I'm trying to build a true RMS volt meter that plugs into my 120 RMS outlet. Since I want to do it safely, I'm using a very handy transformer (ZMPT101B AC voltage transformer module) that takes the 120 RMS wave and outputs 0<>5 volts peak to peak.

Do industry true RMS volt meters have this transformer? It seems the safest option for me, but the hand held meters don't seem to have a bulky transformer. If they don't have a transformer, how do the measure analog voltage safely? Also, do they measure the voltage with an ADC converter?

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    \$\begingroup\$ Double insulation. \$\endgroup\$
    – DKNguyen
    Feb 27 at 16:03
  • \$\begingroup\$ True RMS meters can be made with resistors and heat detection, using no electronics at all. What features of an AC true RMS meter are important? Some variations can benefit from a transformer, but not all. \$\endgroup\$
    – Whit3rd
    Feb 28 at 5:53

2 Answers 2


For your safety using a transformer with that setup is a good idea. You have exposed boards (no insulation) and a connection to USB (no isolation).

Handheld multi-meters are both insulated and isolated so do not require a transformer. They go to great lengths to ensure that you can not contact the voltage being measured and these are isolated from ground. Cases and test leads should be rated with an maximum insulation voltage. This is easy to achieve in a stand alone piece of equipment which has no need to be connected to anything else by using an insulated case and careful design of test leads.

  • \$\begingroup\$ would you mind explaining more about insulated and isolated vs no insulation and a no isolation. I'm assuming no isolation means your'e connected to ground? Is insulated and isolated relatively easy to achieve? \$\endgroup\$
    – Krits
    Feb 27 at 16:02
  • \$\begingroup\$ @Krits assume an voltage spike fries something in your board, and there's now a conductive (because charred) trace from grid to your USB connector. That USB connector connects to your PC's 5V rail, which connects to you keyboard. You don't want to be using that keyboard at that time, right, no matter how well-insulated the cables going into the box with your USB meter are! \$\endgroup\$ Feb 27 at 16:54

In your case, the transformer is not a bad idea. The rest of the circuit somewhat defeats its purpose, since:

  1. You have exposed mains terminals in several places,
  2. You're using alligator/crocodile-terminated wires that are never meant for use on anything but safe low voltage,
  3. The crocodile terminal insulating boots are conductive enough that an overvoltage transient on the mains will go through them as if they weren't even there.

Instead of using a tiny little transformer, I'd suggest using a proper, low-power medical-grade mains step-down transformer. They provide excellent isolation between the secondary and the primary, and I'd trust them much more than a random product from Amazon.

The medical grade mains transformer will have a very low primary-to-secondary leakage current - say tens of μA at most at room temperature with relative air humidity below 90%. But make sure you buy the transformer from DigiKey, Allied Electronics, Mouser, Toroid, or another reputable vendor, never from Amazon, eBay or Aliexpress unless you have the knowledge needed to figure out whether you got a piece of unsafe junk or not. People who have that knowledge will usually be shy of having to use it for that purpose, and just go for trusted parts, with a manufacturer's name attached that can be contacted for support, to report product problems, etc.

Would you mind explaining more about insulated and isolated vs no insulation and a no isolation. I'm assuming no isolation means you're connected to ground?

Isolation is when there is no galvanic connection between the low voltage circuit you can touch, and mains. Isolation is a noun denoting a state, not a chunk of material, although this distinction is often forgotten and lack of awareness can confuse discussion easily.

Insulation is the means of achieving isolation. It is the physical material you use to isolate circuits from each other. In a transformer, it is the insulation on the wire and the insulating material between the windings (such as insulating tape, plastic sheets, etc.) that provide the isolation between the primary and secondary side of the transformer.

Do industry true RMS volt meters use a transformer?

Transformers are somewhat nonlinear so a high precision true-RMS voltmeter will have some trouble keeping its specifications with a transformer in the input circuit. A suitably linear transformer would be quite expensive, and is more trouble than it'd be worth.

Instead, those voltmeters are isolated as a whole. There are no user-accessible parts that are exposed to mains voltages, so there's no need to use transformers. The voltmeter circuits galvanically connect to mains, i.e. there is a conductive path between low-voltage voltmeter circuit and mains. But since the whole voltmeter is in a well-designed plastic box, or in a protectively-earthed enclosure, it is safe. The end user won't ever be exposed to the high voltage.

In your case, since the voltmeter circuit is wide open to the user, using a transformer during the design and prototyping process is a basic safety precaution. The final circuit won't need it as long as you isolate it from the user just like a Fluke multimeter is isolated, say.


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