Computing stuff tied to the physical world

TD – Soldering iron

In Hardware on May 8, 2012 at 00:01

Welcome to the Tuesday Teardown series, about looking inside the technology around us.

Today, I’m going to take a quick peek inside the soldering iron from Conrad, which was suggested as low-end soldering option for a first toes-in-the-water electronics toolkit:

Opening up the base is trivial, just remove 4 screws after taking off a couple of rubber caps:

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On the right: the AC mains feed, with 2 live/neutral wires and the green/yellow ground.

On the bottom: again 2 wires plus the green/yellow ground (as crucial safety feature).

First thing to remark is that there is no temperature sensor in the soldering iron. In other words, this is an adjustable unit, but it’s not temperature-controlled – the 150..450°C scale around the rotating knob is bogus.

Just removing the knob and a washer around the potmeter is enough to examine the board up close:

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A couple of resistors, caps, an inductor, and a little transformer – that’s all. Oh, and a little TRIAC in a TO-92 housing (just beneath the transformer). Here’s the other side:

DSC 3122

A plain single-sided low-cost PCB. No surprises here – this is a very low-cost unit, after all.

So how does it work? Well, it’s basically a simple dimmer. But instead of dimming an incandescent lightbulb, it dims the heater coil inside the iron. The way this works is that the start of each AC mains cycle gets switched off – and then only after a specific time does the TRIAC start conducting. The whole circuit is essentially an adjustable delayed pulse generator, synchronized to the AC mains zero crossings.

Here’s what it looks like on the scope (as measured via a differential probe for isolation):

SCR33

The entire AC mains cycle is 20 ms (50 Hz), half a cycle is therefore 10 ms, and in this mid-range setting, each half of the sine wave is switched on after about 5 ms, i.e. halfway into the sine, at the peak voltage in this case.

Does it work? Sure, turning the knob will definitely adjust the tip temperature – but not very directly. Instead of a feedback loop, we merely control the amount of power going into the iron, and assuming a fairly steady heat dissipation, the iron will then stabilize more or less around a specific temperature. Just like a lightbulb, such a circuit will “dim” a soldering iron just fine this way.

The only drawback is that it’s not tightly controlled. When using the iron and pushing it against a thick copper wire or a big copper surface, the iron will cool off. Real temperature control requires a feedback loop which senses this change and counteracts the effect by pushing more power in when needed.

For simple uses, the crude approach is fine, but if you plan to solder under lots of different conditions (through-hole, SMD’s, PCB ground planes, thick copper wires) then a more expensive type might be more convenient.

  1. The ‘transformer’ is a common core, dual winding choke in series with the incoming mains – that style of switching control makes sharp corners and that means rich in harmonics. Without considerable filtering of all wires in and out of the enclosure, it acts like a mini broadcast station of ‘hash’.

    An alternative is to fire the triac only around the zero-crossing point for a variable number of complete ON/OFF cycles – switching ≈ zero current generates a fraction of the RFI and the thermal inertia of the soldering iron tip integrates the power pulses to a smooth temperature change.

  2. Do you have a recommendation for a temperature controlled one?

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