Mini Project: Magnetic FEA – Current Sense Coil Position Dependency

One method of measuring current through a wire is using a current sense coil. This is a mostly nonintrusive way of measuring current by detecting the magnetic field around the wire, however it only works for AC signals. In this setup we have a coil of wire wrapped around a ferromagnetic toroid. We then pass our AC current carrying wire through the centre of this toroid and measure the voltage induced on the coil. Due to the very high permeability of the toroid, we can assume that the magnetic field inside it is insensitive to the exact location of the wire passing through the toroid.

A voltage is induced on the orange coil when an AC current flows through the red wire

I wanted to investigate this relationship between the sensitivity to the location of the wire inside the toroid and the permeability of the toroid. This can inform how high the permeability needs to be before we can assume that the position of the wire is not important. To do this I used the FEMM finite element magnetics package. This allows me to create an arbitrary geometry of coils and toroid and measure the magnetic field inside the toroid.

But first, a little background on the theory…

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Project: Nixie Clock (upgrade) – Final Schematic and PCB

One big change since I did the first version of the clock is my access to professionally made PCBs. At the time, I was only able to produce PCBs via hand etching or using my home-made PCB mill. A board like this requires at least a double sided design which is not easy using the above methods and so I used veroboard. This is painfully slow and messy.


For the new version I will use a professionally made two layer PCB.

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Project: Nixie Clock – MC34063 Boost Converter (upgrade)

I really like my Nixie Clock that I built two years ago – but I don’t like to leave it on when I’m not around. The reason for this is that the Nixie tubes are powered directly from the mains which means that touching the Nixie tubes could result in a dangerous electric shock. While the tubes are protected under glass I don’t like the thought of someone accidentally tripping over it and getting electrocuted. So I decided to redesign the clock.

I am going to power the clock from a 9V wall supply and boost that voltage up to 180V to light the Nixie tubes. While this is still quite a high voltage, it is isolated from the mains and is much less dangerous.

My design is largely based on Threeneuron’s Pile o’Poo.


The schematic for the 180V power supply

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Micro Motor Controller – Code and Gerbers

For the Robox project,  I needed a board to program and control a servo and a DC motor – I could probably have used something off-the-shelf, but I prefer to have a board specialised for the job.

The board I designed features an ATtiny828 microcontroller with 16 general use IO, two PWM outputs to control two 6V servos and an Allegro A3916 dual h-bridge to control two DC motors (or one stepper motor) at up to 1A per channel. This is more of a general use motor control board and could be useful in a bunch of projects.



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