# Why -5V is 80V compared to GND after a power supply?

I was quite shocked when I realized that negative is not continuous when compared with neutral, and that a negative wire can actually have a positive voltage. For example, this simulation:

I am measuring voltage for L, + and - against GND (symbol)

My simulation makes me think than a DC power supply outputs a constant 5V between + and -, but an alternate current between + and neutral (from the mains socket.)

I went to the bench, using a 5V power supply (RAC05 datasheet.) I was expecting that my multimeter was going to show somewhere between 0 and (-)5V when comparing - with neutral but instead it shows 80V. If I set it in DC mode, then it shows almost 0V.

Why is that?

How can I know the actual voltage between my power supply and neutral?

Are there cases where it is dangerous to touch the 5V+ or 5V- of a power supply because of their voltage difference with Earth? (I understand that my voltage is equal to earth.)

What happens if I connect my power supply negative with mains neutral?

PS: I am in Switzerland with 230V 50Hz

• It's unclear what you measured vs. what's on your schematic. Please provide some more info on that! Commented Oct 25, 2021 at 17:16
• Also, "galvanic isolation" and "ground reference" are search words you are probably looking for. Long story short: voltage is only meaningful between two points. One is commonly ground, but not always. Your supply might not reference ground, but it's "-5V compared to PIN 123" or similar, so you have to make one of them ground yourself. Commented Oct 25, 2021 at 17:19
• @Transistor that is what makes me crazy. I understand that between + and - I will have constant voltage, but I cannot visualize in my head what is that "floating", how hight it can get, and if it is of any danger. Until now every time I see a 5v wire i put my fingers on it Commented Oct 25, 2021 at 17:29
• @CarlosGarcia Let me make a suggestion - In your simulation, attach different nodes to GROUND, and see what happens. Connect the (-) of the rectifier and run the sim. THen move that to one of the ~ inputs and run again. You will find you tie that nodes voltage to 'zero'. See what the OTHER voltages are. You are doing similar when you touch parts of your circuit - Through your body capacitance you are 'grounding' those points. Commented Oct 25, 2021 at 18:00
• If you take a AA battery and measure the voltage between + and ground/earth you will get a nonsense reading with a digital multimeter because the input impedance is 10 megaohm so it measures electric fields in the air. If you ground the battery '-' or if you move the COM lead to battery '-' you'll get sensible readings. Commented Oct 25, 2021 at 18:34

Your power supply output is isolated from input. So there is no DC reference to earth/ground. Not in the simulation, nor not in the power supply at your bench. So since the output has no reference to ground, it is floating, and it can float at any voltage in reference to the input.

Isolation means there is very high impedance.

Very high impedance means even a small amount of stray capacitance between transformer primary coil and secondary coils will cause the output voltage to have common mode voltage.

Your multimeter also has very high impedance input, so thus even small stray capacitance between transformer coils will not get loaded down by the multimeter.

There might even be a Y capacitor inside the power supply for suppressing electromagnetic interference, and it will be deliberately connected between primary side and secondary side of the transformer.

The power supply is also not a linear power supply, but a switch mode power supply that works by driving the primary coil with high frequency square wave, and high frequency square waves get extremely well coupled over a small capacitance.

So, what you measure is true and perfectly normal for a switch mode power supply which has ungrounded mains input.

• And that very high impedance is also the reason why OP can safely put his fingers onto the 5 V line as per the comments, right? Commented Oct 26, 2021 at 5:15
• what is OP? @ThomasWeller Commented Oct 26, 2021 at 11:17
• @CarlosGarcia OP: Original Poster, in this case you Carlos. Commented Oct 26, 2021 at 12:48

A transformer-isolated supply isn’t completely isolated. It will have some leakage from primary to the floating secondary through parasitic coupling in the transformer, and through an RFI bypass cap from primary to secondary if it has one (they're used for switching supplies to capture noise.) You will see this leakage as a common-mode AC voltage on the output when measured with respect to safety ground.

Always make your leakage measurements with respect to safety ground, not neutral.

There are regulatory limits on this leakage: about 3mA for a regular consumer or IT supply, lower than that for a medical-grade supply. It is not harmful, but if it’s a problem for your measurements you can ground the output to safety (earth) ground to stop it. You can also choose a medical-grade supply that has lower leakage by design.

Note: Do not connect anything on the secondary to neutral. This creates a shock hazard.

Why?

First, neutral (the AC return) can have some voltage on it. It's not considered 'ground'. Typically this voltage is allowed to be up to 3% of the line voltage, and it will vary depending on the load of the branch. So if your plug is on a branch that's shared with something that's high current, you could see several volts AC on neutral vs. safety ground.

Second, if there is a break on neutral, the entire primary will be at line potential.

Third, there is a possibility of a miswired socket (hot/neutral swap.)

So, again, never connect the floating secondary directly to neutral. Only connect to safety ground if you need to.

• "Neutral" in what region's wiring scheme? It looks like you're talking about the typical North American split 240V scheme, with neutral and hot at each plug. Commented Oct 26, 2021 at 1:56
• In 240V countries the low-side leg is also connected to ground at the panel (service entrance) and is called 'neutral'. More here: bluesea.com/support/articles/AC_Circuits/87/… Commented Oct 26, 2021 at 2:03
• In any event, this 'neutral' should never have a human-touchable path. It should not be used as a safety ground, and instead treated as if it were a live wire, be it a North American style split feed or a 240V single-ended European style feed. Commented Oct 26, 2021 at 2:06
• For those wondering; neutral can become live if the neutral wire somehow gets cut, and something is plugged into the circuit. Commented Oct 26, 2021 at 8:24
• What @user253751 says counts for NA and 240V wiring, BTW. Commented Oct 26, 2021 at 15:01