Current Transformer Connections - Resistors and Capacitors

[FreshmanNewbie]

I was just going through the purpose of the Current transformer and its working & using.

I found some schematics where I found a common pattern as below. It is a single-phase connection. LINE & NEUTRAL is the input of a single-phase supply.

The LINE CURRENT & NEUTRAL CURRENT connectors are connected to a Current transformer.

The outputs I2P/I2N & IP/IN go as inputs to an energy-metering IC.





Can someone tell me what are the purpose of the Resistors and capacitors in the above circuits and how the values are selected?

Also, I'm not able to understand the purpose of the LINE to VP connection.

I searched about the type of connections, but was unable to find it.

Any idea/guidance on this would be greatly helpful.

 

[KlausST]

Hi,

I searched about the type of connections, but was unable to find it.

can you please give a link to the manufacturer internet page for
* the current transformer
* the metering IC.

Klaus

 

[D.A.(Tony)Stewart]

1. In order to attenuate sampling errors from impulse noise, a low-pass filter with a cutoff frequency of 6.6 kHz (RC=1k*33nF) is employed. This filter ensures a phase shift of less than -1 degree at frequencies below 100 Hz, effectively filtering out current noise.
2. To reduce Line to Vp, it is important to consider the creepage breakdown voltage, which can vary depending on factors such as dust, humidity, and conformal coating. Typically, resistors are rated for 250Vrms or higher voltages lasting less than 1 microsecond. However, nearby lightning strikes can induce common mode or differential impulses, generating high voltages (HV) that can lead to arc breakdown (< 8kV). To prevent such breakdown, it is recommended to use resistors with a higher voltage rating or employ protective measures such as gas tubes.

To enhance reliability, multiple resistors rated for 200~250Vrms are connected in series. This approach avoids fusing open if only a few resistors were used or creating a crater on the PCB the size of a dime when relying solely on a gas tube for protection. Previously, 8kV rated ceramic hybrid resistors with a zener bridge were commonly used in offline linear supplies for AMR (automated meter reading). However, the observed solution might differ from this standard approach. A well-designed layout incorporating an air gap and silicone sealant may have been employed to mitigate impulse safety testing failures caused by dust accumulation in the field.

I was just going through the purpose of the Current transformer and its working & using.

I found some schematics where I found a common pattern as below. It is a single-phase connection. LINE & NEUTRAL is the input of a single-phase supply.

The LINE CURRENT & NEUTRAL CURRENT connectors are connected to a Current transformer.

The outputs I2P/I2N & IP/IN go as inputs to an energy-metering IC.





Can someone tell me what are the purpose of the Resistors and capacitors in the above circuits and how the values are selected?

Also, I'm not able to understand the purpose of the LINE to VP connection.

I searched about the type of connections, but was unable to find it.

Any idea/guidance on this would be greatly helpful.

 

[FreshmanNewbie]

Hi,

can you please give a link to the manufacturer internet page for
* the current transformer
* the metering IC.

Klaus

Hello,

CT: https://5lrorwxhmqqiiik.leadongcdn.com/ZMCT103E+specification-aidirBqoKomRilSjpkmnlkp.pdf
Metering IC: https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-46002-SE-M90E26-Datasheet.pdf

 

[KlausST]

Hi,

* first link does not work for me
* Second link sadly isn't an internet page but a datasheet.

Klaus

 

[dick_freebird]

If those "2.4E" resistors are meant to be 2.4 ohms then they
are likely the CT's "burden resistor".

The rest seems to be HF filtering and common-mode-zero.
Presumably the CT core & winding is the only thing "to the
left of J6 / J7" and isolates both hot and neutral, and all
"figuring" is referred to Earth (ground symbol).