Design advice for energy meter project (230VAC/LVD stuff) - Page 1
Forum section: electronics / projects

author: Entropia
author_rank: Regular Contributor
timestamp: December 26, 2013, 02:54:59 pm
content: Hi. I'm designing a mains energy meter. I need some advice on making the circuit and the PCB safe. The device is meant to be installed after a circuit breaker fuse, so I don't think a separate fuse is needed. The CT SEN1 has proper isolation barrier. I've included a MOV for the voltage sensing part. Do I need other form of protection?Moving on to PCB layout (have not started on that yet). I have marked my best assumption for the board cutout airgap in the schematic. Are they in the right place? What else do I need to take into account besides keeping mains voltage stuff and low voltage DC stuff well apart on the PCB?Edit: Line voltage connected components are thru-hole.

author: Jon86
author_rank: Frequent Contributor
timestamp: December 26, 2013, 03:00:20 pm
content: Where does the data go off to? If you've got any kind of data connection on the device, you'll want to completely isolate it from the mains.

author: Entropia
author_rank: Regular Contributor
timestamp: December 26, 2013, 03:08:05 pm
content: Quote from: Jon86 on December 26, 2013, 03:00:20 pmIt goes off to a microcontroller which then connects to an RS-485 bus via a tranceiver. I was thinking of using a "digital" isolator for the bus. Can't remember the exact TI part no. off the top of my head though.

author: Jon86
author_rank: Frequent Contributor
timestamp: December 26, 2013, 03:22:29 pm
content: I'm not entirely sure, but I think you can get an isolated transceivers for RS485, that might be worth doing.

author: Entropia
author_rank: Regular Contributor
timestamp: December 26, 2013, 08:33:09 pm
content: True. It's easier to go with isolated RS485 transceiver. In addition to air gaps (clearance) I will also have to obey creepage (on surface of the PCB) restrictions. I think 5 millimeters for both should be plenty for equipment that is used indoors but is not potted (pollution degree 2, 250 VAC RMS). Source:http://www.ce-mag.com/ce-mag.com/archive/01/03/ProductSafety.htmlIs this safe enough, or should I still include a board-mounted fuse in series with the CT? The microcontroller system is powered from a separate AC-DC transformer and the bus side of the RS485 transceiver from yet another transformer to maintain isolation.

author: AlfBaz
author_rank: Super Contributor
timestamp: December 26, 2013, 09:39:07 pm
content: If the MOV activates do you really want to wait for the CB to trip? Assuming the circuit being monitored has a load that's not sensitive to voltage spikes then your device will be the first point of failure

author: Jon86
author_rank: Frequent Contributor
timestamp: December 26, 2013, 09:50:50 pm
content: I think that's probably about all the safety measures you're going to have to take, I guess you could stick an isolated ADC in and a few fuses, but it's not really essential at all.You could also use an isolated DC-DC converter module and do away with the extra transformer if you wanted to?5mm would be enough, but a little extra wouldn't hurt. Just make sure everything on the high-voltage side is rated high enough and you should be good to go

author: Jon86
author_rank: Frequent Contributor
timestamp: December 26, 2013, 09:53:59 pm
content: Also, just noticed that 0R resistor going straight to ground off the phase neutral, I'm not sure I see the point of that being there?

author: PA4TIM
author_rank: Super Contributor
timestamp: December 26, 2013, 09:56:01 pm
content: I use seperate powersupplys with separate transformers and optocouplers to separate the mains sensing part (current shunt, and Voltage sense) from the other part (ADC, microprocessor, phase detection, TRMS converter etc)http://www.pa4tim.nl/?p=4771this is my project (i'm still in the proces of modding and improving thing)

author: nctnico
author_rank: Super Contributor
timestamp: December 26, 2013, 11:31:58 pm
content: In one of my recent designs I used a safety rated DC-DC converter and a safety rated I2C isolator from Analog devices to put an ADC on the mains side and read it with I2C from a microcontroller. Not cheap but it works like a charm.Either way the circuit should be designed phase and neutral can be swapped without causing problems. In some cases there may be 2 phases and no neutral.

author: Entropia
author_rank: Regular Contributor
timestamp: December 28, 2013, 06:12:18 pm
content: I have a healthy respect for mains voltage (scared really, since I got shocked pretty bad as a kid) so I was thinking alternative ways to test the meter instead of live mains voltage.In my junk box I found a 230V/12.4V transformer that I back-fed from my signal gen with 50 Hz sine at 10Vpk-pk and measured around 80 VAC from the primary. Nice. I just need to amplify the siggen output a bit and I have a safe way of testing the inputs of the meter.I also noticed that a Euro plug fits perfectly in the 3478A inputs. Probably lots of other meters as well!

author: jn
author_rank: Newbie
timestamp: March 25, 2014, 02:55:39 pm
content: I've done a meter design up to the certification couple years back. Is this topic still interesting (given that the thread hasn't been updated for coupe months already)?