Where is this voltage coming from? - Page 1
Forum section: electronics / beginners

author: windmill john
author_rank: Contributor
timestamp: April 22, 2023, 10:09:09 am
content: Hi All,Okay, so I finished building the case for my Variac. I have the Variac plugged into the mains and then my Carroll & Meynell isolation transformer plugged into that.The earth connection is removed from both output sockets of the isolation transformer; the Variac is earthed as is the input of the isolation transformer.I was doing some initial safety tests. I decided to plug my voltmeter on AC into one of the two output sockets of the isolation transformer. One lead in the neutral, one in the earth. Now, bearing in mind the earth is disconnected and the body is plastic, at about 40volts, my meter showed approx. 2 volts, at about 60 volts, the meter showed approx. 4 volts and at about 240 volts, a very quick test, the meter showed approx. 8 volts.Where is this coming from? I’m new so don’t laugh (you can to yourselves :-). ) Is it some kind of transient voltage?Thanks

author: RoGeorge
author_rank: Super Contributor
timestamp: April 22, 2023, 10:25:49 am
content: Everywhere around is stray capacitance, a parasitic capacitance between the circuit of interest and the surrounding objects. In your case, the circuit you think about is the variac, but the surrounding objects, your body, the Earth, the walls, etc. are all making parasitic capacitors with the body and wires of the variac.The parasitic capacitances are very small, but they are there, unseen, and usually not drawn on the schematic diagrams. The instrument has high impedance, and can sense the voltage coming "through" the surrounding parasitic capacitances.On the same idea (of parasitic properties we usually disregard), any piece of wire is also a parasitic inductance, a parasitic resistance, and a parasitic inductance, it's a parasitic transmit antenna, and a parasitic receive antenna, all at the same time. Same for any other component. Those parasitic values are so small that we usually don't think about them. But sometimes they become noticeable, like in your measurement.

author: windmill john
author_rank: Contributor
timestamp: April 23, 2023, 06:45:49 am
content: Sorry for the delay in replying, I was away. Thank you for the reply and explanation; much appreciated.

author: AndyC_772
author_rank: Super Contributor
timestamp: April 23, 2023, 08:04:07 am
content: It might be worth investing in an electrician's multimeter with a lo-Z AC voltage mode, like the Fluke 289.

author: windmill john
author_rank: Contributor
timestamp: April 23, 2023, 06:17:26 pm
content: Yes I’d read about lo-Z meters, but to be honest, I’m very new and at this stage it would be overkill. Appreciate the comment though.

author: MathWizard
author_rank: Super Contributor
timestamp: April 25, 2023, 03:27:38 pm
content: For higher voltages, like over 90V or so, if you have a little Neon gas bulb and resistor, they draw ~100uA irrc. If it lights that up, I would not touch it by hand, and treat it as "hot"For even higher currents, to really see if there's power on household wires, I also made they same thing, with resistors and an LED. If it can draw 5mA at 240V, yeah I consider it live/hot.