Need stir plate circuit advice

Okay, so i need some advice on my stir plate circuit. Following stirstarters.com i need the following:

2Kohm Potentiometer
330 ohm Resistor
LM317 Voltage regulator
.1 microfarad capacitor

so off i went to radioshack, home of astronomical prices and retail staff who are not at all required to know an electron from their ass. Found most of what i needed but have 2 questions:

1) a 5K pot was the closest i could find. What ill effects is this going to have? I assume that realistically, this will just lessen the chance of burning up the pot since I= V/R and since i've increased R and holding voltage constant, ill be reducing current.

2) they had .1 mfd caps in either polyester film or metal film style. How do i know which is more appropriate?

Thanks!

search "cheap and easy stir plate build"....

didnt even read the post did you? plate is now assembled and works well. the polyester film cap seems to be non-polarized so that's convenient and the pot works A-ok.

Read the post and you posed a question about your stir plate build.... Listed some parts that I didn't think were necessary and I didn't recognize and then I answered your question about how to build an easy one..... Do me a favor and google email tone...,

I tried the cheap and easy stir plate build. In my opinion it isn't good enough. I couldn't get a satisfactory range of adjustability. The LM317 circuit is far superior, so good on you. If it works, I wouldn't be too worried. Test it out with a stirbar and make adjustments if necessary.

1) you'll have a smaller useable rotation range available to you. For most fans, 2k is enough resistance to stop the fan from running completely. You've got a range from 2k to 5k that is not useful for the circuit. This could cause some sensitivity problems, but perhaps not.

This has nothing to do with how much current the fan is drawing, and thus won't impact whether or not the pot burns out. That's just a question of its load rating.

2) doesn't matter

paparker21 said:

Okay, so i need some advice on my stir plate circuit. Following stirstarters.com i need the following:

2Kohm Potentiometer
330 ohm Resistor
LM317 Voltage regulator
.1 microfarad capacitor

so off i went to radioshack, home of astronomical prices and retail staff who are not at all required to know an electron from their ass. Found most of what i needed but have 2 questions:

1) a 5K pot was the closest i could find. What ill effects is this going to have? I assume that realistically, this will just lessen the chance of burning up the pot since I= V/R and since i've increased R and holding voltage constant, ill be reducing current.

Click to expand...

It'll work fine. With the regulator, the current isn't going through the pot, so don't worry about burning up the pot. The regulator has an internal transistor that is dissapating a lot of power through the regulator heat sink. The power that is dissapated is I*(Vin-Vout). screw that tab to a chunk of metal if it gets hot (I'm assuming you are using a TO220 pkg)

paparker21 said:

2) they had .1 mfd caps in either polyester film or metal film style. How do i know which is more appropriate?

Thanks!

Click to expand...

I'd go 10uF or higher. Doesn't matter they type, just make sure it's rated for twice your otuput voltage.

[edit] woops, I see you already have it working. Woot!

Thanks mal, that's actually super helpful- I am having adjustability issues, it almost feels like my pot is an on/off switch and as a result I'm throwing my bar most time I try to start it. Sounds like I need to order a lower resistance pot.

paparker21 said:

Thanks mal, that's actually super helpful- I am having adjustability issues, it almost feels like my pot is an on/off switch and as a result I'm throwing my bar most time I try to start it. Sounds like I need to order a lower resistance pot.

Click to expand...

Don't change the pot. You can accomplish the same thing by changing the fixed resistor above it (I'm assuming you are using the 317 in a typical way - maybe you should post a link to your ckt).

sure, i just followed the stirstarters.com circuit seen here:



I assume you mean that 330ohm resistor that bridges the adjustment and out side of the lm317? How do i figure what resistance is appropriate? The resistors came in a five pack so if just doubling the resistance will help that's an easy change to try.

BTW, I have seen these be VERY unstable without adaquate capacitance on the output side.

Iadj is about 0.0001 amps. So, with a 510 ohm resistor it will amount to about half a volt.

hmm okay, so i need to lower that resistor down to a 240 and pick up a 10 microfarad cap on output. Thanks for the schematic, you've been super helpful! (more so than my electrical engineer buddy lol)

paparker21 said:

Thanks mal, that's actually super helpful- I am having adjustability issues, it almost feels like my pot is an on/off switch and as a result I'm throwing my bar most time I try to start it. Sounds like I need to order a lower resistance pot.

Click to expand...

Passedpawn is correct that you can rescale the regulator output by using a different output side resistor. Also, the output capacitor should help with your bar throwing by smoothing out your voltage changes a bit. If it doesn't, the issue is likely mechanical (magnet placement, flask shape, etc).

well, i've been workin on this thing again today... the first time after putting in the 10 mfd cap and the 240 ohm resistor, i spun GREAT... turned it off, filled the flask to check the pull, and thrown.


It'll spin just fine dry- either directly onto the plate or in an empty flask - add water though, and then i'm throwing. Does that just mean my drive magnets are too weak? they're 8mm neodyniums and based on how hard they are to seperate from each other i think the pull is plenty strong.



edit: worked on it a bit more and realized i still had some gap between the cover and the drive magnents, closed that gap up and kept tweaking. Then i discovered something i wouldn't have expected: the stir bar is not gyroscopically stable at low speeds. I can't turn it on at full tilt, but when i turn it on i have to ramp to moderate speed quickly or the bar will throw. I suspect this has something to do with the teflon pivot ring on my bar.

With the 5K pot and 240 resistor i still don't feel that i've got really good speed control, and my max speed is definitely reduced but i am able to pull about a 2" vortex in a 1000mL so that's just about perfect for a starter.... now i'mma go get my starter...started.

Does it throw even at very slow speeds?

I actually find that there is a sweet spot for fan speed. Too slow or too fast and they throw. I'm assuming that is partially due to imperfect centering of my magnets, but that's a WAG.

I gave up with the small magnets and went with hard drive type and one from a "shake" flashlight (dual stir plate).

Edit: Saw you edited, congrats!

I've built 5 stir plates. My best one had small rare-earth magnets stacked x 3, 2 piles, about 1" apart. The piles have opposite poles facing up.


Here it is spinning.

Nice, i've got the same setup in terms of using 2 stacks that are about 3 high each. I'm still getting a lot of clatter in my bar but i'm sure that's probably just a result of a tiny mis-alignment. It's got about a 1.5-2" vortex in my 1L Erlenmeyer with starter right now so i'm happy enough with it.

How have you guys gone about getting the alignment of your magnets perfect?

paparker21 said:

Nice, i've got the same setup in terms of using 2 stacks that are about 3 high each. I'm still getting a lot of clatter in my bar but i'm sure that's probably just a result of a tiny mis-alignment. It's got about a 1.5-2" vortex in my 1L Erlenmeyer with starter right now so i'm happy enough with it.

How have you guys gone about getting the alignment of your magnets perfect?

Click to expand...

Glue flat surface to fan (round tin piece cut from peanut tin). Tape fan to workbench. Tape magnets to flat surface.

I made the base (cut some plexiglas and hotglued some pvc legs on there) then set it over the fan. Test with flask of water. Adjust magnets until perfect. Glue magnets, then glue fan to plexiglas with some spacers made out of random garage crap.