it is expected to last longer.