5
\$\begingroup\$

I am trying to get my head around using (9v) AC on a circuit. As there is no polarity, I cannot use a multimeter (as far as I know) to determine if both are live or whether one is 0v. The transformer I have has a 3.5mm jack and I'm reluctant to cut only to find there is no difference.

The following circuit is to connect from an AC circuit to power an Arduino.

  1. Are both wires on an AC/AC transformer live?
  2. Assuming it is live/hot, is there no risk in sharing ground across DC and AC components.

AC Transformer Circuit

\$\endgroup\$
2
  • 1
    \$\begingroup\$ There are at least 4 wires on an AC/AC isolating transformer; you ask about two of them. Which two? The 1 uF should be non-polarized. \$\endgroup\$
    – Andy aka
    Nov 30, 2022 at 14:12
  • \$\begingroup\$ @Andyaka I think the voltage on the right of the 1uF is meant to be always higher than the voltage on the left - except for possibly a few cycles during startup? \$\endgroup\$
    – user253751
    Nov 30, 2022 at 14:32

2 Answers 2

17
\$\begingroup\$

Transformers have polarity, indicated by the dots on one side the windings in the schematic symbol. (pic stolen from this page)

enter image description here

What that means is, when measuring voltage on windings between the dot as "positive" and the non-dot as "negative", as is shown on the schematic, all the voltages will have the same sign (positive or negative depending at which point of the AC cycle the measurement is taken).

This is usually not important for power supply transformers, because we don't care about polarity of mains voltage. It can be important for signal transformers, if you want the output signal to be the same polarity as the input signal, then you have to make sure to connect the windings the correct way.

If a transformer has two secondary windings and you want to wire them in series or in parallel, then polarity is important. If they're wired wrong, in series the voltages will cancel, and in parallel they will short each other.

Anyway.

Are both wires on an AC/AC transformer live?

Your transformer has only one secondary winding. Unless one of the wires is connected to Earth (unlikely) then the concepts of "hot" "cold" "live" and "neutral" do not apply. "Neutral" means low voltage relative to Earth, "Live" means high voltage relative to Earth, with the implication that dangerously high current can flow between Live and Earth if it is given a path to do so. That doesn't fit here because the secondary is insulated from Earth.

You simply have two wires with 9V AC between them, and they're both isolated from mains and Earth, ie "floating".

Assuming it is live/hot, is there no risk in sharing ground across DC and AC components.

Since your transformer is floating, you get to choose what the potential of one of the output wires is relative to your circuit ground. Since you're using a half bridge rectifier, and connect one of the secondary poles to ground, then you made that choice. Note the transformer doesn't care, you could use any of the two poles for this.

There would be a risk if you used another circuit powered by the same transformer, and that circuit chose to connect the other pole of the transformer's secondary to ground, which would create a short. This will happen if the circuit has a full bridge rectifier, which connects each side of the secondary to ground in turn on each half cycle.

Otherwise, it's fine.

The 1µF cap should be non polarized, and probably can be removed anyway.

The 120µF cap is most likely too small for a half bridge rectifier.

Polarity of the AC measured at the "voltage sample" point relative to actual mains depends on the first paragraph of this answer.

\$\endgroup\$
0
9
\$\begingroup\$

Question 1: Are both wires on an AC/AC transformer live?

Well there are 4 wires : two for the primary winding (say P1 and P2) and two for the secondary winding (S1 and S2).

When referring to a wire as "live/hot" - we generally refer to the fact that it is directly powered from the mains and implicitally dangerous. However the Collins Dictionnary indicates that a live wire is a wire carrying an electric current . That is technically incorrect because a wire does not have to carry current to be "live". But that dictionnary suggests that if a wire is connected to a power source then it is live. But in the technical world when considering the mains outlet, we have the protective ground, neutral and phase - with the latter being called the live wire (in the US, see all about circuits for instance). In my personal experience in my European countries we refer to the neutral and the phase as the live wires (You can never be really sure the building is wired as expected).

The primary and secondary windings are isolated (for most common transformers) - you can verify that they are by measuring (DC) resistance between each connection.

A 120VAC/240VAC to 9VAC transformer would have two live/hot wires (on the primary) powered from the mains. In my view the secondary wires are not "live/hot" wires, but according to Collins above they are. The secondary wires are "free of potential" (vs. the primary wires) and have a low voltage between them.

"Free of potential" implies that the voltage difference between any secondary wire and the primary is "undetermined" until you make some kind of connection between them. That does not mean there is no voltage difference, but because the impedances are high, you can't tell how much it is and you can easily change that.

Simply measuring the voltage between P1 and S1 will eventually show a value close to 0 because the voltage meter allows a current to flow between P1 and S1 so that their potentials become the same.
Then, measuring the voltage between P1 and S2 will also eventually turn out to be 0.
However you can not conclude from those measurements that the voltage difference between S1 and S2 is 0. P1/S1 and then P1/S2 were simply brought to the same voltage levels while measuring. S1 was kindof "live" by making these measurements, even though there would not be a lot of current flow if you'ld touch S1 because the meter's impedance is high (I do not recommend trying to touch S1 while measuring to verify that statement - I recommend caution with potentially "live" wires).

Question 2: Assuming it is live/hot, is there a risk in sharing ground across DC and AC components?

There is a serious safety risk (electrocution) in sharing ground between "anything" on the secondary winding and the primary winding.

You have a functionnal risk, component failure and related fire risk if you improperly share ground with S1 or S2 on the secondary winding.

Your schematic is perfectly fine as it properly connected in a single ended rectifier. If you were using a double bridge rectifier, the ground reference would be defined on the bridge output and connecting that to S1 or S2 would break the double bridge functionnality.

Question 3: Does it matter if I use S1 or S2 as GND? (The real question you are asking).

The answer is : it does not matter in your schematic where you do not functionnaly depend on the relationship between the primary and secondary phase.

As mentionned before, the secondary is "free of potential", so you can call "anything" 0V. Also, you will measure the same waveform whether you use S1 or S2 as the reference potential. If you measure compare both measurements at the same time, you would find that they have a 180° phase shift.

When it matters, you can expect both the primary and secondary schematics to be provided at the same time and the transformer windings will be marked with a dot. Supposing that P1 and S1 are marked with a dot, that means that the currents flowing into P1 and out of S1 have (essentially) the same phase. Note that I indicate "currents", because a transformer transforms currents, the voltage relationship is kind of side effect. That is because an current change induces a magnetic field change and in a transformer that magnetic field change induces a current. Generally we refer to the voltage relationship of transformers because that is more useful in designs, and because that is generally ok.

\$\endgroup\$
3
  • \$\begingroup\$ Thanks, the clarification on terminology was apprecirated. I certainly intend to avoid the touching the mains. \$\endgroup\$
    – Ryan
    Nov 30, 2022 at 16:17
  • \$\begingroup\$ Suggested phrasing: remove the "no" in title 2, so *... is there risk in sharing ground across DC and AC components?" When I first skimmed this, I had to read a 2nd time to realize the title was a question, not a statement that there was no risk. (The missing question mark didn't help) \$\endgroup\$ Dec 1, 2022 at 22:02
  • 1
    \$\begingroup\$ @PeterCordes Thank you for your feedback. The titles were a direct copy of the original questions, but I agree that it should be clear in itself. I made some updates so that it is easier to read. \$\endgroup\$
    – le_top
    Dec 2, 2022 at 9:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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