AC leakage, does it matter?

[sjl]

I am trying to understand AC leakage from 'floating' power sources when compared to earth/building ground.

I have 4 devices that I am testing:

- Cheapo 5v DC output 2 prong SMPS
- Rigol DP712 Linear DC Power Supply
- Siglent SPD1168X Linear DC Power Supply
- Tripp Lite IS250 Isolation Transformer (converted into a 'technician' isolation transformer: no continuity between input & output, ground & output, chassy & output)

The SMPS was measured leaking 52 VAC RMS to ground, I measured AC current flow to ground at 82 uA

The DP712 was measured leaking 25 VAC RMS to ground, AC current flowing at 15 uA

The Siglent would not leak any to ground. (I guess the SPD1168X wins over the DP712 in this regard, even though the 712 is a more expensive PSU)  I wonder if there is an additional filtering circuit to remove this from the output.

With The Tripp Lite I measured 98 VAC leaking to ground, with an AC current flow of 32 uA
(would be using this when probing mains-powered DUT or for isolating a variac to work with lower voltage AC)


Questions:

1. Is the correct name for this phenomena simply 'AC leakage'? Would this be at all considered ghost/stray voltage?

2. Is there any danger to sensitive components (micros, ICs) from this voltage?  For example if the DP712 was powering a breadboard circuit without ground, and then a ground was introduced via an oscilloscope probe, could a microchip be damaged or affected by the leaking 25 VAC-- even though it is low current?

3. Why do some transformers appear to suffer like SMPS do-- even when there are no capacitors involved. I have seen leakage from transformers out-of-circuit.  Would this come down to more insulation between primary and secondary windings?  I read somewhere one side might be better shielded from the other-- why don't most bench PSU's and audio power supplies seem to do this? Other than the Siglent I have not been able to find a PSU or transformer that does not suffer from some form of leakage (I saw some with a linear audio transformer, torroidal and others).

4. Is it correct to say that in the SMPS situation it is a capacitive coupling issue via an input/EMI capacitor, and in the transformer situation it is also a capacitive issue between primary and secondary?

5. Ultimately should I be concerned with this phenomena in terms of building circuits, testing equipment, or am I wasting my time worrying about it?

Thank you!

[Ian.M]

2. Mains leakage current can be deadly to sensitive components.
e.g. https://www.eevblog.com/forum/beginners/simple-fet-circuit-for-onewire-protocol-doesn_t-work-as-expected/msg1277272/#msg1277272
I wouldn't worry much about a few tens of uA leakage applied to components that have internal ESD protection (e.g. the input protection diodes present in most CMOS logic), but it could well be an issue working with highly sensitive components e.g. many types of RF FETs,  unprotected MOSFETs, etc.

[AngraMelo]

Are you using a low impedance meter to check that? That could be "ghost" voltage

[sjl]

Are you using a low impedance meter to check that? That could be "ghost" voltage

I was using a Fluke 87V, so I believe that would be 10 megohm impedance

[Gyro]

Welcome to the forum. From your questions, you already have a pretty good handle on this stuff.

Questions:

1. Is the correct name for this phenomena simply 'AC leakage'? Would this be at all considered ghost/stray voltage?

The correct name is 'leakage current' (AC or otherwise) - the voltage you see is determined by the source voltage available, the impedance (resistive / capacitive) of the 'leak', and the input impedance of the measuring device. For SMPSs, the term 'Y Cap leakage' is in common use as it results, mainly, from the impedance of the Y rated safety caps typically connected between the input lines and the output / chassis.

2. Is there any danger to sensitive components (micros, ICs) from this voltage?  For example if the DP712 was powering a breadboard circuit without ground, and then a ground was introduced via an oscilloscope probe, could a microchip be damaged or affected by the leaking 25 VAC-- even though it is low current?

Very little on a linear bench PSU. Definitely some danger on a 2-pin SMPS mains adapter though.

The leakage current will be as you measured for a normal rate of voltage change you get on the input mains sine wave. The problems can come from mains spikes or at switch-on. With fast rise-time edges like this, capacitance can couple much more current.

3. Why do some transformers appear to suffer like SMPS do-- even when there are no capacitors involved. I have seen leakage from transformers out-of-circuit.  Would this come down to more insulation between primary and secondary windings?  I read somewhere one side might be better shielded from the other-- why don't most bench PSU's and audio power supplies seem to do this? Other than the Siglent I have not been able to find a PSU or transformer that does not suffer from some form of leakage (I saw some with a linear audio transformer, torroidal and others).

All transformers have some degree of capacitance between their primary and secondary windings. They are either wound side by side in split-bobbin construction, or one on top of the other with insulation between them (including toroidals). There is no real way of getting away from this stray capacitance, some transformers have an inter-winding conductive screen which is connected to ground to shunt most of the capacitance to ground. Some high spec bench meters even have two screens, one ground screen and one guard screen, connected to the floating input guard terminal. Even with an inter-winding screen, the secondary of the transformer will have capacitance to the screen, so assuming, say, a 50V secondary on a variable PSU, you could potentially see 25V leakage current on the secondary - completely unrelated to anything from the primary.

