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I have a 230V to 24V step down transformer. I have not connected the secondary to ground connection of the mains power.

When I measure the voltage between one of the secondary terminals and the oscilloscope I get about 125V. Is it safe to connect the oscilloscope to the secondary to see the AC output signal of the transformer?

I examined the circuit diagram for my setup and I can see that this will not complete the circuit through the oscilloscope ground to conduct a high current, but I just need to make sure if I am making any mistake in my understanding.

enter image description here

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  • \$\begingroup\$ Y capacitor leakage somewhere. What do you use on the secondary to measure it? Floating DMM? \$\endgroup\$
    – winny
    Jul 6 at 14:17
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    \$\begingroup\$ Yes, floating DMM. \$\endgroup\$
    – Ian Ray
    Jul 6 at 14:31
  • \$\begingroup\$ Then probably not Y-capacitor issue. \$\endgroup\$
    – winny
    Jul 6 at 16:02
  • \$\begingroup\$ what is Y capacitor here ? where does it located ? \$\endgroup\$
    – Ian Ray
    Jul 6 at 17:07
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    \$\begingroup\$ Everyone notices this unsettling behavior sooner or later when probing AC voltage between an isolated PSU and some earthed part. $$$$ As others have mentioned, this is usually safe being merely caused by the transformer stray capacitance + potential EMI caps across the transformer leaking microamps of 50 Hz current to the "isolated" side. Up to a few mA of leakage is considered acceptable by regulatory bodies \$\endgroup\$
    – tobalt
    Jul 6 at 17:08

2 Answers 2

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Try this and think about the interwinding capacitance and how it couples primary and secondary: -

enter image description here

Another good example of this happening is on house wiring that uses cable called "twin and earth" in the UK. The twin part is two wires connected to live and neutral. If the earth is left floating you can measure 120 volts between earth wire and neutral with a high impedance meter such as a DVM.

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  • \$\begingroup\$ I connected a 200k resistor in series with the secondary terminal and oscilloscope ground clip. when I measured the voltage across the resistor it reads about 4 volts. will it be safe to measure the secondary voltage using the oscilloscope ? \$\endgroup\$
    – Ian Ray
    Jul 6 at 14:52
  • \$\begingroup\$ @IanRay it will be safe to use your o-scope to measure the output voltage from the secondary and, remember, that in doing so, the scope clip will earth one side of the secondary and bypass the 200 kohm. \$\endgroup\$
    – Andy aka
    Jul 6 at 14:56
  • \$\begingroup\$ 4 volts on 200k is about 20 uA, which is well below the threshold of sensation (about 1 mA), and GFCIs usually trip at about 5 mA, so the scope is safe to use. \$\endgroup\$
    – PStechPaul
    Jul 6 at 21:03
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Recall that multimeters in voltage mode are high impedance devices, and their behavior is thus highly sensitive to stray capacitances.

The mental model of a transformer should include the parasitic primary-to-secondary capacitance. This tiny capacitance (tens-hundreds of pF for small transformers) is plenty enough to act as a good conductor against the 10Mohm input resistance of the multimeter.

In other words: you’ll always measure such voltages. The current the voltage sources produce can be determined by using a lower parallel resistance across voltmeter inputs. Say 1Mohm or 200kohm.

In transformer circuits, there is always some primary-to-secondary common mode leakage current. This current will be shunted to ground via the ground lead of the oscilloscope probe.

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  • \$\begingroup\$ So, it will be safe to measure the secondary voltage using the oscilloscope as long as the secondary is isolated from the mains ground, right ? \$\endgroup\$
    – Ian Ray
    Jul 6 at 14:46
  • \$\begingroup\$ I connected a 200k resistor in series with the secondary terminal and oscilloscope ground clip. when I measured the voltage across the resistor it reads about 4 volts. \$\endgroup\$
    – Ian Ray
    Jul 6 at 14:48
  • \$\begingroup\$ 4V across 200k is 20uA. Plenty safe to shunt with anything, including an oscilloscope ground clip. \$\endgroup\$ Jul 6 at 16:14
  • \$\begingroup\$ if you connect the oscilloscope probe's ground clip to the transformer secondary then it will not be isolated any more. but if the transformer is working correctly this will not be a problem. (unless some other part of the secondary circuit is also grounded but your 125V measurment strongly hints that this is not the case) \$\endgroup\$
    – Jasen
    Jul 6 at 23:41

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