Old DC current measuring device - Page 1
Forum section: electronics / beginners

author: nForce
author_rank: Frequent Contributor
timestamp: January 04, 2019, 06:35:16 pm
content: This is a very old concept, maybe in the 70' were these devices used for measuring DC current:https://imgur.com/a/D9fjKFYDoes anyone understand how it works? In this schematic I1 is a DC current we are measuring, U~ is AC voltage. I think other signals are clear enough.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2019, 06:44:26 pm
content: This looks like a kind of transductor cor magnetic amplifier circuit. It usually requires inductors cores that are easy to saturate and high µ. An external current (usually rather large) drives the cores more towards saturation. This increases the AC current flowing through them. Using the 2 Transformers in series, but with opposing primary keeps the AC voltage appearing at the measured circuit small.

author: coppice
author_rank: Super Contributor
timestamp: January 04, 2019, 07:00:42 pm
content: The diagram is rather minimal, but this looks like a flux gate sensor. There has been a resurgence of activity with these, based on recent techniques for making them very small.

author: nForce
author_rank: Frequent Contributor
timestamp: January 04, 2019, 09:27:56 pm
content: Thanks, I have a few questions:What does the single phase rectifier do?And that ammeter, so here we are measuring this DC current I1?QuoteWhy is there an increase of AC current, when we saturate cores?

author: glarsson
author_rank: Frequent Contributor
timestamp: January 04, 2019, 09:57:55 pm
content: The rectifier turns the AC current (I2~) into a DC current (I2=) that can be measured by the ampere meter. The current is proportional to the big I1 we are trying to measure.The AC current through a cored coil or transformer is restricted by the magnetic field created by the current itself. When the core saturates the current increases dramatically as there is ni longer much to stop it. This is what happens when a 130V Weller soldering station is connected to 220V.

author: nForce
author_rank: Frequent Contributor
timestamp: January 05, 2019, 11:15:11 am
content: I didn't understand Kleinstein what he means by:QuoteAnd another thing, how do we get then this proportionality constant, because the original I1 current in proportional to the measured one at ammeter?

author: mikerj
author_rank: Super Contributor
timestamp: January 05, 2019, 11:30:32 am
content: Quote from: nForce on January 04, 2019, 09:27:56 pmCore saturation causes the inductance (and therefore inductive reactance) to rapidly drop to a small value.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 05, 2019, 02:20:25 pm
content: Quote from: nForce on January 05, 2019, 11:15:11 amThe proportionality constant would depend on the core material. It is even well possible to get nonlinear response especially at low currents.It would also depend on the amplitude of the AC voltage, especially if one wants to detect small currents.The setup is a little similar to a flux gate sensor or direct current transformer , though simpler and less sensitive.The flux gate sensor is different in that it is not just looking in the change in AC current, but uses a separate sense coil and is usually looking at the harmonics (or some features of the waveform) of the driving AC.

author: nForce
author_rank: Frequent Contributor
timestamp: January 05, 2019, 07:08:16 pm
content: What about DC magnetic flux and AC magnetic flux in the cores? I have read somewhere that the cores are trying to create zero total magnetic flux, so AC flux - DC flux = 0.

author: nForce
author_rank: Frequent Contributor
timestamp: January 07, 2019, 05:46:05 pm
content: Another thing: When does the secondary winding equal 0 V?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 11, 2019, 02:35:57 pm
content: There is a tendency for the flux to go towards 0, but not very strong. The more important point is that the flux can not go higher than a saturation value. So when there is already some DC flux, the AC part will eventually high that limit and than there will be a significant AC current.There will never be a true zero at the secondary, as even with rather soft cores (which should be the right choice) there is some magnetizing current, though not much. So there would be some offset.One could probably simulate such a circuit in something like LTspice. There are provisions to also simulate nonlinear magnetic cores - however I don't know how.

author: iMo
author_rank: Super Contributor
timestamp: January 11, 2019, 04:08:00 pm
content: ..a lot of magnetic core simul stuff in Bordodynov library..