For a project I am doing I need to measure high currents. I was thinking if I could somehow use core saturation to measure the magnetic field, and thus the current in a close conductor. The idea is to have two the same LC oscillators, and then magnetise the ferrite core of one of those, and mix the output, use the difference frequency in a micro, a PIC to be precise. Either that, and the system should no use a shunt, or use one of those Hall sensors. So I did set up a quick experiment, using the small strong magnet I have as a simulated magnetic field from some turn of high current wire. The result is fascinating, the frequency deviation is *much* bigger then I expected!
Here is the test circuit: --------------------- +5V from USB | | | | |---- d | ------------------------>| BF245 | | | |---- s === || ( === 680p | --- N[]S || ( |_____________ | | 47 uF magnet || ( L1 | | | || ( 10mH === 1500p [ ]1k5 | | | | | /// /// /// ///
I measure 13313 Hz without magnet, and 95071 Hz with magnet close against the inductor! So say 13 kHz to 90 kHz, or more then a factor 7 frequency change! The amplitude does *not* change! By using two of these oscillators, and influencing only one, temperature effects get cancelled, a simple dual gate MOSFET mixer... lowpass, and you have a nice signal you can do all sorts of things with in a micro.
I think I do not have to explain this oscillator type here. As to the coils: One day I asked in the shop for 10 uH coils, the guy gave me 10 mH, these... (I think they are 10 mH, could be 4.7 too, have not measured these, do your sqrt LC if you like), I did not want to call the guy a moron, and though these would be fun to play with, as they are :-)
Here is a picture of the circuit with magnet far away: ftp://panteltje.com/pub/dc_current_sensor/osc_without_magnet_img_1790.jpg
The resulting scope picture: ftp://panteltje.com/pub/dc_current_sensor/freq_without_magnet_img_1794.jpg
Here is the picture with magnet against the ferrite cover of the coil: ftp://panteltje.com/pub/dc_current_sensor/osc_with_magnet_img_1797.jpg
The resulting scope picture: ftp://panteltje.com/pub/dc_current_sensor/freq_with_magnet_img_1795.jpg
The scope is connected to the BF245 source. The frequencies were measured at the gate with freq_pic :-) :
Any non linearity will not be an issue, as a lookup table is easily implemented in the PIC.
I love it when an idea works:-)
Copyright (c) Jan Panteltje 2010-always All Rights reserved. Nothing of this can be used without written permission from the Author. Certified Usenet prior art. Beware of the watchdog.