On the quest to my stir-plate build

[podz]

I have a 5 litre erlenmeyer flask just dying to be used for making a big lager starter, so I decided to build a stir-plate.

First off, I extracted an 8 cm square fan from an HP ML-350 hot-swappable power supply. Let me just make it clear that this is no ordinary computer fan; it's a jet engine operating at 12v DC, 9.1 watts. It's strong enough to blow itself sideways and quite noisy at full power.

Secondly, I extracted hard drive magnets from one of the hot-swappable SCSI hard drives. Again, I suspect that there is a big difference in size and thickness between the normal hard drive magnets and the SCSI ones. I attached one magnet to my 16kg kettlebell, and while lifting that didn't quite succeed, I was definitely able to life my 2kg cast-iron fondue kettle.

My testing power supply has a fine-step speed control (probably 20 steps from zero to full-speed), but I'd like to find a true variable-speed control without steps. Recommendations welcome.

For the stir bar, I plan to use 1, 2, or 3 geomag sticks connected to each other lengthwise, depending on the size of the flask being used. Anybody used geomag sticks before?

All suggestions and recommendations are very welcome. Keep in mind that I live in Finland and, as in most of Europe, the DIY culture is basically prohibited by the lobbying of professional labor unions. I can't just trot over to the local electronics store and purchase a variable speed control - I'd either need to salvage one from something or mail order one for an ungodly high price, assuming they would even sell it to a private person (or else we are talking import from US).

 

[podz]

Moving along, I decided to hack an old computer power supply into an adjustable voltage DC power supply.

So far, I managed to create a little distribution panel with multiple connections for 3.3v, 5v, and 12v. Next, I am going to mount banana jacks to the power supply box itself. On the project box for the stir plate, I will attach negative and positive power wires with banana plugs at the ends so they can plug directly into the power supply.

Inside of the stir plate project box, I am going to build an adjustable voltage regulator based on an LM317. Or I might make a separate little box for the voltage regulator and simply plug it in between the power supply and the stir plate, then it can be used for other things as well.

Probably, I am using this tutorial to build the adjustable voltage regulator.

 

[cannman]

SCSI

Ahhhhhhh........ Nothing can take away from the smell of skuzzy making a transfer to 1.44 ^_^ the good ol days

 

[Minbari]

Moving along, I decided to hack an old computer power supply into an adjustable voltage DC power supply.

So far, I managed to create a little distribution panel with multiple connections for 3.3v, 5v, and 12v. Next, I am going to mount banana jacks to the power supply box itself. On the project box for the stir plate, I will attach negative and positive power wires with banana plugs at the ends so they can plug directly into the power supply.

Inside of the stir plate project box, I am going to build an adjustable voltage regulator based on an LM317. Or I might make a separate little box for the voltage regulator and simply plug it in between the power supply and the stir plate, then it can be used for other things as well.

Probably, I am using this tutorial to build the adjustable voltage regulator.

just use the 12VDC output of the supply and put a 20 ohm rheostat in series with it. no need for all the complication with multiple voltages and regulators and all that. you arent going to need to stir it at mach II

if you do use the LM317, make sure you use a big heatsink. Those things get HOT.

 

[podz]

just use the 12VDC output of the supply and put a 20 ohm rheostat in series with it.

I've been trying to find one inside of my old electronics, but no luck so far. I got some bass, treble, and balance controls out of an old amp but they don't really work properly - they only turn on the motor when turned almost to the max position (not really useful).

 

[Minbari]

ya, most of those are 10k audio taper and not gonna work very well as power control. They might burn up anyway, certainly not designed to pass 9 watts of power.

 

[thekraken]

Have you thought about PWM? I've been looking into using one of these 555 timer pwm circuits to control the fan speed for my build. However, with my fan, at the right distance from the stir bar, it will spin just fine at 12V (so long as there is enough liquid in the flask). I haven't decided if I even need/want the speed control.

 

[podz]

Yeah, I would also use a PWM if I could just find one. Scavenging electronics is not possible here as we have government run electronic recycling centers and that's where everybody takes their stuff. Same all across Europe. I managed to get some old devices by posting local ads in facebook that I could come and pick things up and take them to the center, so I managed to get some dc motors, etc but haven't managed to find a single linear pot or PWM. Anything mail order is extremely expensive and comes from half way around the world.

 

[Minbari]

only other thought would be to make a multi speed version instead of variable speed.

get a 3 or 4 position rotary switch (or even a series of toggles) and put a different value resistor on each one. this would allow a "off", "low", "med", "high" kind of setting. if you used 4 toggles you could even wire them for up to 16 different speeds (2^4th)

the speed of the stir plate doesnt seem like it needs that fine of a tune. once it sucks the whirlpool down, its good to go.

 

[podz]

I've got a 4 position rotary switch from my wife's dead blender. But when you're dealing with a 9 watt fan, this is not your average computer fan... I mean, it's strong enough to chop off the tip of your finger if you mess around. I would like to gradually ramp up the speed of the stir stick instead of throwing it through the side of my glass.

 

[Minbari]

Then have it go OFF-1-2-3. (or a toggle and speed 1-2-3-4)

speed one is the slowest and have it charge through a large cap. this will ramp up the voltage slowely and give the fan a soft start. The other speeds might not need a soft-ramp once it is moving, but if they do, use a similar approach with a large RC time constant.