STPM33 RMS calculation

[Michal Szulc]

Posted on January 24, 2017 at 13:54

Hello,

I'm working on an energy meter system using STPM33. I designed and assembled PCB. For testing I wanted to use STPM3x Evaluation Software. I can communicate with STPM, I can read and write registers. To see if it works properly I wanted to measure power and energy drawn by 100W resistive load (with phase angle close to 0 or cosp ~= 1) from ACmains (230V RMS, 50Hz). Below is a screen from evaluation software:

 

Voltage RMS and power (both WB and FUND) calculation appear to be OK (230V and around 100W). However current RMS value is too small, approximately 2 times smaller than it should (430mA measured in with am

pere m

eter in circuit). I'm using 10mOhm shunt resistor as current sensor. To verify that I took C1 RMS Data [16:0] from dsp_reg14 (entire register value is 00D319E7) and using Table 15 (STPM3x current voltage LSB values) from STPM3x datasheet and chapters

1.4.2.1

and

1.4.2.2

from UM2066 User Manual (Getting started with the STPM3x) and calculated I

RMS

=

0.21707A

.

I tried to measure different values of current (from 80mA to 2A) and every time RMS value calculated by STPM33 is around 2 times smaller than value shown by ampere meter. Why is that?

I also don't understand why active power is calculated correctly (100W) while current RMS value is wrong.

Kind regards

Michal

#stpm33 #stpm

[Nesrine M_O · Accepted solution]

Posted on January 24, 2017 at 15:29

Hi

mszulc

,

Due to the nature of the STPM33 energy metering product, we don't offer support for it on this Forum at the moment.

We recommend you to get in contact with the STMicroelectronics office closest to you.

Here you can find the contacts:

/external-link.jspa?url=http%3A%2F%2Fwww.st.com%2Fcontent%2Fst_com%2Fen%2Fcontact-us.html

-Nesrine-

[Yang Shufan]

Posted on May 02, 2017 at 09:52

Maybe you should calibrate the device

[Gary schoombie]

Posted on July 25, 2017 at 15:27

This is an old thread, but in case someone else has the same issue - there is a silicon bug in Rev BA of the STPM33 (chip marking CHN 413), which outputs the RMS current of half the true value. The current silicon revision is BB (chip marking CHN 610) - I found this out the hard way.

It also has nothing to do with calibration.