Stir plate woes!

[Brewer Gerard]

Starting to pull my hair out on this one. Started by trying to use a 12v adapter to power this because i used a 12V fan from a PC. This was way over gunned and nearly took off when i powered it up. Ordered a 9v adapter and tried that in the circuit to find that this was still too much.

So i introduced a 10K pot to try and control the speed which either kills the fan or once i adjust beyond a certain point the fan accelerates to full speed just like the pot is not in the circuit at all.

Having gone back and actually read a thread on a build i noted that a 7.5V adapter was used. Ordered one and when i connected it to the fan it hardly revved at all.

Any tips. Would a 100Ω pot provide the sensitivity i require?

 

[jflongo]

100 ohm for 7.5V is way too much. I use a 25 ohm with my 8V, and works well.

 

[deadfall]

I took a simpler route to make my stir plate. I skipped the pot all together. I had a transformer similar to this one.

Mine works pretty good on 3volts.

 

[Brewer Gerard]

100 ohm for 7.5V is way too much. I use a 25 ohm with my 8V, and works well.

That a 25Ω pot right?

 

[ekjohns]

You need a 50 ohm pot at radio shack. The problem with larger pots is the range that they add usabkd resistance. Because the fans pull such low amount of watts at 12v something like 10 ohms will drop the voltage by a few volts. On my build if I used a 9v power supply with 40 ohm resistor the fan would not spin. So 100k ohm pot would give you a very small workable range. Aka on or off

 

[Brewer Gerard]

Tried the 50Ω pot but the circuit still behaves the same. Just to clarify the wiring, I have done something along the lines of whats below.

Am i way off?

 

[ekjohns]

you connected the resistor in parallel it needs to be connected in series. As in + (power supply) - resistor - fan - (-) power supply

 

[IslandLizard]

you connected the resistor in parallel it needs to be connected in series. As in + (power supply) - resistor - fan - (-) power supply

NO, he has the correct circuitry. But the 50 ohm pot maybe too large, so he has a very small range where it works, like 1/10 of the total scale. Plus regular pots may not have enough wattage to carry the 40+ mA required to run the fan.

There is a schematic around using an LM317 voltage regulator. That would be my preference.

 

[mbbransc]

25ohm with 6V power supply works great for my stir plate. Got the 25ohm pot from Radio Shack.

 

[HDIr0n]

This is why I went with one of these fans.

http://www.amazon.com/dp/B00080G0BK/?tag=skimlinks_replacement-20

The plug and play aspect was very appealing.

 

[day_trippr]

I would disconnect the wire from the "bottom" leg of the pot and the DC return. There's not only no advantage having that connection extant, even with the fan turned "off" the pot is sinking current, and I would suspect it makes the adjustability that much more "touchy"...

Cheers!

 

[Brewer Gerard]

I would disconnect the wire from the "bottom" leg of the pot and the DC return. There's not only no advantage having that connection extant, even with the fan turned "off" the pot is sinking current, and I would suspect it makes the adjustability that much more "touchy"...

Cheers!

day_trippr wins the prize!! Can't believe this did not occur to me. Many thanks! My stir plate is at long last fully functional. What are these things for again?