Transcript Based on a 12V 100mm computer fan and a variable speed

Magnetic Stirrer
Based on a 12V 100mm
computer fan and a
variable speed controller
Original Design Idea from
http://www.experimenten.nl/magneetroerder.html
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Controller
Parts
 12V Fan 0.2A 100mm dia.
 Skill Drill 7.6V powerpack no load
~ 9V
 Heat sink - overkill salvaged from
old computer
 Variable speed controller derived
from web site
http://www.cpemma.co.uk/reg.html
See next pages
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Variable Speed Controller 1
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Variable Speed Controller 2
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Variable Speed Controller 3
The circuit was built on a piece of veroboard, 7 strips wide, 15 holes long. Two track cuts as shown by the red spots.
A 0.25" (6mm) drill bit can be used to make these, a few turns by hand will remove enough copper to break the track.
I used a slim conductive polymer potentiometer for VR1; but a 16mm PCB-mounting carbon type will fit, the link (and resistor, if
necessary) will go under the pot barrel.
The 317T requires a heat-sink; a 25°C/W type will run fans rated at up to about 10W (at 12v) as at the worst case it will be dropping 6v
at 0.42A, i.e. 2.5W.
I like the Maplin RN77J type shown on the photo at the bottom of the page, they're rated at 13.5°C/W but very neat (only 20mm tall), or
their clip-on KU50E is cheaper and at 25°C/W is suitable for a lot of fannage. (More on choosing a heat-sink)
Fit the heatsink and dry-assemble the other components to check there's no interference, then cut the tracks and start soldering, lowest
components (jumper links, then R2, etc). If there is any danger of the heatsink touching the j1 link it can be moved to columns 2, 4 or
14.
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Variable Speed Controller 3
Check the polarity of the tantalum electrolytic C2 before soldering – there's usually a plus
sign (+) and a line by the positive lead.
One warning – the tab of the 317T is connected to the regulator's output pin, so don't allow the heat-sink to touch any grounded metal
case parts, circuit-board links, etc, or a short-circuit will occur.
Photo shows an earlier design to use up some rather long heat-sinks – I had to bend the regulator legs to fit a drive bay, the Mark#2 is
simpler to build and neater.”
Note
I used 680 ohm resistor for R1 to suit stirrer under load
Used a bar magnet 50 mm long 15mm wide 10mm high - found that the rare earth
magnets didn’t rotate the flea as well
Flea and 2 litre erlynmeyer flask from Test Tubes
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Flea (Stir bar)
Flea or Stir Bar
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Construction
 Heat belt to keep yeast warmish ~ 23oC
 Blutak to hold magnet
 Scrap perspex
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In Operation
Click on image to run video
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Result – Happy Yeast ready to pitch
Click on image to run video
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