potentiometer for stir plate

[beerboar]

I am hoping someone can help me. i am building a stir plate and keep buying the wrong potentiometer. I am using a DC12V  1.60A computer fan with a 12v phone charger. what one should i buy?  the 47k one i got keeps sparking.

thanks in advance.

[Kevco5]

This is what I used when building my own stir plate.

http://www.ebay.co.uk/itm/290930404661

[johnrm]

I used more-or-less the same thing too.
On my fan I use a curved magnet form an old Hard Drive.
I find that the fan motor struggles with a heavy yeast.
Also, the Stirbar tends to fly off.

[LordEoin]

i remember trying to pick a potentiometer for my stirplate and it wrecked my head. So much maths!
I really can't rememeber what resistance I went for in the end, but a bit of Googling suggests this one:
http://www.maplin.co.uk/p/47k-andohm-miniature-log-single-gang-potentiometer-jm78k

This page tells you how to work out the resistance of your potentiometer based on the power supply and the fan:
http://fearless-assassins.com/tutorials/article/259-making-your-own-computer-fan-controller/

[JD]

OK guys, things are a little more involved that just picking the resistance of the potentiometer. Bearboar's pots are sparking and seem to do so no matter what resistance he uses. This is because his problem is not about resistance but about power rating.

Lets have a look at his numbers: a 12 V supply and a 12V fan that draws 1.6A. This fan consumes approximately 19 watts (12V x 1.6A = 19.2W). 

Let's look at the fan's resistance. We calculate this by dividing the voltage across the fan by the current flowing through it (not entirely accurate but good enough as a first approximation). Lets assume there is no potentiometer in series with the motor to begin with. This gives us a fan resistance of  12/1.6 or 7.5 ohms.

Now, we have a 12V supply and the fan is 12V fan, the assumption is that if we reduce the voltage across the fan, the fan will slow down. Let's assume the fan will stop when we reach 4V (total guess). This means that we need to lose 8V. We will use a potentiometer to do this. What resistance should it be?

The pot will drop 8V and the motor will drop 4V. Assuming the motor's resistance doesn't change as it slows down, the 4V on the fan and the 7.5ohms fan resistance means 0.5333A flows through the motor. Since the pot is in series with the fan, the same current flows through the pot. The pot has 8V across it, so its resistance must be 8V / 0.5333A or 15ohms.

So, what's needed is a pot that has a max resistance in the region of 15ohms and a power rating of about 20W. Something like http://www.amazon.com/Amico-Wirewound-Potentiometer-Variable-Resistor/dp/B0087YIQ6Q/ref=sr_1_2?s=industrial&ie=UTF8&qid=1398968961&sr=1-2&keywords=potentiometer+wirewound. As you can see, it is not cheap.

This type of thing is a going to get quite hot. A far better alternative is to use a proper motor speed controller.

/JD

[Will_D]

+1 JD

The typical "pot" that you buy is meant to act as a voltage divider and not a power sink!

Typically they are like fixed resistors and are usually rated at less than one watt! (typically 200 mWatt)

They are used to provide a control voltage to something like a variable voltage regulater!

No wonder some are "sparking"

[beerboar]

thanks to all for all the suggestions. particularly to JD.
@JD seeing as you seem to really know can i ask a few more questions.
I don't mind forking out the 10$ for the pot you linked to but you mentioned it would get pretty hot! will it be safe in the enclosed wooden box which i hope to make my stir plate out of?

also if i went down to route of a proper motor speed controller..do you have any suggestions about which one and where to get them

sorry for my general ignorance but technical things really baffle me.

thanks

   

[beerboar]

i forgot to add that i am happy to buy a new fan if needs be. the one i have came out of an old desktop. i just don't want to get the wrong thing. i have already bought two pot from maplin and both spark.

so if i was to buy a new fan such ashttp://www.maplin.co.uk/p/thermaltake-80mm-high-performance-fan-zt87u
which is 12v but 0.18 amp
going by your calculations i would need a much small pot. probably one of the ones i already own.

[LordEoin]

- link the pots you bought from maplin
- post all information printed on your power supply
- post all information printed on your power adapter

Without these bits of info it's all guesswork

[Sorcerers Apprentice]

Here's an alternative guide to building a stir plate including circuit details


http://www.stirstarters.com/instructions.html

[JD]

Doing the same calcs for the new fan, we get a power usage of 12 x 0.18. This amounts to 2.16W. That's still outside the range of most pots you'd buy in Maplin.

Heat produced by electronics is proportional to the resistance of the circuit and the square of the current flowing through it.  We write this as H = K x R x I². K is some constant which we'll not be able to quantify without looking at the material the parts are made of etc. To allow comparison, we'll just carry it along as we calculate. R is the resistance and I is the current.

Your standard 100W light bulb draws about 0.3A. It has a resistance of 800ohms. Its heat output is is K x 800 x 0.09 or 72K. Your 25W pot has to deal with 1.6A and has 20ohms. This means that, at worst, it will produce heat = K x 20 x 2.56 or 51.2K.

Because the value of K is not the same in both cases, we'll handwave things and say that the two are the same. Your set-up, then, would be as hot as a 75W light bulb give or take 50%.

Imagine putting such a bulb inside a wooden box. How likely is that to be dangerous? I wouldn't do it and leave it running overnight, lets say.

Amazon sell a range of fan speed controllers for PCs. Use your small fan and a workable fan controller will be fairly cheap.
This http://www.amazon.co.uk/gp/product/B000292DO0/ref=s9_simh_gw_p147_d0_i2?pf_rd_m=A3P5ROKL5A1OLE&pf_rd_s=center-2&pf_rd_r=0WVXF5P1GZ6GDXB8E3NQ&pf_rd_t=101&pf_rd_p=455344027&pf_rd_i=468294
might do and is cheap.

[LordEoin]

i never has a problem with my maplin bought pot

[Shane Phelan]

This is what I used when building my own stir plate.

http://www.ebay.co.uk/itm/290930404661

I use something similar. They are designed to work with case fans and works perfectly throughout its range.