Inside pink shoe/boot dryers/warmers with "UV" sterilisation.

There's nothing colder than wet steel-toecap boots at sub-zero temperatures. So I went online to see if I could find some boot heater and dryers. A whole range came up, from the nicely designed American Peet boot dryer to the clumsy looking heater and hose versions to the little mains powered ones that push into your footwear.
I was particularly intrigued by the UV sterilisation function mentioned in some of the Chinese listings. This can be achieved with germicidal lamps that use a mercury vapour discharge in a quartz tube to generate shortwave ultraviolet light that has a direct sterilising effect and creates ozone. (And is an eye hazard if viewed directly.)
I was secretly hoping that these units would have the mercury vapour based lamps using the heaters as ballasts. But its just two blue LEDs with a less than stellar power supply pretending to be UV.
The heaters are based on PTC (Positive Coefficient Thermistor) blocks that self regulate by increasing their resistance as they get hotter. They have interesting resistance characteristics, since they have a lowest resistance value of about 1000 ohms which actually increases as they get either hotter or colder. This makes sense, since if their resistance kept going down as they got colder, then they could experience quite a high current flow in freezing conditions. In a cold house they measured almost 3000 ohms. This means that when they are first connected to the mains they show a modest current that increases and then decreases as they heat up to their state of equilibrium. When they stabilised at about 10W total (for two) that equated to an equivalent resistance of about 12,000 ohms each.
The peak central temperature on the heater assembly was around 150C graduating out along the metal dissipation plate to about 80C.
While not having a real UV sterilisation function, these units are actually well worth buying for the self regulating heater assemblies.