Auto Heat Limiter For Soldering Iron

auto heat limiter for soldering iron

Usually a soldering iron takes a couple of minutes to get adequately heated up to melt the solder, after which the heat generated is much above the requirement and is wasted. The solder itself also produces toxic smoke which needs to be extracted with the likes of Integrated Air Systems extraction devices. Moreover, excessive heat decreases the life of the bit and the element, causing serious damage to the delicate ICs, capacitors and PCB tracks. With this in mind, it is strongly recommended that you choose the best soldering station from this site or other similar online soldering resources. This way, you can be sure that you are investing in a quality soldering iron that will stand the test of time.

Furthermore, a common solution to most soldering issues is to use a diode in series with the mains and the load, so that the iron gets only half the AC cycle. But this process has one serious drawback, that it slows down the normal rate of heating. Thus, after switching on the power, one has to wait for a long time to start work. This can create a lot of annoyance especially when one is in a hurry.

Auto Heat Limiter For Soldering Iron circuit diagram

R1 220?
R2 82K?
R3 10K?
R4 150K?
C1 100µF 25V
C2 100µF 25V
C3 220µF 16V
D1 IN4007
D2 IN4007
D3 IN4007
D5 IN4007
D6 12V 400mW Zener
T1 SL 100
T2 BC 108
RL1 6V 300? Relay

The circuit described here solves this problem in a simple and inexpensive way and can be used with various types of loads of up to 80 watts. As the mains is switched on, an approximate 15V drop of the positive half cycle across R5 is detected and supplied to T1, which acts as a voltage regulator.

Zener diode D6 together with diode D4 (yellow LED) stabilises the emitter voltage of T1 at 13.2V DC which is then delivered to the relay circuit built around T2 and C3. Capacitors C3 charges through the base-emitter path of T2 and causes the relay to actuate. which in turn allows both the half cycles of the AC mains to flow through diode D3 and R5 to the load to heat it up at a normal rate.

After a certain lapse of time (about 2 minutes preset ), C3 saturates and T2 stops conducting through the relay, thus switching on series diode D2 to allow only half of the AC cycle through the load.

After switching off the system, C3 discharges very slowly through R3 and R4. Before C3 gets completely discharged, if the power is switched on again, C3 takes a shorter time to reach the saturation level, thus switching series diode D2 much earlier than the preset time to prevent double heating of the load.

However, if the circuit is switched on only after a few seconds of switching off, C3 gets no time to discharge and the relay does not actuate at all. MOreover, if the relay circuit fails due any reason and T2 does not conduct, no harm is done to the load because in that case D2 remains in series with it. Thus the circuit offers complete protection to the load.

As stated earlier, the given value of C3 gives a delay of 2 minutes. However, a 1000µF capacitor can also be used to produce a 41/2 minutes delay. R5 maintains a drop of about 15V across itself. So, for use in different load conditions, its value changes as shown in table 1.

wattage of load 10W 18W 25W 35W 65W 80W
Value of R5 (in ohms) 330 180 136 (68+68) 100 56 44 (22+22)
wattage of R5 (in watts) 1 2 2 4 5 6.5

The whole circuit can be mounted on a PCB and fitted in an adapter case (7.6cm *5.1cm * 6.4cm) and used as a mains plug. Since R5 gets heated up during the operation, it should be kept well isolated from other components.

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