Although 60/40 solder
melts at about 200°C;, the tip temperature of a soldering iron should be
at about 370°C;. This is necessary to make a good quick joint, without
the risk of overheating delicate components because the iron has to be
kept on the joint for too long. Unfortunately, at this temperature, the
tip oxidises rapidly and needs constant cleaning. That’s where this
circuit can help – it keeps the soldering tip to just below 200°C; while
the iron is at rest.
Oxidisation is then negligible and the iron can be brought back up
to soldering temperature in just a few seconds when needed. In addition,
normal soldering operation, where the iron is returned to rest only
momentarily, is unaffected because of the thermal inertia of the iron.
Two 555 timers (IC1 & IC2) form the heart of the circuit. IC1 is
wired as a monostable and provides an initial warm-up time of about 45
seconds to bring the iron up to temperature. At the end of this period,
its pin 3 output switches high and IC2 (which is wired in astable
configuration) switches the iron on – via relay RLY1 – for about one
second in six to maintain the standby temperature.
The presence of the iron in its stand is sensed by electrical
contact between the two and some slight modification of the stand may be
necessary to achieve this. When the iron is at rest, Q1’s base is
pulled low and so Q1 is off. Conversely, when the iron is out of its
stand, Q1 turns on and pulls pins 2 & 6 of IC2 high, to inhibit its
operation. During this time, pin 3 of IC2 is low and so the iron is
continuously powered via RLY1’s normally closed (NC) contacts. Note that
the particular soldering iron that the circuit was designed for has its
own 24V supply transformer. Other irons may need different power supply
arrangements. The warm-up time and standby temperature can be varied by
altering R2 and R5, as necessary.
Author: Alan March – Copyright: Silicon Chip Electronics