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Building a Soldering Iron Controller



Soldering irons are available in many power ranges. The smallest sizes, around 15 watts, are recommended for very small and precise work, such as surface-mount-component (SMC) work . Medium s izes, about 30 watts, are recommended for most general electronic work, including PC board soldering. Larger sizes, from 60 watts and up, are for large soldering jobs, such as making solder connections to large bus bars or stud-mount diodes. If you run into situations where you need to do a variety of different types of soldering, there are several solutions. The obvious solution is to buy several different types of soldering irons. Another solution is to buy a soldering station, with an automatic tip temperature regulator (starting at about $100.00). A good middle-of-the-road solution is to buy a 60-watt soldering iron, and use it with the circuit illustrated in Fig. 9-5. In addition to being a useful and convenient tool, this circuit will help illustrate most of what has just been discussed concerning thyristors and power control.Referring to Fig. 9-5, the incoming 120-volt AC power is applied across the triac through the load (lightbulb and soldering iron, in parallel). Assuming SW1 is turned on at the instantaneous point in time that the AC voltage is at zero, the triac is off (nonconductive). As the AC voltage begins to increase through a half-cycle (the polarity is irrelevant because the diac and triac are both bilateral in operation), all of the AC voltage is dropped across the triac because it looks like an open switch. Similarly, the same voltage is dropped across the firing circuit, or trigger circuit (P1, the diac, and C1), because it is in parallel with the triac.



C1 will begin to charge at a rate relative to the setting of P1. As the AC half-cycle continues, C1 will eventually charge to the specified break over voltage of the diac, causing the diac to avalanche, and a current pulse (trigger) to flow through the gate and M1 terminals of the triac. This trigger pulse causes the triac to turn on (much like a closed switch), resulting in the remainder of the AC half-cycle being applied to the load (lamp and soldering iron). When the AC power has completed the half-cycle and approaches zero voltage (prior to changing polarity), the current flow through the triac drops below the holding current and the triac returns to a nonconductive state. This entire process continues to repeat with each half-cycle of the incoming AC power. There are several important points to understand about the operation of this circuit. The diac will reach its breakover voltage and trigger the triac at the same relative point during each half-cycle of the AC waveform. This relative point will depend on the charge rate of C1, which is controlled by the setting of P1. In effect, the setting of P1 controls the average power delivered to the load. P1 can control the majority of the AC half-cycle because C1 also introduces a voltage-lagging phase shift. Without the phase shift, control would be lost after the peak of the AC power cycle was reached. Throughout the entire power control range of this circuit, the power wasted by the triac is negligible, compared to the power delivered to the load.PL1 is a standard 120-volt AC three-prong plug. If you build this project in an aluminum project box, the ground prong (round prong) should be connected to the aluminum box (the chassis in this case). For safety reasons, the 120-volt AC hot lead should also be fuse-protected. P1 is mounted to the front panel of the project box for easy access. I used a flat, rectangular aluminum project box large enough to set the soldering iron holder on. I also connected the soldering iron internally to a phenolic solder strip with a strain relief to protect the cord. This, of course, is a matter of opinion. You might want to wire the circuit to a standard 120-volt AC socket for use with a variety of soldering irons. The triac, diac, and C1 can be assembled on a small universal perfboard or wired to a phenolic solder strip.The 15- to 25-watt lamp is a standard 120-volt AC incandescent light bulb of any style or design you like (it might also be any wattage you desire, up to 60 watts). It is mounted on the outside front panel of the project box and serves several useful indicator functions. First, it indicates that the power is on and that the circuit is functioning. Second, with a little practice, the brightness of the bulb is a good indicator of about how much power you are applying to the soldering iron. For example, if you’re using this circuit with a 60-watt soldering iron, and you adjust P1 until the bulb is about half as bright as normal, you’re supplying about 30 watts of power to the tip. Third, the light bulb makes a good reminder to turn off the soldering iron when you’re finished working. (I can’t count how many times I have come into my shop and found the soldering iron still turned on from the previous day.) The best value for C1 will probably be about 0.1uF. After building the circuit so that it can be tested using the light bulb as the load, try a few different values for C1, and choose the one giving the smoothest operation throughout the entire power range. C1 must be a non polarized capacitor rated for at least 200 volts. In addition to controlling the power delivered to a soldering iron, this circuit is a basic light-dimmer circuit. You might use it to control the power delivered to any “resistive” load up to about 150 watts. For controlling larger loads, you will need to use a larger triac and, depending on the triac, you might need to use different values for P1 and C1. For controlling large loads, it’s also a good idea to place a varistor (MOV) across the incoming 120-volt AC line (such as an NTE2V115).