4. Is it correct to say that in the SMPS situation it is a capacitive coupling issue via an input/EMI capacitor, and in the transformer situation it is also a capacitive issue between primary and secondary?

Yes exactly. Y-Cap leakage and capacitive leakage across the high frequency transformer windings.

5. Ultimately should I be concerned with this phenomena in terms of building circuits, testing equipment, or am I wasting my time worrying about it?

With Linear bench PSUs you are probably worrying a little too much - I don't know the two particular models you're talking about, but many include a ground terminal that you can connect the (normally negative) output to, to remove any remaining leakage when working on sensitive parts. 2 Pin SMPS adapters, you should definitely worry about when working with sensitive parts.

[sjl]

Thank you for the detailed answers, Gyro! That helps fill in the gaps for me. Very much appreciated.

[Gyro]

You're welcome. 

One (potentially useless) thing I forgot to mention - of all the standard types of mains transformer, the split bobbin ones typically offers the lowest leakage. Side-by-side windings have the highest capacitive coupling to the grounded steel core, and present the least surface area, and therefore capacitance, to each other. Even the connection 'polarity' to the transformer have an effect - with the highest voltage swing portion of the winding closest to the core, you get the lowest primary-secondary leakage current.

Un-screened torroidals tend to present the highest inter-winding capacitance - windings wound, one on top of the other over the largest area, and no grounded metal core to mitigate it.

[Brumby]

3. Why do some transformers appear to suffer like SMPS do-- even when there are no capacitors involved. I have seen leakage from transformers out-of-circuit.  Would this come down to more insulation between primary and secondary windings?  I read somewhere one side might be better shielded from the other-- why don't most bench PSU's and audio power supplies seem to do this? Other than the Siglent I have not been able to find a PSU or transformer that does not suffer from some form of leakage (I saw some with a linear audio transformer, torroidal and others).

All transformers have some degree of capacitance between their primary and secondary windings. They are either wound side by side in split-bobbin construction, or one on top of the other with insulation between them (including toroidals). There is no real way of getting away from this stray capacitance....

Just running with the theory: There is capacitance between any two conductors in the universe and the value of that capacitance is described in maths.  There is capacitance between the active and neutral conductors in a mains cable; there is capacitance between your car's body and the Earth; there is capacitance between the steel ruler on your bench and the car keys in your pocket.

Fortunately, most cases are significantly small that they can be ignored for all practical purposes - but sometimes that thinking can cause us to not objectively consider situations where the effects may show up.

[Kevin.D]

I think it can still be a potential problem at just 10uA+ leakage if the source voltage is large enough.

Mains leakage (or any other ac noise) can get rectified, it's then much
more of a potential problem because it can charge capacitors (rising until charge current = any load or capacitor leakage current ) up to possibly high peak voltages .

It only takes a couple of diodes which happen to be oriented to form a voltage doubler
 ( looking for an example of something that one might actually connect to a bench supply  would be a bridge followed by a capacitor then into a fixed voltage regulator the type that has a power saving disable pin so it only uses a few uA in shutdown followed by your precious circuit which is earthed say by a scope probe).
That capacitor  will now potentially charge upto twice the peak voltage (2 * 1.41 * Vrms) of the ac leakage when the regulator is disabled. In this example  If we are lucky the cap will start leaking and limit the peak voltage before the reg gives up the ghost and goes s/c with the resultant surge taking out whatever is after it.
In the example here bridge diodes are used to rectify  but transistor pn junctions and MOSFET body diodes may also happen to be oriented to form a voltage doubler (even internally inside ic's when you earth one pin via a scope probe)) so this may not be as improbable as one might think, I haven't tested this hypothesis of possible internal rectification damage to ic's so i got no idea how probable it is.

As the previous posters mentioned for bench supplies that use 50-60Hz transformers the best solution is to use transformer with an inter-winding shield  or split bobbin transformer, the shield can reduce primary to secondary inter-winding capacitance (Cint) from a typical 100pF to < 1pF .
If shielded tranny not used then there should at least be a capacitor fitted from the secondary side circuit to earth (47n-100nF 200V+ which is usually put near the supply output) , this forms a capacitive voltage divider with Cint reducing the leakage Vpeak to a safe couple of volts .
I can recall a thread about the rigol dp832 where a.c leakage voltage concern was raised and I mentioned the absence of those earthing caps as problematic, I don't know if they fixed it yet.(https://www.eevblog.com/forum/testgear/rigol-dp832-firmware-updates-and-bug-list/msg640067/#msg640067)
If you have any supply with excessive voltage a.c leakage then try the bridge rect followed by say a ~1uF cap (rated 100v+) and earthed one side experiment, a minute or so depending on a.c leakage current should see the cap charge to 2*Vpeak (though when connecting a meter the measured Voltage will quickly fall from the Vpeak as cap is partially discharged and the source is now loaded through the 10meg input resistance of the meter) . I ought to mention that disabling supply output with a switch which usually only breaks the +out would not disable the charging of the cap by the a.c leakage current (which is 'common mode', that is it's equally present on both + and - output's)  which can still continue to charge via the -Vout socket.

Regards