Soldering iron – temperature control failure

I wanted to modify a soldering iron to insert brass threaded inserts into holes drilled in plastic parts, and for this application looked to eBay for an inexpensive temperature controlled soldering iron that could be adjusted down to around 200°.

Well first check was of its temperature when set to 200°.

Ouch, that is a fail. The Chinese cheats have supplied product that does not comply with its description.

Now the iron is rated for 200-480°, and a simple model implies a power ratio of 0.42:1.

Let's explore how it works.

Above is the current waveform when set to 200°. It is ordinary full wave phase control and the conduction angle is about 65°.

The chart above shows that the RMS voltage at 65° is about 48%, and that the power into a constant resistance would be 24% of maximum.

A small detail, simple phase controllers such as this can probably not achieve conduction angle much over 130°, and this one is of that type. The relative power at 135° into constant resistance is 90%.

Power into a constant resistance at 24%:90% or 0.27:1: is well below the 0.42:1 implication of the specified temperature range.

So, why does it not work?

The heating element is clearly not a constant resistance, but is a positive temperature coefficient (PTC) resistance probably to provide a temperature regulating mechanism.

When this is switched on set for 480° (but it only achieves 440°), initial current is 0.3Arms, and quickly falls to 0.1A, 69W to 23W.

We have in cascade, a mechanism to adjust the RMS voltage and a non-linear resistance which tries to compensate changes in temperature (including the effect of reduced RMS voltage).

The PTC element doesn't quite win. In fact the observed temperature range is 365:440 or 0.83:1 even though the spec is 0.42:1 and the phase controlled power range (into a constant resistance) is 0.24:1.

A total failure of design, it cannot deliver the claimed 200° which is needed for my application… and the seller thinks I should accept 30% refund as compensation. Very Chinese!