Choices for Potentiometers - Stir Plate

[Jay-Brew]

Hi,

Attempting to build a stir plate. Have looked pretty extensively on HBT and the web. My understanding is that a 25 ohm 3 watt potentiometer, like those sold at Radio Shack is a good pot to go with.

Problem is that I am in Canada and can't order that. Haven't been able to find pot with these specs anywhere online.

What other specs for a pot would work (ohm range and watt range)? Not very knowledgeable in this area and any insight would be very appreciated.

Thanks!

 

[EarlyAmateurZymurgist]

Another option is to use a low-cost pulse-width modulation (PWM) controller. A PWM controller will not get anywhere near as hot as a potentiometer because PWM works by turning the voltage source on and off very quickly instead of through resistive loss. The duty cycle of the PWM circuit determines the voltage. The duty cycle is the ratio between the time that the voltage is on and the time that the voltage is off. For example, if the power supply voltage is 12V and the duty cycle is 50%, then the voltage that the motor sees is 6V. By varying the duty between 0% and 100%, we can very the voltage the motor sees between 0 volts and the power supply voltage. The only heat that we have to dissipate is the heat produced by the switching transistor or transistors. Heat can be reduced significantly by using a power FET with a low Rds(on).

http://www.ebay.ca/itm/12V-40V-10A-...552?pt=LH_DefaultDomain_0&hash=item5d44d25ce0


Note: an LED dimmer is a PWM controller.

 

[nickmv]

OP, I just grabbed one of these bad boys, disassembled it, and mounted it into a homemade wooden stirplate setip.

Works amazingly well.

 

[Al-Hala]

Greetings, fellow Canucklehead here.

A potentiometer (or "pot") is just a variable resistor (same as a rheostat) that drops the voltage across it from the input to what you want on the load.

Since there is no such thing as a free lunch, the "unwanted" voltage, (combined with the current = power) dissipates as heat, no matter what wiring configuration the pot is in.

EarlyAmateurZymurgist covered the pulse width side is excellent detail, so I won't add anything to that, other than some fans have their own controlling circuitry (rare, I know) that this would mess with.

Nickmv's idea is also the route I was planning to go; saves all the work of locating and paying for components, shipping (digikey.ca, or one of the other electronic supply houses).

Having said all that, the key point in the rating is the wattage. If the pot rating is higher than what it is likely to dissapate, the only downside is the loss of energy (whatever your per kilowatt rating is). If you do exceed the rating (thing running in a hot room with no cooling), it'll eventually cook and become inert. As you've seen, several people have no issues at the miniscule power the fans draw regulating with such.

While it's possible to work backwards from the load of the fan you intend to use to come up with another possible pot, I'd recommend doing just what Nickmv did; re-purpose the cheapest existing control you can find.

 

[EarlyAmateurZymurgist]

OP, I just grabbed one of these bad boys, disassembled it, and mounted it into a homemade wooden stirplate setip.

Works amazingly well.

That product looks like a fan with it's own USB connectable PWM controller. It's a cool idea.

 

[nickmv]

That product looks like a fan with it's own USB connectable PWM controller. It's a cool idea.

Here's my build using that fan. It works quite well, and works just fine on starters of 3L size or even larger, if needed.

 

[Jay-Brew]

Thanks everyone for your input. I truly appreciate it.

Not long after posting my question I stumbled on the Thermaltake mobile fan that nickmv mentioned. I have since seen some builds online using this and it sounds like it works fine. Seems like a no brainer route to go and will ultimately be cheaper I suspect too. Will be ordering one.

Thanks again.

 

[AtticBrewing]

The only downside to using the ThermalTake fan w/ built-in speed control is that the speed control does not go all the way to zero - minimum speed is 1300rpm (about 43% of max speed,) which makes starting the system a bit finicky.

I have one of these setups and I'm in the process of swapping in a proper pot for this reason.

 

[Jay-Brew]

I ended up having pretty good luck with the Thermaltake mobile fan. At first I was using a laptop rare earth magnet that wasn't working so great. Bought some rare earth magnets and rigged up a couple on the fan and it works very well with my one inch stir bar. The two inch stir bar not so well. In a two litre flask that is basically full the one inch easily gets the job done - I can get pretty much get to full speed without throwing the bar. So in the end it seems to work quite well for me so far.