Friday, September 26, 2014

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Soldering Iron Tip Preserver

Although 60/40 solder melts at about 200&degC, the tip temperature of a soldering iron should be at about 370&degC. 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. Thats where this circuit can help - it keeps the soldering tip to just below 200&degC 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. 

Circuit diagram:
soldering-iron-tip-preserver circuit diagram
Soldering Iron Tip Preserver Circuit Diagram

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, Q1s 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 RLY1s 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

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