Electronic – High AC voltage on Touch Sensitive Switch

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I recently got a 120V/240V On/Off Touch Switch kit (ZIJIA P12-L56). They are a cheap way of adding Touch Lamp functionality to a standard lamp. It works great and I immediately started poking around inside it. While doing this I noticed that when I used my multimeter to measure the voltage between the touch switch and ground it read at ~99.8VAC.

That voltage differential seemed very large to me and definitely something I should feel when I touch the switch.

Why don't I feel a shock from a relatively high voltage?

I don't have a full diagram of the circuit inside but the Switch lead comes directly off the center pin of a Mosfet.

Best Answer

You haven't provided a link or a schematic but the general scheme of things should be similar to that of Figure 1.

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Figure 1. A touch dimmer circuit. (I have used this on a previous answer and have lost the source reference. It is drawn in the unique style of Elektor which I subscribed to over many decades and still do.)

Note that this has 9.4 MΩ resistance between the touch pad and the chip input and another 4.7 MΩ resistor pulling that to the '0 V' line. (The whole circuit must be treated as live. The '0 V' is just the reference point from which all voltage measurements are taken.)

  • The 9 MΩ is very close to the usual 10 MΩ input impedance of most digital multimeters. That means that if pin 5 is close to mains voltage you would expect to measure about half of that on your multimeter.
  • The worst case current from the touch pad would be \$ \frac {V}{R} = \frac {230}{9M} = 25\ \mathrm{\mu A} \$. This is too low to cause feel.
  • In normal operation the input is sensitive enough to detect this small current when the circuit is coupled to ground through the capacitance of your body when you touch the switch.
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