Can I measure mains voltage and supply the meter using the same transformer? - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: December 27, 2017, 06:07:52 pm
content: I had a USB light switch. And I am stepping it up to a mains-rated version with energy measurement, USB and Ethernet control interfaces. Now for the energy measurement part, I need a way to sense mains voltage. (For the mains current I think Allegro ACS758LCB-050B would work.) Now can I sense the mains voltage by tapping off the output of the mains supply transformer for the logic circuitry directly?For the logic on the board, I can either supply it with a mains to 5V isolated converter module and sense the mains voltage using a sense transformer, or use a mains-rated transformer to get a supposed 6VAC, and both measure this AC and draw power from it. Will the latter bite me?Also, after I finished the circuit, how do I calibrate it? Do this call for isolation transformers, high-cat-rating multimeter and probes or something similar? I am using this in China, so 220V 50Hz mains. (Treat it as 240V though as mains here, if not in Shanghai, can sway a bit.)

author: Kleinstein
author_rank: Super Contributor
timestamp: December 27, 2017, 09:09:28 pm
content: It depends on the required accuracy, transformer size and power needed for the supply. Small transformers are pretty poor and thus even without a load, a 1 VA transformer will not give a good voltage signal. Transformers start to get reasonable quality from about 5-10 VA on. With not too much and a well behaved (more sine like and not pulsed, or even with DC component) load, it might work with a >= 5-10 VA transformer.It also help if the transformer is a low standby loss type, so that there is not too much magnetizing current.Much of the distortion could be compensated for but would need an increasing amount of math and calibration.For current sensing a simple current transformer should be enough if there is no need for DC current sensing.If no high accuracy is need, one could bet away with something as simple as a 100 W incandescent lamp for calibration. An isolation transformer would be nice, but is not absolutely required. In theory a CAT II meter would be enough - but in China even a CAT 4 mark on a meter might not mean much.

author: technix
author_rank: Super Contributor
timestamp: December 28, 2017, 04:52:17 am
content: Quote from: Kleinstein on December 27, 2017, 09:09:28 pmSo for a varying load the tapping off power transformer idea may not be that appropriate? Also for the current sensing how well does a Hall-effect sensor like ACS758 work?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 28, 2017, 05:23:07 pm
content: The hall effect sensor will also work. However compared to a current transformer the price and noise level is usually higher. Especially small currents like modern lamps can be a problem.The transformer use depends a lot on the required accuracy and size of the transformer - a variable load is not a general problem.

author: capt bullshot
author_rank: Super Contributor
timestamp: December 28, 2017, 06:27:16 pm
content: I wouldn't recommend a normal small power transformer as voltage sense.If you do everything right, there's no need for isolation here, just high enough impedance. Build a differential amp with >1MOhm single ended input impedance and use appropriate resistors for the high voltage side of the voltage divider. Use identical resistors to connect to Line and Neutral, feed that into your differential amplifier (or maybe the differential input of your power metering IC) and you're fine. That's the way many of these professional power analyzers are built.I'd recommend to use a small and suitable CT to sense the current, open loop Hall sensors tend to be drifty and noisy.