Isolation of STM32 USB from mains

Hardware
#1

Dear all,

I would like to isolate the USB connection from mains because I would like to attach a Raspberry Pi instead of the nanoPi. I was thinking about a chip like the ADUM3160 https://www.analog.com/en/adum3160 or ADUM4160. They provide an isolation up to 2.5kV rms and 7.5kV respectively.

They provide a maximum data rate of 12Mbps as specified as full speed in USB 1.0. Which data rate is needed for the openzmeter/STM32? Does it use 1.5Mbps (Low Speed) or 12Mbps (Full speed) or even USB 2.0?

Best regards

#2

Hi,

Single phase version of openZmeter (DeviceV1_x) USB is isolated from mains. Current sensor is isolated and voltage is sensed with optocoupler. Power supply is also isolated and most part of PCB can be touched without risk.

Three phase version of openZmeter (DeviceV2_x) USB is NOT isolated from mains. Current probes (CT, Hall… are isolated by design) but voltage is sensed directly. Power supply is NOT isolated. We get power with buck converter.

In both models we use USB2.0 full speed (12Mbps) and ADUM isolator should work. If you make and ADUM module from scratch you can take power for ozm side from ozm, and from USB for isolated side.

Please, send some information of your progress to help us and other users.

Best regards

#3

Thanks for the answer. I will gladly share my design once it is done.

Why did you decide to change the design for the 3-phase meter? Was it space? Why not just duplicate the existing design?

Did you note any difference between 1-phase and 3-phase voltage measurements?

Best regards

#4

Hi,

Isolation in 3phase version require 3 isolation amplifiers, this raise the cost a lot. Isolation is not nedded, becose ozm is not designed to manipulate it while running.

Ozm 3 phase version allow up to about 340vac in mains. we cant find integrated power supply with inputs that high.

Voltage readings behave allmost identically in both models.

Certainly isolation is a great feature while development but it can be solved easily with ADUM isolators.

#5

For future revisions we maybe use a custom flyback power supply with isolation and some alternative to expensive isolation amplifier, for example i make some promising tests with IL300 linear optocoupler. Future version may also use other linux board with less power and heat waste.

#6

I ended up using a readily made adapter. It also provides isolated 5V power:

USB Isolator Module ADUM3160 USB Digital Isolation USB to USB Voltage Isolator Board Protection (2.5KV MAX) Support 12Mbps

#7

Here is a working solution of isolation of the USB lines with the ADUM3160 for example for use with the Raspberry Pi.

IsolatorForRaspberry
IsolatorForRaspberry5940×4198 825 KB

No liability whatsoever and not recommended for use with mains.