Simple coiled wire current transformer . - Page 1
Forum section: electronics / projects

author: SL4P
author_rank: Super Contributor
timestamp: March 17, 2022, 07:35:11 am
content: I’m thinking about checking whether a n AC load is drawing any current (240V mains), and figured that because I have very limited space, it’s long as I ensure electrical isolation, run a few turns of enamelled wire around the live conductor (or either), feed that into a tiny bridge and measure the output voltage to identify if the load is present.Maybe a small cap on the rectifier output to smooth the waveform going in to an (0-5V) analog input.Is this the right way to go for a simple solution, or is there a better way ?What sort of turns / voltage should I be expecting ?

author: Ian.M
author_rank: Super Contributor
timestamp: March 17, 2022, 08:05:43 am
content: That would give you insignificant magnetic coupling. A wire carrying current induces a magnetic field ROUND the wire not along it. To induce voltage in a coil you need changing magnetic voltage THROUGH the coil not round it. Its equivalent to putting coils at right angles to minimize their coupling.Have you got any space to sneak in a ferrite core you could wind say 50 or 100 turns of thin magnet wire on then pass your insulated mains conductor through to actually implement a current transformer?If not the next best option would be a pair of analog hall sensors on opposite sides of the wire oriented to detect the circumferential field and a differential amplifier to cancel out environmental magnetic fields, feeding an OPAMP` precision full-wave rectifier, then a low pass filter then your analog input.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2022, 08:39:40 am
content: For a current transformer the magnetic core is around the conductor. For only detecting the presence of a current and this no high needs for accuracy one would not need a a special core or much copper for the secondary. The core could be a bend iron wire with a small coil around (can be on one side of the mains wire).The expected voltage depends on the number of turns, loading and inductance. Ideally the current would transfer like the turns ratio the resistor sets the voltage. With too high a resistor / to low an inductance, the inductance would set the load and may also saturate the core. Directly to 5 V would be quite high. Normally one keeps the ouput voltage of the CT small. With many turns it can be enough to overcome a diode drop. Chances are one would need a little AC amplification to than get to a 5 V level.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 17, 2022, 10:21:01 am
content: If you've got a busted beyond repair SMPS, use the common mode choke as a CT. Join windings in series.

author: SL4P
author_rank: Super Contributor
timestamp: March 17, 2022, 10:44:20 am
content: Thanks for the thoughts.They’ve given me some ideas.Worst case! I have a few small ‘proper’ CT toroids - maybe it’s worth finding that extra 2cc of volume to squeeze it in !

author: jonpaul
author_rank: Super Contributor
timestamp: March 17, 2022, 05:41:54 pm
content: A mains freq CT needs a laminated iron core or tapewoundAir core or ferrite will not work.See the excellent Tektronix Circuit Concepts Oscilloscope Probes for info on current probes, much of that applies here.https://w140.com/tekwiki/images/6/62/062-1146-00.pdfJon

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2022, 05:58:25 pm
content: A soft ferrite core is not so bad for a current transformer. The main downside is a lower ( ~ 1/4) saturation and thus a lower maximum voltage for the output.Without a core, the CT transformer becomes a Rogowski coil and the signal is not current proportional to the primary current, but votlage proportional to the derivative of the current.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 17, 2022, 06:22:02 pm
content: It may be less bother to use a low current hall effect device like a DRV5032.