19 Apr 202319 Apr 2023 11

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

Original Design Idea from

Magnetic Stirrer

http://www.experimenten.nl/magneetroerder.html

19 Apr 202319 Apr 2023 22

ControllerParts

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

19 Apr 202319 Apr 2023 33

Adjustable Linear Regulators

The Other Voltages page showed how a fixed voltage regulator like the 7805 could be rigged to give a variable output. The morecommonly used (but slightly more expensive) adjustable regulators such as the LM317 or L200 are essentially similar, the LM317 beingat heart a 1.25v regulator, the L200 a 2.77v device.

Their advantages include higher current handling (up to 1.5 amps with a 317T, 2A with the L200) and a much lower Iq value, allowinghigher value resistors to be used without large errors in calculated output voltage due to Iq variations. This means readily-available"knob" potentiometers can be used to adjust the output.

Most published regulator circuits give an output down to the minimum possible – with fan control there's little point in going below about6v, as you're wasting a lot of potentiometer track that could be used for better sensitivity. Adding a fixed resistor in series with the potachieves this.

The LM317 has a Reference Voltage of 1.25v between the output and adjustment pins, and with the circuit shown, in similar vein to theprevious example,

Variable Speed Controller 1

19 Apr 202319 Apr 2023 44

Vout = 1.25(1+ [R2 + VR1] / R1) + (Iq * [R2+VR1])

As said, the Iq value is very much lower (typically 50uA = .00005A) and can usually be ignored in calculating the output.

With the values shown in the circuit above, the calculated output will be from 6.5v up to 11.7v, ideal for fans.

As pointed out earlier, some volts are lost across the regulator transistor junctions, so from a 12v input the output is limited to around10v in any case (10.25v on mine, with a 0.2A fan) . I've designed for 11.7v because potentiometers have a pretty broad tolerance range(+/-20% is common), so there's a bit of leeway if the actual pot resistance is on the low side of nominal.

Construction

Parts List

IC1 LM317T

C1100n (0.1uF) 16v ceramic ormylar film.

C2 1uF 16v tantalum bead

R1 240R 0.25W 5% or better

R2 1k 0.25W 5% or better

VR11k lin, 16mm pcb mountingpotentiometer

MiscTO220 heat-sink & paste, controlknob

Variable Speed Controller 2

19 Apr 202319 Apr 2023 55

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, ifnecessary) 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 6vat 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), ortheir 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, lowestcomponents (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 or14.

Variable Speed Controller 3

19 Apr 202319 Apr 2023 66

Check the polarity of the tantalum electrolytic C2 before soldering – there's usually a plussign (+) 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 metalcase 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 issimpler 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 earthmagnets didn’t rotate the flea as well

Flea and 2 litre erlynmeyer flask from Test Tubes

Variable Speed Controller 3

19 Apr 202319 Apr 2023 77

Flea or Stir Bar

Flea (Stir bar)

19 Apr 202319 Apr 2023 88

Construction Heat belt to keep yeast warmish ~ 23oC

Blutak to hold magnet

Scrap perspex

19 Apr 202319 Apr 2023 99

In Operation

Click on image to run video

19 Apr 202319 Apr 2023 1010

Result – Happy Yeast ready to pitch

Click on image to run video