Confused on Stirplate Speed Control

[Will_the-new-brewer]

I use 2 of the 4-wires on a PC fan. I have it wired to an on/off switch. I think the fan is spinning very fast so I am trying to find a way to fix that. I see all kinds of posts that say I need a PWM controller to adjust the speed on my PC fan; however, there a also posts of a standard AC potentiometer (dimmer switch) being used.

What is the real deal? I don't want to have to order something, but I live on Guam so will probably have to--not much dealing with PWMs on island.

 

[Amadeo38]

The potentiometer is the key here, more so than an on/off switch. I don’t use an on/off switch since at lowest setting the potentiometer turns the fan off. You’ll want to get one that jives well with the fan you’re using in terms of the ohms...I had a 10k ohm potentiometer and it pretty much only had two speeds (medium-high and full blast). I got a 300 ohm one and now it works at all speed levels!

 

[Will_the-new-brewer]

You’ll want to get one that jives well with the fan you’re using in terms of the ohms...

How do I determine which one to get for my fan?

 

[Amadeo38]

An engineer friend came over and calculated what I needed based on my setup (consisting of the power supply I had feeding my fan, and the fan itself). I don’t quite understand what he did, so I wish I could help more. I would bet a 300 ohm potentiometer will well for most applications, but perhaps someone else can chime in and be more helpful.

With what are you powering the fan?

 

[Will_the-new-brewer]

With what are you powering the fan?

AC/DC adapter...
- Output: 12V 1.5A

Powering:
12V 0.4A computer fan.

 

[Amadeo38]

Same exact setup as mine. The only difference would be the quality and/or lifespan state of the fan itself compared to mine, which was an old ball bearing style LED lit computer fan. I’d bet the 300ohm potentiometer is what you need.

 

[Will_the-new-brewer]

Awesome, thanks for the info!

 

[Will_the-new-brewer]

Now I can't seem to find what I think they look like....what I need. Where did you get yours? Can you post a link of one so I know what I am looking for?

 

[Amadeo38]

The same engineer who told me what I need actually had an extra of what I needed and gave it to me. Not helpful for you.

25W 300 Ohm Ceramic Wirewound Potentiometer Rotary Resistor Rheostat https://www.amazon.com/dp/B00JR3SBS4/?tag=skimlinks_replacement-20

 

[Will_the-new-brewer]

25W 300 Ohm Ceramic Wirewound Potentiometer Rotary Resistor Rheostat https://www.amazon.com/dp/B00JR3SBS4/?tag=skimlinks_replacement-20

Thanks. That is the one I was looking at too.

Potentiometers seems rather expensive/hard to get when I only want one speed. I may wire in resistors to get that speed and save some money:

https://www.radioshack.com/products/radioshack-330-ohm-1-2w-5-carbon-film-resistor-pk-5

 

[Amadeo38]

They are quite expensive so that’s probably a good thing to try first. I like having the option to start it slow and ramp up the speed so it doesn’t throw the stir magnet.

 

[ba-brewer]

Even though you could pick a single resistor to set the speed, having the ability to adjust is very helpful. I sometimes need to turn up or down as the starter progresses. I also use different size vessels and they need different speeds.

25W seems a bit high, but you would not have to worry about it over heating. Your fan has a resistance of 30ohm and 4.8W of power consumption.
If you get a 5W potentiometer you will be safe, I would not go lower than that. There is a general rule of thumb to use parts twice the expected power requirement, which means you probably should use some thing closer to 10W.

If you put a 30ohm resistor in series with your fan the voltage will drop in half and use 2.5Watts, if you need to reduce speed below half voltage it will have more power on the resistor.

Power resistors are not cheap either.

I have a stir plate with a potentiometer and one with a adjustable wall transformer, the transformer is a bit coarse but it works OK, the price is about the same as the pot Amadeo38 linked, but with looking you may find something cheaper.

 

[Will_the-new-brewer]

Even though you could pick a single resistor to set the speed, having the ability to adjust is very helpful. I sometimes need to turn up or down as the starter progresses. I also use different size vessels and they need different speeds.
....
There is a general rule of thumb to use parts twice the expected power requirement, which means you probably should use some thing closer to 10W.

Good point. I thought of using multiple resistors tied through power switches to "adjust" the overall speed, but I am not a specific fan for that idea. I do have a buddy of mine looking locally for a 300 ohm potentiometer.

Also, thanks for the rule of thumb...I wasn't tracking that one.

 

[Will_the-new-brewer]

I tried two AC/DC power supplies. Attached are photos taken--It is obvious the 9V is going slower; however, I don't think it is going slow enough.

Keep in mind that I do not have a flask yet so this old measuring bowl worked for a test--it has 1L of water in it.

 

[Will_the-new-brewer]

Found my solution...adjustable universal DC power supply (1.5V to 12V...7 settings). The 4.5V is the minimum setting to get the fan to actually move.

 

[Beer666]

I bought a pre wired pot and LCD readout from ebay for £3.50. Also has an on/off switch. I can try and find the link if anyone is interested.

Edit. One of these 12v regulators.

 

[piojo]

1 W is all you need for the potentiometer. Specifically can use a pot with a power rating equal to the fan's power rating. Multiply it by five or ten if you want to be positive it'll stay quite cool to the touch. And know that a potentiometer and simple PWM aren't the most sophisticated power control you can do with a 4-wire fan, but it's okay. You can also use PWM to send a signal that tells the fan which speed to turn at, but that needs a circuit and soldering. It costs a few bucks for parts, then a few more for the box that holds the circuit.

The sophisticated speed controller is what I made, but maybe I shouldn't talk because it stopped working after three months! I suspect a wire is loose, but I won't know until I take it apart.