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Are the following zener diode based non isolated power supplies safe to use with a touch based application?

I would assume, the circuits are safe if they completely sealed (including the touchpad). Would the TT6061A circuit be safe, if y-Capacitors would be used for C4 and C5 and the touch pad be exposed?

e.g. SGL8022W SGL8022w

e.g. TT6061A TT6061

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  • \$\begingroup\$ Zeners make a good reference, but a crappy regulator. \$\endgroup\$ Apr 18, 2017 at 18:32
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    \$\begingroup\$ Sure, but my question is about safety. \$\endgroup\$
    – someonr
    Apr 18, 2017 at 18:36
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    \$\begingroup\$ Safe to touch has nothing to do with zener diodes. Safe to touch means that the supply is mains isolated meaning, current from the mains (120V/ 240 V) cannot pass through the user. This requires a transformer. When used properly, zener diodes seldom fail. If a zener does fail as open, the circuit relying on the zener could be damaged. And that's it, the circuit might break. The circuit will not become more or less safe to touch as that is a property of the power supply (transformer: probably safe, no transformer: not safe) . Not the way the supply voltage is regulated. \$\endgroup\$ Apr 18, 2017 at 18:39
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    \$\begingroup\$ Circuit 1 is very bad and not safe, as the bridge rectifier is directly mains connected. The "safety" of the touch point relies on a proper ground which is asking for trouble. Circuit 2 is slightly better because of the 2 1nF capacitors in series with the touch point. The rest of the circuit is not safe to touch, only the touch point is, you might still feel a slight "tingle" though when you touch it. \$\endgroup\$ Apr 18, 2017 at 18:46
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    \$\begingroup\$ Nice Chenglish: TT6061A IC IS A KIND OF CMOS TECHNOLOGY PREMITTED TO DESIGN :-) In China they generally take a more liberal approach to user safety concerning mains connected circuits. \$\endgroup\$ Apr 18, 2017 at 18:52

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Absolutely not.

In many locales, non-grounded and non-isolated electrical devices are required to be double-insulated. An exposed touch pad would probably fail the double-insulation requirement.

Further, alternating current can flow through a series capacitor. It doesn't matter what the output voltage of your non-isolated supply is; because it's non-isolated, it's always referenced to mains voltage, and there is always a direct path to mains voltage on every half-cycle (which is what makes non-isolated circuitry so dangerous). A capacitive touch pad could couple this mains path directly to the person touching it, leading to a possibly lethal situation, regardless of whether the power supply is regulating correctly.

Use an isolated supply or use a button instead of a touch pad.

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