DIY Stir Plate Questions

[wolves63]

So far I have been able to acquire for free an 80mm case fan, the hard drive magnets needed, a cigar box, and a 12v 500mA AC to DC adapter and a stir bar.

The one thing I am needing is something to regulate the voltage. I have been looking at pots as an option, but most of those I am looking at are 2W at the highest. Based on calculations the AC/DC adapter will be pushing out 6W of power, thus making the pots wattage limit exceeded. What other suggestions to you guys have in regards to this or where can I find one that will meet my needs?

 

[DesignatedDrinker]

I would get a smaller power adapter. I use a 9v with a potentiometer and with 2L of water in the flask and the setting at half, it creats a vortex that touches the bottom.

 

[wolves63]

what is your A output? and potentiometer limits?

 

[DesignatedDrinker]

what is your A output? and potentiometer limits?

not sure about the potentiometer. But the 9v power supply says output 300ma

 

[charliehorse]

I bought a Pulse Width Modulator like this one off of eBay for about $7 delivered:
http://www.rakuten.com/prod/12v-40v...P&adid=18179&gclid=CPeuqqCuu74CFYcDOgodTzIAfA

 

[wolves63]

not sure about the potentiometer. But the 9v power supply says output 300ma

You will be putting out about 3 watts which if you are using a 1w potentiometer (most likely with radioshack), you are running it hot and it has the chance of setting things on fire.

I might look into the PWM for CPU fans.

 

[motleybrew]

Not to toot my own horn...well, maybe:

https://www.homebrewtalk.com/f51/another-stir-plate-447814/

 

[DesignatedDrinker]

You will be putting out about 3 watts which if you are using a 1w potentiometer (most likely with radioshack), you are running it hot and it has the chance of setting things on fire.



I might look into the PWM for CPU fans.

Ive used it probably 50 times and it hasn't had a problem yet. But why not just get a bigger potentiometer?

 

[wolves63]

Ive used it probably 50 times and it hasn't had a problem yet. But why not just get a bigger potentiometer?

They are more expensive than a PWM, that is the main reason. The problems associated with using a potentiometer with a lower watt rating that what you are using is that is might get too hot and cause problems, at the very least it will shorten its life. Most likely you won't have a problem, but the risk is increased when you work outside of tolerances.

 

[byrone]

When I created mine I used a simple 555 timer circuit for PWM. You can find tons of sample circuits on google and it only a few dollars in parts. Just search "555 timer pwm circuit". Using pwm is much better way of controlling the fan speed compared to a pot inline with a power supply.

 

[Litebread]

When I built me stir plate I used an LM317t variable voltage regulator. They are just a couple dollars and all you really need to complete the circuit is a resistor and potentiometer, both sized based on your input voltage, and a small heatsink. Total parts cost less than $10 and all the needed parts can be had from radioshack.

PWM is nice and I even built one but found that the control wasn't noticeably better on the 12Vdc fans I had on hand and the simplicity of the LM317t circuit won out in the end.

-Litebread

 

[MaxStout]

The trouble with using potentiometers in series is that the power dissipated at the pot is a function of voltage drop, not voltage of the power supply. (voltage drop)X(current)=watts dissipated. So, the lower you run the fan, the greater the voltage drop. If you run your fan at or near maximum, there will be relatively little power loss at the pot. That might explain why some people have been able to use those little 1W pots with 12V power supplies with no problems.

That said, either get a pot that is rated for the maximum amount of wattage it will be dissipating. Otherwise, PWM is the way to go.