HELP! Magnetic stirrer

There are so many posts about building your own magnetic stirrer and it seems so easy but I am having a hard time! I'm not electronically minded and I'm trying to be with disastrous results. I need some advice/HELP!

So I have a 3W 25ohm rheostat which starts smoking after the device has been on for a while.

Fan is 12V DC, 0.32A, 3.84W
Adapter is Input 120V AC, 60 Hz, 13W, Output 12V DC, 600mA

I think it's the adapter or I need a bigger Watt Rheostat maybe, well I actually have no idea obviously.

Your rheostat wattage is less than your motor wattage, however your motor will only pull full wattage at full power, so you might get away with it.

Can you post a schematic or describe how its connected, I wonder if you've got a wire wrong.

It shouldnt be difficult to get that to work.

You know you might be able to get away without having a rheostat altogether, if you can rig up a means to move the rotating magnet away from the stirrer magnet then that will give you a speed control, the stirrer magnet will slow down as it gets further away from the motor magnet, then just run the motor flat out.

I haven't googled what a magnetic stirrer is but from your information you need a bigger reostat.

As it is, it may be dissipating up to 4.8W depending on where it is set.

Gobbledok said:

I haven't googled what a magnetic stirrer is but from your information you need a bigger reostat.

As it is, it may be dissipating up to 4.8W depending on where it is set.

Click to expand...

It is just a motor that spins under a small pad, a beaker can be placed on top of the pad and a small magnet placed inside the beaker. The spinning motor in turn spins the magnet, thus stirring the contents inside the beaker. This is a device a chemist would use.

Given a choice I would just build a small PWM circuit. We were trashing one of those stirrers at work several years ago so I gutted it out of curiosity. Small board driving the motor using PWM, not much to it. AC in to a small full wave bridge, some filter caps, then a LM7812 regulator. They use a LM324 so I would guess PWM starting with a triangle wave and then a comparator. Finally an output transistor so not much to it. Something like that or as mentioned a higher power pot.

Ron

At any rate, a rheostat is not the best way to control motor speed. Something more on the lines of what Ron suggested.

There are fans made which have a PWM input specifically for speed control. I probably wouldn't PWM the supply voltage to a normal brushless fan without a speed control input though, the fans internal electronics may behave unpredictably since they're designed to be powered by a specific DC voltage.

The unfortunate part is you said you were not electronically minded. This https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_120539_-1 kit will do it for you. I've used it, but not for a fan. I will assume you have a 12 VDC power supply.