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Old 08-14-2012, 06:24 PM   #1
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Default Mounting Rheostat in Stirplate

I know this is a stupid question, but I gave up trying to figure it out on my own.

I am in the process of making my own stirplate, and decided to go with the 3W 25Ω resistor from Radio Shack/The Source mentioned in many threads. I drilled a hole in the side of my project box and am trying to mount the rheostat tightly. Im having issues because one side of the rheostat has a metal tab, but the other one doesn't. So when I go to mount it flush, it ends up on an angle.



I can't mount it further inside the project box because the hole I drilled isn't thread-tight, and it only comes with one bolt.

From the pictures I've seen, it appears people were able to mount it without any additional hardware or gluing, and it doesn't appear crooked. It also appears to be mounted flush with the side of the project box.

I can always remove the tab, get another bolt or glue the thing in place but I would rather do it with the included hardware if possible. Am I missing something here? Does anyone who has made one of these want to fill me in on what I'm doing wrong?

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Old 08-15-2012, 04:09 AM   #2
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Oh, that's normal, generally speaking.

It's so that the device doesn't rotate with the knob. Drill another hole for the tab. Or break it off and apply some glue.

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Old 08-15-2012, 01:45 PM   #3
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Yep file that down and tighten the outside good!! Thats what i did and it works fine

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Old 08-15-2012, 02:07 PM   #4
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+1 for filing it down. Don't drill a hole in your nice new stir plate!

Just FYI, I used the same rheostat when building my stir plate, and I found that for the fan I used, the highest resistance still let too much current flow. It would pull a hard vortex all the way to the bottom of the quart jar I usually use for my starter. I ended up rewiring mine with 5 parallel resistors glued to a small Al heatsink to scrub some power off before hitting the rheostat. I used a 5 pack of the el cheapo Radioshack 10 Ohm, 1/2 W resistors, and one of the small voltage regulators' heat sink. It doesn't get hot, even when run overnight due to the fan constantly spinning. Just make sure to drill a vent hole or two in the back or side.

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Old 08-15-2012, 02:29 PM   #5
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Thanks for the replies! I ended up filing it down and bolting it tightly. Now it's nice and flush!

Quote:
Originally Posted by KBentley57 View Post
I used the same rheostat when building my stir plate, and I found that for the fan I used, the highest resistance still let too much current flow. It would pull a hard vortex all the way to the bottom of the quart jar I usually use for my starter. I ended up rewiring mine with 5 parallel resistors glued to a small Al heatsink to scrub some power off before hitting the rheostat.
Thanks for the advice KBentley57. I haven't finished wiring and testing yet, but if I notice the same thing, I will try this out. I have ample space in the box for these additions if I need them.
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Old 08-15-2012, 06:29 PM   #6
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Sounds good, if you need any help just let me know.

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Old 08-17-2012, 06:58 PM   #7
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Quote:
Originally Posted by KBentley57 View Post
Just FYI, I used the same rheostat when building my stir plate, and I found that for the fan I used, the highest resistance still let too much current flow. It would pull a hard vortex all the way to the bottom of the quart jar I usually use for my starter. I ended up rewiring mine with 5 parallel resistors glued to a small Al heatsink to scrub some power off before hitting the rheostat. I used a 5 pack of the el cheapo Radioshack 10 Ohm, 1/2 W resistors, and one of the small voltage regulators' heat sink. It doesn't get hot, even when run overnight due to the fan constantly spinning. Just make sure to drill a vent hole or two in the back or side.
One problem with reducing the voltage, is that the min speed is sometimes too high. Even for achievable low speeds, many times the fan won't start without increasing the voltage to get it spinning, then decreasing it.

There is a thread where a guy has already done all the engineering for a very cheap PWM speed controller for typical muffin fans. The instructions, parts lists, and schematics in the thread are very thorough. If I wasn't so lazy, I would track down the link.

For a ready made PWM controller with true speed control, there are PC fan controllers that use a fan's tach feedback wire. There are even ones with fancy touch screens for $50 that will control ~6 fans and display temps as well.
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Old 08-17-2012, 07:27 PM   #8
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I think there is probably an rpm range for the fans that is ideal, and it's probably common to 'quiet' fans, but it would be good to figure out what that range is.

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Old 08-18-2012, 01:23 PM   #9
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Quote:
Originally Posted by cwi View Post
One problem with reducing the voltage, is that the min speed is sometimes too high. Even for achievable low speeds, many times the fan won't start without increasing the voltage to get it spinning, then decreasing it.

There is a thread where a guy has already done all the engineering for a very cheap PWM speed controller for typical muffin fans. The instructions, parts lists, and schematics in the thread are very thorough. If I wasn't so lazy, I would track down the link.

For a ready made PWM controller with true speed control, there are PC fan controllers that use a fan's tach feedback wire. There are even ones with fancy touch screens for $50 that will control ~6 fans and display temps as well.
Thanks, I didn't realize there was already a thread covering the issue. I'll give it a look just to see what his design is. I was kinda in a pinch when I made mine, and it only cost a few bucks, seeing as how I already had most things from my electronics hobby stuff. I was about to ask if a touch screen stir plate was being excessive, until I looked across the room at all my stuff!
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