AC Current measurement / CT's for power Analyser - Page 1
Forum section: electronics / beginners

author: diag7919
author_rank: Contributor
timestamp: October 05, 2022, 08:12:14 pm
content: I scored a cheap (broken) power Analyser (Metrel MI2092).. Now repaired I want to employ it to carry out what Dave's just done in his latest video, ie, find out what add's up to my whopping 410W @ night.. I have a number of split core CT's best is a 20A/1V but I can only measure to minimum of 95VA due to the 0.02V minimum of the Metrel, no good for looking at standby currents.. I could really do with a 1A/1V CT making my minimum 5VA.. I know it also effects the maximum current too, due to Metrel 1V maximum.. I do have a 20A/20mA CT and wondered if I could alter the load resistor, thus, loading it 2kohm means I would have the 1A/1V ratio.. Adding a rotary switch I could add different loads that equate to 2A, 5A, 10A and 20A to 1V.. Can this be done? The CT I have is a PCB job, LCTA21CE 20A/20mA..

author: srb1954
author_rank: Super Contributor
timestamp: October 05, 2022, 08:33:06 pm
content: Quote from: diag7919 on October 05, 2022, 08:12:14 pmYou can boost the sensitivity of a CT by threading multiple turns of the conductor through the core although threading 20 turns through to get your desired sensitvity might not be that practical.If you are switching any load resistor to a CT make sure that there isALWAYSsome load on the CT otherwise the output voltage will rise very high into an open circuit load and might damage the CT or surrounding circuitry. CTs are normally operated into a very low resistance load.

author: coppice
author_rank: Super Contributor
timestamp: October 05, 2022, 09:36:47 pm
content: Quote from: diag7919 on October 05, 2022, 08:12:14 pmCTs are fine when you reduce the burden resistance from the specified value, but performance goes downhill as you increase the resistance. You can get a phase shift that varies with current, and the readings can start to become non-linear. The rate at which accuracy degrades varies a lot between CTs. If you are only looking for a rough estimate of where your power goes, you can probably get good enough results.

author: IsUniqueUserNameTheIssue?
author_rank: Newbie
timestamp: October 12, 2022, 09:45:35 am
content: Hi,Not sure if this is the right place to be asking, but some of you seem to know about current transformers. And searching the forums has not revealed much of relevance.I'm in the midst of making a battery terminal spot welder from a microwave transformer.I'm keen to have a way to test the output current during welding. I expect a fair amount of loss in the transformer so measuring the current in the MOT primary seems like a good thing to keep an eye on, but a bad reference for current in the secondary. The biggest clamp meter I have goes to 200A, so i'm keen to make a CT which can measure 1000A or 2000AMy thoughts thus far are to use a ferrite toroid of a suitable size for the high current welding cable. Have the secondary of the CT be maybe 100 turns. Aim for a ratio of 1A Pri : 1mA Sec, so 1000A Pri = 1000ma Sec. Calibrate it by running 10A through it to a dummy load and adjust the number of turns until I get 10ma.Has anyone got any experience doing this? Are there any pitfalls to lookout for? I know enough to make sure the secondary of the CT is always shorted to avoid high voltage.