Built Stir Plate need Assistance

So I built a stir plate from scratch.

I bought the following

8"x6"x4" project box
5K Ohm Potentiometer
Led light
On/Off Toggle Switch
Wire
Heat Shrink Tubes
9 Watt 300M Power Supply
Power Jack Adaptor
Power Jack

Had:
2 RE Mags
1 80mm Fan (from really old comp)

Cost $35

Hooked all together and it worked but not like I wanted it to . The fan only works if the POT is set on high. Then I can adjust it lower but if I go too low the fan stops spinning.

The original plans call for an 80mm fan with a 500 ohm Pot. As you can see I got a 5K and 80mm.

Is it the POT or the fan? Or do I need to get both?

Any help would be great...

Thanks

- WW

the resistance of your pot is 10 times larger than recommended, and even the recommended one was probably higher than needed... So the range in which you can really adjust the fan speed is only like the first 10% (or less) of the adjustment region of the potentiometer, which explains why the fan stops on you once you adjust past a certain point.

Using a potentiometer is really a hack-job kind of way to control the speed of a fan to begin with, but at the very least you need to use a potentiometer of the right value (and you definitely need to use one with a sufficient power rating so it doesn't go up in smoke)

If you're interested, I think I have some basic voltage regulator boards around, which would be a much more robust solution, and should allow you to use your existing potentiometer to control it.

Thanks for the advice. That would be great.

I know its a hack job but it is what I had. Ok so what needs to be done next, if I use a basic voltage regulator board and how would I attach it?

Should I get a much smaller POT?

- WW

A voltage regulator is pretty easy to hook up: power in (from your power supply), power out (to the fan), and connection to the potentiometer. With a voltage regulator board your existing pot will work fine, it will be used for adjustment, not directly controlling the fan current.

Gotcha! Thanks again for the tip. Can you get a VRB at say Radioshack?

- WW

Will I also need a heat sink?

- WW

A voltage regulator board, at its simplest, would consist of a voltage regulator, a resistor, potentiometer, and a couple of capacitors. You may be able to buy all the parts at radio shack, though lately (at least in my area) they haven't been stocking hardly any components, so YMMV. Look at the datasheet for the LM317 voltage regulator for a circuit.

As I said, if you're interested, I have some small circuit boards around that fit this purpose, and the necessary parts to populate one as well. Would be a lot cheaper than radio shack prices.

You may or may not need a heatsink - it depends on what the voltage of your power supply is, and how much current your fan draws. It certainly can't hurt to use one just to be safe, but these voltage regulators. Speaking of which, what IS the voltage of your power supply? The specs you gave in your original post aren't that clear, if you meant 9W 300mA that would mean a 30V power supply, which seems pretty high. Ideally you'd want something somewhere around 12 volts.

Hack job or not. Which route is the cheapest? Won't they both work exactly the same once the box is closed? turn on and dial up or down? Then if you get a voltage regulator sometimes you need a heat sink. It sounds like, to me(remember I'm not a electronics guy) to get the same result it'd double the price and be much more complicated than it needs to be. I'm not trying to flame anyone, at all. I'm not attacking anybody either. Just don't know alot about electronics beyond what i picked up in electronics class in high school 15 years ago. =) Sounds like a tuner vs. muscle car type of debate... each might have 500 horses and 700 foot pounds of torque but the muscle car seems easier to build than a computer driven highly modded 4 banger but in the end both go really, really fast.

Grimsawyer said:

Hack job or not. Which route is the cheapest? Won't they both work exactly the same once the box is closed? turn on and dial up or down? Then if you get a voltage regulator sometimes you need a heat sink. It sounds like, to me(remember I'm not a electronics guy) to get the same result it'd double the price and be much more complicated than it needs to be. I'm not trying to flame anyone, at all. I'm not attacking anybody either. Just don't know alot about electronics beyond what i picked up in electronics class in high school 15 years ago. =) Sounds like a tuner vs. muscle car type of debate... each might have 500 horses and 700 foot pounds of torque but the muscle car seems easier to build than a computer driven highly modded 4 banger but in the end both go really, really fast.

Click to expand...

I agree, once the box is closed it doesn't matter too much... But, I'm an electrical engineer, and I sort of cringe at the thought of using a potentiometer as a variable current limit for a motor, just because it's a hack job.

But the other half of the story is that a voltage regulator circuit is much more flexible - it will work the same whether you are using a 12v power supply, 15v, or whatever - and it will allow you to control any regular 12v computer fan, without needing to change component values around.

A potentiometer used as a fan speed control will perform differently depending on the power supply voltage you are using, and also on the particular fan you are using. If you pick a potentiometer that is too far from an ideal value for your PS/fan combo, the adjustment range won't be very good - you might have the fan go from fully on to fully stopped in just a fraction of the rotation range of the potentiometer, as happened to the original poster.

Also, since you mention price, based on prices at Digikey (a major online electronics retailer) a 3W potentiometer is about $3.50, and parts for a voltage regulator circuit are about $3-5. Of course, one requires more soldering than the other, but the price isn't much different - and besides, it's still a small part of the total cost of the stir plate in either case.

Not to say that nobody should use the simple potentiometer method for stir plates, all I'm saying is that as an engineer, I simply would not personally recommend it to someone - it's just not good engineering practice. But if it works for you and you're happy with it, great.