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.

  • \$\begingroup\$ Do you mean the part of the lamp that you are supposed to touch by hand (to turn the light on and off), relative to the ground/neutral of your mains, is reading that AC voltage? Or do you mean that the hot-side switch itself, which is supposed to be buried inside where people don't normally touch it but you are touching it "just because" to see if you can feel it? \$\endgroup\$ – jonk Oct 19 '18 at 23:28
  • \$\begingroup\$ The touch switch is showing a high voltage in respect to ground. I am confident that the live and neutral wires are painful without the need to touch them. :) \$\endgroup\$ – Technoloft Oct 19 '18 at 23:32
  • 2
    \$\begingroup\$ I've measured 20 VAC coming from the Microsoft Surface Book, itself. Not the power brick. The laptop case. After long email discussions with Microsoft "support" I got a letter telling me this was "normal." (I could easily feel it.) Turns out, newer brick power supplies include capacitors that are balancing tradeoffs but are for emitted radiation restrictions, I think. (It's discussed here on this site in several places. Here is one of them.) \$\endgroup\$ – jonk Oct 20 '18 at 0:17
  • 1
    \$\begingroup\$ You can't 'feel' voltage - it's current that you can 'feel'. If the resistance inside that switch is high enough it won't allow enough current for you to 'feel' even though you measure a high voltage. \$\endgroup\$ – brhans Oct 20 '18 at 0:23

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

enter image description here

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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.