Isolated voltage sensing using transformer

[richard_jay]

Hey,

I would like to design a voltage measurement circuit using a step-down or isolation transformer and I would really appreciate some advice on practical implementation of a solution.

The input voltage is nominally 120 V RMS (max 180 V RMS) and an Instantaneous output voltage between 0 - 5 V biased at 2.5 V DC is required for sampling by an ADC.

I have initially considered the type of transformer to employ – isolation or step-down.

Step-down transformers are much more readily available and therefore seem the most appropriate selection.

The data sheets for these step-down transformers tend not to provide detailed design information. They simply state the typical step down voltage (e.g. 120 to 6 V AC) and the current rating (e.g. 58 mA). How do I determine what value of load resistor to select for my application? Where do I begin?

Thanks, Rich

 

[cdh7]

Hi Rich,

I'm not sure but I guess you don't need any load resistor. Is there anything in the datasheet saying one is required?

You should get a center tapped transformer and use an ADC with differential inputs to make things easier for measuring AC voltage. There are plenty of application notes for transformer coupling of ADCs.

What are you trying to measure anyway? RMS or peak voltage? Keep in mind that a 180 VRMS signal will have a peak amplitude of over 250V.

Regards,
Chris

 

[richard_jay]

Hi Dave,

Thanks for replying to the post.

Agreed, a load resistor is not required but unfortunately I’m limited by the ADC configuration of my sampling microprocessor which is single ended.

Assuming I use a 120 to 5 V AC (24:1) step down transformer what about using a voltage divider circuit configurations to offset the output and range it correctly?

I have an maximum input voltage range of 500 V peak-to-peak (180 V RMS). This needs to be scaled to a 5 V range. A scaling factor of 100:1 is appropriate.

As the transformer will step down by a factor of 24:1 a further division of 100/24 (4.167) is required to give an overall scaling of 100:1.

The circuit must then be filtered for anti-aliasing purposes and then buffered before being routed to the ADC.

What do you think of this circuit proposal? Pick holes please!


Thanks, Richard

 

[cdh7]

Hi Richard,

That looks alright to me. You'll lose some accuracy though due to the relatively weak voltage divider supplying the offset. Maybe stiffen it up a bit with 1k resistors instead of 100k.

You might want to add protection diodes to ground and Vcc at the input of the opamp to prevent any "surprising" voltages there. Actually, it might not be a bad idea to add a transil diode or some other kind of overvoltage protection device across the secondary winding of the transformer.

Regards,
Chris

 

[richard_jay]

Hi Dave,

Excellent. I've added a bi-directional transient voltage suppression (TVS) across the secondary winding and reduced the value of the voltage divider resistors according to your recommendations.

Thank you very much indeed for the advice about this circuit construct.

Rich J

 

[dsg]

Could you provide the transformer part number...