Here below I draw a simplistic model for stray capacitance issue in SMPSs:

enter image description here

Basically pF level Y capacitor across the AC line forms an RC voltage divider when measured by a voltmeter with respect to the earth. So the voltmeter will measure around 80V AC but as soon as the R1 reduced to zero Vm will approach to zero as well. And the currents are tiny. This phenomenon is sometimes called ghost-voltage.

My question is related to a similar one which happens: when we hook up scope probes to the scope and we dont measure any signal probes act as antenna and we see some hundred mV 50Hz signal. And when we touch the probe this 50Hz signal on the scope even increases a lot. Here is a related question why it is happening.

My question is not why it is happening but how can we model this as a circuit like in my SMPS leakage example. I want to see a circuit which shows this phenomena where the components should represent the scope probe, human body and the scope input impedance and maybe the earth or ground. How can it be modelled?(Simplistically)

  • \$\begingroup\$ Mike's answer had link to article on scope probes and their cables. The secret sauce is the LOSSY cable: 200 ohms resistive loss per meter (visit the link in Mike's answer). \$\endgroup\$ Sep 14, 2017 at 1:18

1 Answer 1


The scope probe should be modeled as a lossy transmission line with the resistor/capacitor divider (10x probe is assumed).

The ground lead inductance should be added and you may couple this inductor to the inductance of your AC power wiring (50 Hz) [C2 in the image below].

You may also add capacitive coupling for same said AC power wiring (50 Hz) [L2 in the image below].

Forgive my poor and lazy image editing skills, your model may look similar to the attached image.

Note the added parasitic capacitance (C2) and indutctive coupling (between L2 and the ground clip) form the AC power line.

Scope probe model

Please read Doug Ford's: "The sectet world of oscilloscope probes" founrd in the following link, which is also the source for the image attached, for the model's detail.


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