2 ch voltmeter with different grounds - Page 1
Forum section: electronics / beginners

author: franciscoPT
author_rank: Contributor
timestamp: February 02, 2021, 06:17:43 pm
content: Hi guys, im building a power supply with 2 independent channels 0-30v 0-5A, i will have the measures on 20x4 lcd, to do that i need to measure both voltages and currents on the same microcontroller (i will use atmega328p because i have 3 at home without using it), the microcontroller circuit will be powered by 1 channel of the supply, my trasnformer is rated for 6,5A on each channel because i will make power supply max at 5A, 1,5 its more than enough for microcontroller +2 lcd, my problem its 1 channel will share ground with microcontroller, but the other doesnt, its any way to read the voltage on isolated way such like optocoupler?

author: Ian.M
author_rank: Super Contributor
timestamp: February 02, 2021, 07:18:00 pm
content: Probably the easiest option would be an isolated SPI ADC for the second channel, however this would require four optocouplers + your choice of ADC chip. If you use another ATmega328P for the second channel you could use their UARTs for serial communications to transfer the readings via two optocouplers.Otherwise, its possible to subtract out the common mode component of the measurement using an OPAMP + precision resistor network configured as a differential amplifier, but the common mode range is limited by design and you may have difficulty designing it to work with inputs up to 50V outside the rails.

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 02, 2021, 07:49:37 pm
content: Another option is to use an ADUM (or comparable) isolator including power to do the SPI ADC.Another option is to do Analog -> PWM -> opto -> PWM -> analog (or directly in the micro if that's possible)

author: franciscoPT
author_rank: Contributor
timestamp: February 02, 2021, 08:34:33 pm
content: yeh i was thinking in analog to pwm than opto. it seems the easiest solution to me for i will consider the other options.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 08, 2021, 12:34:25 pm
content: I would make all the channels equally in the executive part, and the control part and the display by a separate device with its own power supply. There are a lot of isolation options, in addition to those listed earlier, look at the analog optocoupler, there are good examples of its use in the datasheet:https://lcsc.com/product-detail/Optocouplers_AVAGO-Broadcom-Avago-HCNR200-500E_C23785.html

author: Kleinstein
author_rank: Super Contributor
timestamp: February 08, 2021, 02:04:35 pm
content: Quote from: franciscoPT on February 02, 2021, 08:34:33 pmEspecially with only positive readings, the old style voltage to frequency converter and than an optocoupler is an alternative. An ADC with serial interface is also an option.There are analog Ok and also isolation amplifiers, but thes tend to be relative expensive of not very accurate. So it depends on eht resolution needed and also the supply available.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 08, 2021, 02:56:12 pm
content: I agree ADUM are a good choice as a drop in solution. NVE isoloop deviceshttps://www.nve.com/webstore/catalog/index.php?cPath=30are easier on your pocket and you can buy direct from NVEs webshop. These devices are often overlooked they but are very handy. Have a look at the full product range. GMR, brilliant!Opto solutions can get unexpectedly expensive and bulky in comparison with digital isolators.Look forward to seeing picture of your finished PSU.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 08, 2021, 02:56:45 pm
content: But OTT?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 08, 2021, 03:54:21 pm
content: Don't. Really just don't.Instead just design a single channel power supply (Including PCB etc) and then build two of them.It will need some extra components, each will have it's own microcontroller and some other things, but those components are cheap. You will also save time because the resulting PCB's are simpler, smaller and cheaper.The microcontrollers can communicate with each other over some isolated channel. UART + Optocouplers is a simple example. I2C or SPI can also be used, but UART is the simplest.Also: If you have a communication protocol to set and request values, then adding a separate project for the interface is also an option. (Possibly even in a separate box and connected with a cable or even RF).