1 MEG resistor to earth at isolated flyback SMPS secondary......

[treez]

What are the various situations where you would not be allowed to do this? (add such a 1 MEG res)
The secondary is not earthed......but wish to connect it to earth via 1 MEG.
Its a SELV secondary.

 

[KlausST]

Hi,
* for some applications, like medical devices
* unsuitable resistors, with too small voltage specification
* other effects (inductive, capacitive, ohmic) that increase earth current to a too high level
...

Klaus

 

[treez]

* other effects (inductive, capacitive, ohmic) that increase earth current to a too high level

Thanks, do some equipments measure the current to earth, for whatever reason, and so it would not be allowed for this reason?

 

[KlausST]

Hi,

Regulations ask for total earth current (or impedance/resistance), not only the "installed resistor current" .
There will be capacitive current in AC applications, there will be (resistive) leakage current.

So usually every test measurement equipment should measure the total current (impedance, resistance).

******
Maybe you don't ask for "test equipment" but "installed, continously measuring equipment".
Yes there are. Especially for isolated parts with dangerous high voltage.

Example: In industry often electric systems are safety isolated. Touching any (only one) wire won't harm.
But in case of an accidental short of some wire to Earth ... note: this won't cause high current, no fuse blow, it won't stop the machine...
It's just not "safe" anymore. If then some one touches a wire it may harm, even lead to death...

Thus there is special "safety measurement equipment" checking for earth impedance. And in case impedance is too low it will cause an alarm and/or (timed) power off the machine.

Bender is a manufacturer for such equipment. https://www.bender.de/en

Klaus

 

[Easy peasy]

If it is SELV as you say, i.e. under 60V and the sec is properly isolated - there appears to be no safety issue with earthing the 0v sec via a 1 Meg resistor, worst case dissipation is 3.6mW so quite a small resistor would suffice ...

 

[treez]

Thanks, the Bender equipment looks quality, and I am also wondering if there are small PCB-mount modules that get used to measure earth current? I suspect medical equipment have such devices, and I think electric car chargers too? Does anyone know if such small pcb mount components for measuring earth current exist?

By the way, if you have an isolated secondary (SELV), and you add Y capacitor from the secondary to earth ground, then surely there is a danger of them getting overvoltaged if the secondary ground kind of floats up in voltage?

 

[treez]

Sorry i must have accidentally clocked "SOLVED" on this when it wasnt.

the idea of a ground or PE, protective earth, is that is cannot kind of float up ...

Thanks for this reply in "conversations"...
Yes but would you think that the secondary ground of the Flyback SMPS could "float up"?...i agree that the chassis wont float up because its directly connected to earth via the green and yellow wire in the mains power cord.

The Y caps are from secondary 0V to chassis. Secomdary 0V could float up (or down), and overvoltage the Y caps?

 

[treez]

Surely any "floating" offline flyback SMPS secondary should not really be allowed to flaot, otherwise its voltage would tend to drift up to the positive mains peak voltage?
(due to the charge pumping action via the transformer interwinding capacitances?)
So a 1 MEG (or so) resistor across the isolation barrier would always be needed?...or better still, a resistor to earth from secondary 0V, if an Earth connection is available?

 

[betwixt]

Do an experiment (very carefully Treez). Connect a typical 'floating' output voltage, say 200V, through a 1M resistor and hold one side of the voltage in your hand. Now take a wire from the 1M resistor and place it in your mouth. It's a crude simulation of a dental situation. You will see why sometimes even tiny currents are significant!

Regulations vary from one type of equipment to another but in general, a resistor to the primary side will limit the absolute output voltage to the range of input voltages. My preference would always leak to ground rather than the input side but consider that many SMPS do not have an Earth connection through the line cord. In most cases where an Earth exists, the resistor would effectively be wired between Earth and part of the primary circuitry so safety wouldn't be an issue and the resistor may actually be to discharge the primary side when the power is turned off.

Brian.

 

[treez]

If it is SELV as you say, i.e. under 60V and the sec is properly isolated - there appears to be no safety issue with earthing the 0v sec via a 1 Meg resistor, worst case dissipation is 3.6mW so quite a small resistor would suffice ...

Thanks, and in cases of transients involving local earth, a high voltage resistor, eg a 2W axial 1MEG resistor, with some 1kV pulse voltage rating would be needed? (for connection between secondary 0V and earth)

It just seems wrong to connect eg a 50V, 4n7 ceramic 0603 cap between secondary 0V and earth?

 

[treez]

Hi
The attached is an offline flyback in LTspice...it shows 10 MEG resistors from pri to sec.....the secondary return has ended up being 333V above earth potential....this shows how floating offline flyback secondaries float up in voltage to near mains peak.

Do you thereby agree that it would be a bad idea to put a 50V 4n7 MLCC cap between secondary 0V and earth?

This isnt to mention transients involving the local earth rising up high in voltage and blowing up any 50v caps connected from secondary 0V and earth

 

[c_mitra]

the secondary return has ended up being 333V above earth potential....this shows how floating offline flyback secondaries float up in voltage to near mains peak.

What will happen if these two 10M resistors are omitted?

Consider an ideal case: The secondary output is floating. That means that the potentials at individual terminals are floating or undefined; that your software is seeing the voltage float up to the mains peak is most probably due to some leakage currents.

That the secondary output is floating means that you can connect them in series (or in parallel) and can get very high voltage outputs.

Connecting both ends with 10M resistors is unwise; a DC resistor helps define the potential of the connected end. We usually want the secondary ground to be the same as the primary ground.

This is essential if you want to regulate the secondary voltage with respect to the primary side. No, I do not mean connecting to the earth point (that is a safety feature) but to the circuit DC ground on the primary side. Connecting the secondary side to the primary side via a capacitor is more for noise and AC coupling.