Armed with a components list typically being rheostat, PC case fan, rare earth magnets,12v mobile phone charger I set about getting them together. I made my way to Maplins. After 5 mins remembered how much I dislike Maplins and headed for my local independent electronics shop. Best move I made, why? instead of the rheostat, which he pointed out could burn out if left on too long, he recommended instead using diodes placed in series to reduce the voltage to get the desired speed. Diodes typically cause a volt drop of 0.9V each and dont heat up. He also told me I could get these from the circuit boards contained in PC power supplies, just heat them up (in a well ventilated space) and remove them. Bought a 12 way switch and a chicken head knob (had to be bought just for the name)
So next move, went to my mates PC shop and took a variety of case fans, duff power supplies, old hard drives and a cd-drive. Stripped the power supply and got say 10 diodes and soldered them together. Stripped the cd-drive to get at the stainless steel spindle, this would be my stir bar. Then mocked it up to gauge the distance required between vessel and magnet. By soldering the diodes together, I could quite easily find out what voltage worked best, by simply placing the supply lead at different numbers of diodes.
Tip one - use diodes to regulate the voltage
Now for the fans, there are two types single pole (two wires) and two pole (four wires). Most designs I have seen use the single pole, the disadvantage is that they require pretty much 12v just to start. Once started you need the rheostat to reduce the speed for stirring. I found that by using the 2 pole they need a much lower voltage to start. Hence no need for the rheostat. Secondly , the single pole fans have a lot of play between the fan case and part that carries the blades, 2 pole are a better quality and have very little play. This is important because if your magnets are really strong they pull the two components together and the fan wont spin.
Tip 2 use 2 pole case fans
I got a 12 way switch which meant I could have a variety of speeds, as with the rheostat. This I found necessary as different vessel bottoms required different speeds to stir correctly.
So for a princely sum of £2 I have a 6 speed magnetic stir plate that works with a number of vessels. I used it for the first time last week and it was running constantly for 36hrs and in my living room. It was really quiet.
Different speeds shown in the movie clip, the bowl by the way is 10in in diameter.
Yeast growth from 1 sachet of SO4 after 24hrs
