Stir Plate Assistance

[Tricerahops220]

I built a stir plate today but it seems to have some issues with it. When I first go to turn the stir plate on, it appears as though the magnets prevent the fan from starting to spin. Has anyone else has these problems? Is the fan too weak? The size of the wire on the fan is fairly small so I imagine the draw can’t be very strong. Also, once I help start the fan to spin, I don’t get much of a vortex even when I have the potentiometer is maxed out (photos attached).

Any help would be much appreciated

Thanks

 

[Jag75]

Is the fan up against the top too much ? Did you turn it on before you enclosed it to see how well it spun and if the magnet was centered?

 

[mongoose33]

On my stir plate the flask ended up actually touching the magnets, which is NG. The flask wouldn't sit properly on the screws holding the fan to the box, so I took a couple thin pieces of plastic, hot-glued them to the top of the stir plate to life up the flask just a little bit, and that was that.

 

[IslandLizard]

What's the fan's power? Voltage + Amperage.
Are you regulating the power to it to control the speed? If so, how? <== OK, potentiometer.
How easy is it to stop the fan by hand once it's spinning?

I have computer fans that if I (accidentally) stick my hand in them it ends with blood and pain.

 

[Tricerahops220]

Is the fan up against the top too much ? Did you turn it on before you enclosed it to see how well it spun and if the magnet was centered?

The fan sits about 1/8"-1/4" from the backside of the plastic. Would cutting a hole in the top of the plastic help? I turned it on before I enclosed it and yes the magnet is centered. I know it is centered because I had screwed that up the first time and it did have a wobble to it.

 

[Tricerahops220]

What's the fan's power? Voltage + Amperage.
Are you regulating the power to it to control the speed? If so, how? <== OK, potentiometer.
How easy is it to stop the fan by hand once it's spinning?

I have computer fans that if I (accidentally) stick my hand in them it ends with blood and pain.

its a 120mm x 120 mm x 25 mm Uxcell fan from Amazon. 12VDC and 0.16A (ding ding ding) so it is certainly too small. Its real easy to stop the fan when it is spinning and there is no blood involved.

Any suggestions on other DC fans?

 

[Jag75]

Sounds like you found your issue. Sounds like @IslandLizard might have to bolt his stir plate to the counter to keep it from lift off lol.

 

[balrog]

I've made 3
Used PC fans of varying sizes/speeds
All can be stopped easily when running, without too much issue.

You would hear it if it were scraping on the enclosure; but it might clear without the flask/liquid load on top, but bind once the lid's loaded with weight.

 

[kh54s10]

I scavenged my fan from a very old computer so I don't know what it was. I got a potentiometer from Radio Shack when they still existed. I think I got one to specs in a thread. It is very touchy. For power I tried "wall warts" from old answering machines etc. It took several tries to find one that gave the right power. That may be a place to look.

 

[balrog]

That's a good point. I did not use old cell phone wall warts, I used an old laptop 2A - 12v power brick. So maybe the supplied current is the issue.

 

[IslandLizard]

12VDC and 0.16A

That should be sufficient power. Now it's a fairly large size fan, could that be the issue?
Most builds use 80mm fans, like in @mongoose33's post, which looks like being a ThermalTake fan.

Do you have a spacer between the fan and the case? You need enough space to clear the magnets, but not excessive as you'd loose coupling power with the bar.

So maybe the supplied current is the issue.

Good point. It needs to deliver 160 mA and 12V, or at least 9.
The thinness of the feed wires is pretty much immaterial here, unless they're 100 feet long.

How does the fan operate outside the case?

 

[kh54s10]

That's a good point. I did not use old cell phone wall warts, I used an old laptop 2A - 12v power brick. So maybe the supplied current is the issue.

The one I ended up with is 9vDC - 500mA. I don't remember what the ones that didn't work as well were. That was years ago. I still have the ones that I didn't use but they are in storage so I can't look.

 

[Tricerahops220]

Here is what i have for a power adaptor: https://www.amazon.com/gp/product/B06Y1LF8T5/ref=ppx_od_dt_b_asin_title_s00?ie=UTF8&psc=1 . I didn't have one laying around the house that I could cannibalize.

Then a 1k potentiometer. There is about 1/8" to 1/4" to the underside of the top surface of the magnet. I don't think it has anything to do with the spacing though. It seems like either the magnets are affecting the fan or the fan is just weak.

 

[IslandLizard]

Have you used the power source directly on the fan, without the potentiometer? That's the most direct. If that fan doesn't spin well that way, something is wrong.

Multimeter?

 

[BrewerBrad82]

By simply slapping a potentiometer inline with the power supply will only really serve to limit the current applied to the motor. Using a cheap PWM module from ebay or wiring a super simple circuit with a LM317 adjustable linear voltage regulator will effectively limit the voltage to the fan while maintaining full current draw. Done this way, the fan will have all the power it needs (current) to start, and the speed is then controlled by adjusting the voltage. Google LM317 schematics for the circuit. All of these parts are very cheap.

 

[Tricerahops220]

Have you used the power source directly on the fan, without the potentiometer? That's the most direct. If that fan doesn't spin well that way, something is wrong.

Multimeter?

When I was building it originally, I removed the potentiometer temporarily and tried this and the fan ran at about the same speed and with the same resistance as

I have a 120V AC fan at work which I can take that is about the same size but the thing is a monster weight wise. And my whole stir plate is running on DC with the power converter so that is pretty much out of the question unless anyone has any other ideas.

 

[Nagorg]

I used to have problems with the stir-plate I built. Then I got this and now those problems are a thing of the past..

 

[Jag75]

I used to have problems with the stir-plate I built. Then I got this and now those problems are a thing of the past..

Man that's nice . I got like a 60 dollar one at morebeer . It works but wish I had temp control on it