Current control in inductor / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Current control in inductor

1 2 next

Move Thread

LAN_403

Tinkerer

Fri Sept 03 2010, 03:15PM

Registered Member #3141 Joined: Wed Aug 25 2010, 03:11PM
Location:
Posts: 6

I have 2 inductors in series.
Inductor #1 is 300uH
Inductor #2 is 15uH and is 180 degrees out of phase from #1

I need to control the current in #2
The total current is 10A at 10kHz

Can I reduce the current in inductor #2 by inserting a bypass between the 2 inductors?

Tinkerer

radiotech

Fri Sept 03 2010, 06:34PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

In a series circuit , AC or DC the current is the same everywhere.

Tinkerer

Fri Sept 03 2010, 11:49PM

Registered Member #3141 Joined: Wed Aug 25 2010, 03:11PM
Location:
Posts: 6

Thank you for the feedback.
I guess I have not described my problem well.

I started out with a low power, pulsed circuit. By adding a Potentiometer, between the two inductors, bypassing some of the current from Inductor #1, I could regulate the current in inductor #2 to some degree.

As I increased the current in the circuit, I changed the potentiometer for a Mosfet that I used as a variable resistor. This still worked fine.

Now I looked for a way to recycle some of the power with a LC tank. The recycling of the power works fine. The function of the circuit works fine, however, since the current now goes in both directions, the Mosfet does not work anymore as a variable resistor.

So I need to find a different way to regulate the current in #2 inductor.

The #2 inductor serves as an induction balance. The regulation of it's current is for fine adjustment of the induction balance.

any help is most appreciated.

Tinkerer

radiotech

Sat Sept 04 2010, 02:04AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Could you post a schematic for the project showing voltages, currents, frequency and values of the parts used along with description of what values you need, and what you have measured.

Tinkerer

Sat Sept 04 2010, 03:23PM

Registered Member #3141 Joined: Wed Aug 25 2010, 03:11PM
Location:
Posts: 6

Thanks for coming back.
Attached is an EAGLE schematic with the original circuit that worked OK

Then there is a newer design, recycling the power, using a Mosfet.
The timing is different.
This works beautiful, but I can not use the current control on the 15uH coil anymore.

Another design using an IGBT instead of the Mosfet. The simulation looks quite different, however with the real circuit there is not much difference. This may be because of the choice of the IGBT, which is the HGTG12N60C3D, for which I do not have the simulation.

This is the TX of a extremely deep going, discriminating metal detector. It works very well so far in the lab.

Tinkerer

radiotech

Sat Sept 04 2010, 05:07PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

I might suggest you redefine the problem this way. A method of controlling the number of ampere-turns in inductor #2, while keeping the inductance constant is needed .

This detector looks like a Hughes Balance which if disturbed by external induction will shift the ouput as a phase retard/advance signal depending on whether the metal is ferrous or not.

this might be of interest

Tinkerer

Sat Sept 04 2010, 07:28PM

Registered Member #3141 Joined: Wed Aug 25 2010, 03:11PM
Location:
Posts: 6

Thanks for the link.
It is indeed the Hughes Induction Balance principle that is being used. The precise balance is achieved by the right proportion of Ampere Turns in the coils. (There is also a RX coil).
However, very slight movements of the coils during assembly, as well as highly mineralized soil, upset the balance.
My Mosfet variable resistor bypass solved this problem perfectly. The resistor could be MCU controlled.

But, with the bipolar current this does not work anymore. so I need to find a method to replace the
Mosfet variable resistor.

Tinkerer

radiotech

Sat Sept 04 2010, 07:37PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The bipolar transistor may be conducting C-E on the opposite polarity of the AC voltage and damping the coil.Try using the transistor as a unipolar load for a bridge rectifier whose AC terminals are connected across the coil.

Tinkerer

Sat Sept 04 2010, 11:22PM

Registered Member #3141 Joined: Wed Aug 25 2010, 03:11PM
Location:
Posts: 6

Thanks for the suggestion.
The Bridge rectifier's switching noise spoils the signal. The signal is in the microvolt range.
But I looked at the numbers again. A simple non inductive 1k Ohm 5W potentiometer would do.
Not so easy to get.

A 1K Ohm digital potentiometer of 5W?

The largest I have found so far is 100mW. But how are they built? could I build a 5W one?
A matched pair of P and N Mosfets? A HalfBridge?

Tinkerer

radiotech

Sun Sept 05 2010, 03:05AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The Spectrol 860 is a 10 turn 8 watt 0.25% linearity potentiometer.

Occasionaly coilmakers alter the resitivity of portions of their windings. Antenna makers too. Your resitivity is controlled by skin effect to an extent.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob