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With capacitive touch sensing and transparent materials, it seems like it would be possible to run small digital devices from A/C mains without a transformer isolated supply.

Is there a flaw in this thinking?

Would it make a capacitive dropper power supply from mains power safe, if the packaging is sealed up water-tight?

Is this how modern stoves etc are designed?

Picture from a different question. Putting UI into the switch plate form factor.

enter image description here

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  • \$\begingroup\$ touch panel overlays wear out \$\endgroup\$
    – jsotola
    Apr 4, 2020 at 20:15

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‘Dropper’ supplies don’t provide isolation (well, enough anyway). Anything they power needs to be considered ‘live’.

If the supply and its load are contained in a safety enclosure, then it’s acceptable to use this kind of supply (think light dimmers, motion sensors and such.) If not, you need a supply that does provide galvanic isolation from the line.

Given how cheap and compact USB chargers are these days, leveraging this approach would be the go-to answer for all but the simplest of line-powered projects. USB charging has spawned a whole ecosystem of inexpensive off-the-line switching ICs and small transformers.

If space isn’t an issue and power is modest, a transformer supply with a low-cost linear or switching post-regulator solves a lot of problems. You get isolation, efficiency and can use easy-to-find low-voltage components in the secondary.

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  • \$\begingroup\$ Yes, I'm thinking about something in the dimmer-like category. Added an illustration. \$\endgroup\$ Apr 4, 2020 at 21:47
  • \$\begingroup\$ Yes, it's acceptable to have live components behind a plate. That said, typical dimmers have the whole circuit in an enclosure and treat the stuff inside as 'double insulated'. The dimmer knob shaft itself is often plastic so that it is isolated from the stuff inside. \$\endgroup\$ Apr 4, 2020 at 22:13
  • \$\begingroup\$ OK, thanks. That's the kind of answer I was looking for. I like the analogy to the plastic dimmer shaft. \$\endgroup\$ Apr 5, 2020 at 3:56
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Some devices can be made that way. However little things like the keyboard or an LED or a crack in the display exposing conductors could kill the user if they are not insulated to a high standard, so it is difficult to make a device of much complexity that meets modern safety standards.

Also the low supply current can be a major issue. For that reason, even devices that have no UI at all (eg. Wemo WiFi controlled outlet) will still use an off-line switching power supply.

For something like a stove, a relatively expensive appliance, I would wager that the control circuitry would almost certainly be galvanically isolated (but check before assuming that, of course, if it could cause danger).

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  • \$\begingroup\$ Understood. I was thinking that it might have a thin but solid layer of polycarbonate atop the UI. I think that would offer some good protection. \$\endgroup\$ Apr 4, 2020 at 21:53
  • \$\begingroup\$ When the polycarbonate is attacked by some cleaning chemical the hapless homeowner uses, or deteriorates from decades of aging and sunlight (UV) exposure it would be good for the appliance not to kill too many people. \$\endgroup\$ Apr 4, 2020 at 21:59

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