Working of this protective circuit

[FreshmanNewbie]

Below is a circuit which I found that is used for Protective Earth Neutral detection.

The PEN_ON comes from MCU and PEN_dET does to an MCU.

P20, is a current transformer.



Can someone explain me the working of this circuit and why it is required?

This is an application of an EV charger module that takes mains input and then meters, & then AC chargers the EV battery. Between the AC main and EV charger connector, where would this CT come? There's a Relay (which is controlled by an MCU) in the line between AC main and the connector.

 

[KlausST]

Hi,

What the circuit does:
The first stage is a (poorly designed) amplifier and rectifier.
The second stage (also poorly designed) is a comparator.

But I don´t know how it is meant to work in combination with a current transformer.

Maybe the current transformer type and the current transformer wring could give more insight.

*****
I´ve done some "isolation fault detection" for an isolated high power application.
It basically introduces some very low frequency (far below mains frequency) signal into the isolated net (voltage limited, current limited) and checks whether there is current flow.
"Very low frequency" instead of DC is used to validate (correlation) the signal. So the output is more fail safe an thus more reliable.

Here the "very low frequency" may be generated by the PEN_ON signal.
But indeed it´s nothing more than guessing.

Klaus

 

[betwixt]

I agree with Klaus on all points. It's some kind of precision rectifier and comparator with an offset at its input provided by the "PEN_ON" voltage. It is DC coupled so the added signal from the current transformer needs to be known before a true analysis can be made.

Please show how the current transformer is connected and in particular whether it reads current through a single primary or two. This may be looking for imbalance between two primary wires, possibly one being the feed and the other the return.

Brian.

 

[danadakk]

Should C36 be connected directly to OpAmp 2 U9B NI input, eg. to create a LPF
with R69 ? Also allow R69 to decouple the load of C36 from OpAmp # 1 U9A.

And if U9B is a "comparator" sure looks like it has no hysteresis and really
low G to function as a comparator.

Something real fishy about circuit.


Regards, Dana.

--- Updated Jun 14, 2023 ---

I do sim of circuit, first stage operating as expected, but second stage
saturated high. One only has to look at dc coupling thru both stages
and wonder........

Regards, Dana.

 

[FreshmanNewbie]

Hi,

What the circuit does:
The first stage is a (poorly designed) amplifier and rectifier.
The second stage (also poorly designed) is a comparator.

But I don´t know how it is meant to work in combination with a current transformer.

Maybe the current transformer type and the current transformer wring could give more insight.

*****
I´ve done some "isolation fault detection" for an isolated high power application.
It basically introduces some very low frequency (far below mains frequency) signal into the isolated net (voltage limited, current limited) and checks whether there is current flow.
"Very low frequency" instead of DC is used to validate (correlation) the signal. So the output is more fail safe an thus more reliable.

Here the "very low frequency" may be generated by the PEN_ON signal.
But indeed it´s nothing more than guessing.

Klaus

Thank you for the answer and thanks for letting me know it is poorly designed. I intended to take this circuit as a reference for a design that I am going to develop.

But Can you tell me why the first and second stages are poorly designed? So that I can understand the logic and try to make the necessary changes in my reference design. I will learn based on the mistakes in this design. Hence, please let me know how you say it is poorly designed and what should I take care of to make sure it is correct.

Also, the current transformer part number is ZMCT103E & specification is https://5lrorwxhmqqiiik.leadongcdn.com/ZMCT103E+specification-aidirBqoKomRilSjpkmnlkp.pdf

Another question is that, the PEN_ON is from a MCU which goes into pin 2 of the CT. Does pin 1 of the CT give a voltage? I am not sure how would this CT be implemented on the PCB and how would the PEN_ON signal be present with respect to the transformer. Can you please help to clarify?

 

[betwixt]

Just a thought to confuddle matters further - could PEN_ON be a pulsed or alternating signal in phase with that in the current transformer so PEN_DET is a 'match or no-match' indicator?

Brian.

 

[FreshmanNewbie]

Just a thought to confuddle matters further - could PEN_ON be a pulsed or alternating signal in phase with that in the current transformer so PEN_DET is a 'match or no-match' indicator?

Brian.

This circuit mentions that it is a PEN fault indicator. I looked up and saw that it is to identify Whether Earth and Neutral are connected together? I am not sure. Any idea on how this circuit helps to do that? How it works>

 

[danadakk]

Here is what a sim looks like :

Does look like a poor mans comparator in the second stage, the input stage is working fine.

The input stage design saturates at I neutral = ( 5A / 2.5mA ) x 7.5 mA =~ 15A

The comparator stage tripping at ~ 1 mA = ( 5A / 2.5 mA ) x 1 mA =~ 2A in the neutral line.

The comparator stage with no hysteresis susceptible to noise and false triggers.

I simed it with a PEN-ON assumption its a 3.3V logic signal....maybe PEN_ON provides the hysteresis....?


Regards, Dana.