Recommendations for logging AC apparent/reactive & RMS power draw & IC solutions - Page 1
Forum section: electronics / projects

author: bsodmike
author_rank: Regular Contributor
timestamp: September 10, 2020, 09:35:11 am
content: Hi all,For future projects, I want to include the ability to record power draw at the AC input end of any circuits by way of IC solutions.Another aspect to this is building an AC "pass-through" wall-connector, and log power draw details via the use of say an ESP8266 and Wifi. Applications for this are pretty wide!Please recommend any ICs that you may be aware of or any solutions I could refer up, thanks!Going slightly off topic: Secondary question.When I started taking a look at some of the approaches out there, I stumbled across this "Energy Monitoring PICtail™ Plus Daughter Board"http://ww1.microchip.com/downloads/en/devicedoc/51933a.pdfSee the attached schematic (which is also at the end of the PDF). The PDF shows PCB layout and component list as well.Mains AC is passed through a 200mA fuse, a small inductor/MOV, 1.5UF film safety capacitor, a 33R 5W resistor and then subject to half-wave rectification. This is then straight away supplied to a 5V regulator.Is this possible due to the low-voltage (24V) on C24, the 470uF cap? I'm not sure how the 240VAC input voltage is effectively reduced to a low-enough voltage to be fed into the 5V regulator.How "safe" is this circuit -- suffice to say, this is the first time I've seen mains power being used to provide regulated 5Vdc output.

author: ledtester
author_rank: Super Contributor
timestamp: September 10, 2020, 12:23:07 pm
content: Here is a Atmel/Microchip app note about designing an AC energy meter with a microcontroller:http://ww1.microchip.com/downloads/en/AppNotes/Atmel-2566-Single-Phase-Power-Energy-Meter-with-Tamper-Detection_Ap-Notes_AVR465.pdfThis video does a teardown of a AC power meter module and the chip at the heart of it is a Vango V9811a:https://youtu.be/G0AlH3vtT0k?t=8mThe Vango chip is a 8052 mcu designed for power metering applications.

author: coppice
author_rank: Super Contributor
timestamp: September 10, 2020, 12:36:36 pm
content: Quote from: bsodmike on September 10, 2020, 09:35:11 amIts a standard RC supply. It is not isolated, so its unsafe to allow random things to come into galvanic contact with it. However, with a proper double insulated barrier its a VERY common way to do mains energy measurement. As long as the high voltage capacitor, used to drop most of the mains voltage, is a suitably rated type, and the resistor is raised well off the board, it can be a very reliable and safe design. The reason for raising the resistor is you arrange that its rating allows it to run quite cool under normal conditions. However, if the mains becomes very distorted the capacitor will appear as a rather low impedance at the high harmonic frequencies, and the resistor may get very toasty.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 10, 2020, 01:18:18 pm
content: It is a capacitive dropper supply. There are however 2 bad points:1) it is unsafe to combine it with the alternative low voltage input.2) the capacitor dropper usually needs a shunt regulation, as the voltage in the filter cap would other wise rise too far. So the normal way is to have zener diode to limit the voltage.A 1.5 µF is allready quite large - this is a power region, where one may consider a different solution (e.g. SMPS). One could combine the capacitive droper to some 12-20 V with a buck converter, so one could use a smaller capacitor.

author: bsodmike
author_rank: Regular Contributor
timestamp: September 10, 2020, 02:26:06 pm
content: https://hackaday.com/2017/04/04/the-shocking-truth-about-transformerless-power-supplies/#commentsNice readup on these nasty little buggers (aka TPS devices). I'd rather go the iron core route for galvanic separation and safety. Thanks for the heads up about "Capacitive dropper supplies"!!