Having made a platform to mount them on, it’s time to start thinking about the electromagnets themselves. It doesn’t take me long to realise I know next to nothing about this area. My starting point is just what I remember from high school physics lessons – which were quite some time ago! (What did we do before Google?)
If nothing else, I have plenty of questions; how strong? how many? bought or home made? what voltage? how much current? wire gauge? number of turns? core material? power dissipation? positioning? mounting? The list goes on and on…
Fortunately, I’ve got somewhere to start – and that’s the original resistance unit, which uses a single neodymium magnet. I know where the magnet is positioned relative to the flywheel, and I if I can measure the strength, I should know what I’m aiming for with the electromagnet(s).
To measure the magnetic field, I can use a hall effect sensor. I happen to have an Allegro A1302 in my parts drawer which fits the bill. This is a very simple to use 3 terminal device, requiring just 5V power and ground, and producing an output voltage that is proportional to an applied magnetic field.
Recording the output voltage for various magnets will provide a means to compare the magnetic field strengths. The output of the A1302 is specified at 1.3mV per gauss, but the units aren’t a consideration for me at this stage, just the relative differences.
I’ve created a very simple breadboard circuit to provide the regulated 5V for the sensor using a LP2950 LDO regulator, and 12V power (from my bench supply) to an electromagnet. To help with consistency, I’ve routed a small groove in a block of MDF, and stuck the sensor to the bottom with double sided tape. This provides a gap of 2mm between the face of the sensor and the top of the groove, meaning I can place any magnet against the block, and will always be comparing the same air gap between sensor and magnet.
The following graph illustrates the relative field strengths of various magnets. Just for starters I have the original neodymium Tacx magnet, another neodymium magnet salvaged from an old hard disk drive, a fridge magnet, and a small £5 electromagnet from eBay.
It soon became clear that the strong neodymium magnets were saturating the sensor at a distance of 2mm, so for the purposes of this initial test I increased the distance to ~8mm with a small block of foam.
Hmm, that eBay magnet isn’t off to a very promising start! Next I’ll try winding some of my own electromagnets to see how they compare.