STPM10/STPM32 unit mettering and ISOLATION with microcontroller

Francisco SinapseEnergia · ‎2017-12-22

Posted on December 22, 2017 at 21:03

Hello all.

We want to start a design for mettering with STPM10 or STPM32 mettering IC. (STPMx)

We want to tie the control lines from these mettering devices to one STM32 microcontroller. We have seen that if we want to tie communication lines from STPMx with one RS232 for connecting with one PC or laptob is needed to put one isolation device in order to avoid reverse currents or for preventing typical non-isolation problems.

Question is..?

If we only want to tie STPMx with STM32 microcontroller , is it really need to use isolation IC, in example ,in SPI or UART pins? Anyway, is it convenient?

Thanks in advance.

Best regards.

Francisco.

#stpmx #stpm10 #isolation #stpm32 #mettering

T J · ‎2017-12-22

Posted on December 23, 2017 at 02:54

The reason for the RS232 chip is when the connector from the LIVE PC ground.

the power supply you have, is usually a plug pack with two wires, no ground connection.

So your circuit is isolated and thereby your circuit 'Ground pin' may in fact be 400v above the PC ground.

The problem is that when you connect the connector,

which pin is going to discharge the 400v dc static potential from the plug-pack ?

that's the shroud around the DB9 trying to discharge the ground potential first.

The RS232 chip will also withstand 2500v static discharge for a very short time.

but for your question.. do you need isolation on the SPI,

The E4622 current sensor provides isolation from the mains voltages.

Since all of your circuits are at the same ground potential, there is no need.

Francisco SinapseEnergia · ‎2017-12-26

Posted on December 26, 2017 at 13:47

Thanks Marsch.

I am not going to use E4622 current sensor. I will use shunt resistors for current, and for voltage measurement, one resistor divider, as shown in below image.

Then, I will tie SPI ports to another microcontroller. I understand that if I use the same VDD-GND that powers the meassurement unit for that another microcontroller, I will not need isolation. But If I use another power supply, Will I need isolation for SPI port or it is enough by using a good isolated power supply?.

In general, problems with PC RS232 connector is because of PC power supply is not isolated??

Thanks a lot.

Francisco.

T J · ‎2017-12-26

Posted on December 26, 2017 at 23:13

Yes, 2 microcontrollers, with a good ground between them, there is no need for isolation.

Yes, the PC has a proper earth-ground outside your house. full of noise...

Your isolated power supply's negative terminal will drift upto 400V above ground until it is tied to the PC ground.