question about analog isolation techniques

[mamech]

hello

I am dealing nowadays with a project that needs analog isolation of mains voltage.
I got the following circuit from a schematic of educational kit that uses IC ADE7953 for power metering.

they use attenuation circuit to attenuate the mains voltage to sample it using ADC in the IC.

the problem of this configuration,the mains is connected directly to the IC and also that the ground of IC will be connected to neutral (or even line, in case the user swap the plug) , so this is not good from protection point of view.


it is suggested to use potential transformers to get the mains voltage totally isolated to the metering IC ADE7953.

the problem is that I do not now what are potential transformers. Are they different than normal Ac transformers? and do they exist in miniature package?

I searched the Internet using the name of potential transformers, but it seems that this is not the commercial name of it.

in another post here, I found someone talks about signal transfomer, is it the same as potential transformers??

any recommendations ?
thank you very much

 

[treez]

so you are trying to measure the mains voltage, and struggling since you need the processor oviously to be isolated from mains.
So in that case, what I would do is convert the mains voltage to a digital number, (with a processor on mains side) then pass this through a digital isolator to your processor which is isolated from the mains...digital isolator eg by silabs.com.

alternatively, there is a way you can do it with a differential amplifier, and just use loads of series melf type resistors, of a very high ohmic value as the input resistors, and then you can literally port the anlog signal directly across the isolation barrier like that...(because you have those high value resistors to respect the isolation barrier with)........the signal may get attenuated, so you may have to amplifiy it up first.

 

[mamech]

thank you for reply . the first idea would be good if I am using ordinary processor in low voltage side. but in case of metering IC ADE7953, they have embedded processor and I think that the way to make it able to do the necessary calculations on voltage signal, is to digitize this signal through its normal adc input.


for the second idea, I want to be sure that I understood well, you meant that I do something like buffering using opamp with high value resistors??

 

[treez]

Actually its a diff amp stage, and the two input resistors are very high value, so you can literally take them across the isolation barrier....an electric drive co I worked for did this all the time...and a lighting company who wanted to pass dimming signal across isol. barrier.

 

[D.A.(Tony)Stewart]

Instrument transformers are a class of transformers for measurement, namely CT & VT

Potential transformers (PT) are also called voltage transformers (VT)

There are inductive transformers, capacitive , hybrid, active , passive and many other types