Suggestion for reading AC voltage variation with level shift - Page 1
Forum section: electronics / projects

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 22, 2018, 06:30:13 pm
content: Hi All,I am trying to read the voltage variation on the ac line when a LOAD such as a motor is switched on in my Aquaponics system. The simplest form i could come up with is attached.Second is used to measure the current consumption by the motor using an opamp with level-shifter. This was another forum. I wish to know what needs to be added when used at the mains side. I could think of an optocoupler.Suggestions are welcome.Thank you.

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 22, 2018, 08:41:42 pm
content: Incomplete circuit that is probably unececeraly dangerious!

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 22, 2018, 08:43:22 pm
content: I haven't really completed it. Just came across a hurdle so seeking some help.

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 22, 2018, 08:49:52 pm
content: The help you need is to learn how to design circuits that are not dangerious. Anything that connects to a mains or grid power supply should include galvanic isolation, your circuit does not.

author: drussell
author_rank: Super Contributor
timestamp: February 22, 2018, 09:03:15 pm
content: Quote from: fourtytwo42 on February 22, 2018, 08:41:42 pmI assume you mean "unnecessarily dangerous?"You don't even know what he is doing with it, nor under what circumstances.Why is it "unnecessarily dangerous?"

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 23, 2018, 11:20:17 am
content: Quote from: drussell on February 22, 2018, 09:03:15 pmBecause it is always better to err on the safe side and the OP has not provided suficient information for anybody to determine if the circuit is safe or not!

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: February 23, 2018, 12:00:15 pm
content: Depending on the speed, accuracy and resolution the OP wants, it might be easier to do things on the mains side. Of course this needs to be taken into acount in terms of safety, and proper fuses need to be in place, isolation spacing has to be respected, and components must be selected that can handle those voltages safely.Every circuit is dangerous to some extent - you can kill someone with a coin-cell battery if you can just stab the current close enough to the heart. Having respect for the possible dangers involved is far safer than just going "oh no this is dangerous so I'm just gonna run away from it".The OP might not have provided suficient information for anybody to determine if the circuit is safe, but the OP has also not provided sufficient information to claim that the circuit is " dangerous". So before we go run and scream danger, perhaps it is better to ask why the OP does it the way they do, and if they are aware of safety concerns.

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 23, 2018, 03:59:54 pm
content: Thank you all for the concerns. For the record am very well aware and in senses about what am doing here.I've just partially completed the circuit on a bread board but yet to have it drawn on CAD to share it here. The first schematic is actually to check if the load is ON. The load here is a microwave transformer (MOT) used for Spot welding and not a motor. Well i had the motor in mind for another project. But latter figured that the MOT suits better for this application. Here is how the first circuit comes into play, When the MOT is OFF there is a small current flowing at the primary and a voltage drop is seen when the secondary with the electrodes are shorted. I could use that change in voltage as an Auto-Switch ON feature where touching both the electrodes on a nickel strip would activate the MOT via an SCR which is controlled by an AVR.