David Marx
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Corning hotplate - speed regulator repair
My corning hotplate has no speed control on the stirrer. It heats fine, it spins but it is on/off. The markings are all meaningless, once you get to
about 3 it turns on, and rapidly accelerates to maximum speed.
Has anyone repaired such a problem before?
alea iacta est
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Al Koholic
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I have a pc-420 that had a similar problem. When I first got it, it did something like what you are describing and once on, would rapidly go to full
speed. I called Corning to get a quote on the speed control/led/heating control pcb so maybe I could get a replacement since they'd be easy to
swap, but they wanted like 130 dollars for a new board and said I would have to send it in to be repaired at thier site.
Not wanting to spend the money (on top of already buying it for 150 (guess you get what you pay for)) or send the thing through the mail, I took it
totally apart including popping all the programmable chips out of their sockets, and cleaned everything with acetone. When I put it all back
together, I was at least afforded a range of settings from 0 to 3 in which the speed was controllable up to full speed. Sure, it is more sensitive to
adjustment but I'm fine with that as it is pretty easy to adapt to.
Edit: Also, as far as I know you also need to have an account with corning to do any business with them directly. At the time, my site of employment
had one so that's why I quoted the pcb. They don't seem too friendly to private individuals.
[Edited on 20-12-2004 by Al Koholic]
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Mr. Wizard
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There are two ways to approach it. One way is to fix the old board, and check the potentiometer that controls it. You will have to check the
individual circuit parts. If you are handy with a digital VOM you should be able to figure it out. It sounds like it might be the pot because it does
work a little. Plan "B" would be to buy a variable light switch for $5 and use it right out of the box. I had a fancy speed controller on my
fireplace fan that quit, and I didn't fool with it, I just bought a light dimmer. Problem solved.
I just noticed the second post is the one that works a little. I'd check the pot in that one. If it just works on and off, it probably isn't
the pot but in the board, an SCR or transistor. I'd try wiring a dimmer switch directly in series with the motor. You can only lose the price of
the dimmer ;-)
[Edited on 20-12-2004 by Mr. Wizard]
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unionised
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The last stirrer I saw just had a wire-wound pot for the speed control. The problem I had was the same as you describe and I fixed it with a drop of
conductive paint where the wire had worn through.
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Rosco Bodine
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The technology of commercially available magnetic stirrers in general just plain sucks . To get a decent stirrer having the components needed to do
the job right in any medium scale or large scale work , you basically have to build it yourself .
What a stirrer really should have is a non-magnetic spindle bearing assembly equipped with a rare earth bar magnet structure about 1" square by
4" long oriented horizontally , and a reverse polarity arc shaped wafer beneath it at the axis to shape the field in an upward region . This
would result in rock solid coupling to the stirbar which would also self center itself in the field , and would be effective through even several
inches of height above the stirplate surface , enabling the use of mantles or insulated baths without any problem . Coupling would probably be
observed even with a half meter distance between the stirbar and the drive magnet , perhaps even more . The spindle would be belt driven from a DC
pancake motor equipped with a digital tachometer encoder output for the
electronic active sensing feedback speed control . Stirring speed would be variable from 100 - 2500 rpm . The hotplate surface would also be
thermistor controlled and optionally equipped with an auxillary input jack for a probe sensor in the reaction vessel and at the least a switchable
function to select and maintain a target temperature registered on an accurately calibrated selection dial and actual sensed temperature readout
display , for the plate surface or the vessel monitoring .
If someone builds such a stirplate , it may have a couple hundred dollars worth of parts , but you can bet the sale price will be ten times that .
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neutrino
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| Quote: | Originally posted by Rosco Bodine
...a rare earth bar magnet structure about 1" square by 4" long oriented horizontally... |
Do you have any idea just how insanely powerful this magnet would be? The currents induced by this thing would probably destroy the electronics, as
well as screwing up the motor. As for getting the stirbar away from the plate…
edit:spelling
[Edited on 23-12-2004 by neutrino]
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Rosco Bodine
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| Quote: | Originally posted by neutrino
| Quote: | Originally posted by Rosco Bodine
...a rare earth bar magnet structure about 1" square by 4" long oriented horizontally... |
Do you have any idea just how insanely powerful this magnet would be? |
In direct contact with equal mass of
iron , in the neighborhood of 100 kilos force . However , at a distance of perhaps 4 cm away , with the lower core mass of alnico in a stirbar , the
coupling force would be much lower . By the use of such a massive drive magnet set at a safe working distance initially , the useful coupling height
above the surface of the stirplate would be much increased without the need for use of expensive rare earth stirbars which are also limited to low
temperature use . If a 1" X 4" drive magnet is overkill , then reduce to a 3/4" X 3" . Somewhere thereabouts should work fine
. | Quote: | The currents induced by this thing would probably destroy the electronics, as well as screwing up the motor. As for getting the stirbar away from
the plate...
edit:spelling
[Edited on 23-12-2004 by neutrino] |
The field will be nearly entirely captive between the drive magnet and the stirbar , through a ceramic top plate . The field could probably permeate
a thin section of stainless steel or aluminum without excessive eddy currents up to certain speed anyway , to enable the use of water baths ect.
contained in metal pans . Here again it is the distance of the air gap to the drive magnet which will be the determinant for the flux density and
operation limits . The reason for using a nonmagnetic bearing spindle being belt driven from the motor is to get the motor housing away from the
field of the drive magnet . The control circuitry could easily be shielded within the confines of a sheet metal box which would provide the protection
of a "Faraday cage" for the components within , similar to the shielded component protection protocols for electromagnetic pulse
countermeasures and even more commonplace shielding of tuner sections , power supplies , ect. from stray magnetics emission or absorption . The power
supply in your computer is inside a Faraday cage . The case for the computer itself is often yet a second Faraday cage itself . A second possibility
is to further isolate the electronics in a remote control box cable connected to the drive and heating unit to get some extra distance . This can
actually be a handy feature when working in a fume hood for example , because the hood need not be opened to reach inside and make adjustments to
controls mounted directly upon the base unit .
Basically what I am talking about is a high end Mirak unit on rare earth steroids .
Think of the capability this way . Suppose you wanted to use a bunsen burner under a flask you also wanted to magnetically stir . With a unit like I
have described , you could set your burner upon the top plate , tape it down so it doesn't spin from the eddy currents in its brass and stainless
parts , and you can couple the stirbar in the flask above the burner , through the height of the burner and the gap for the flame . And if there are
some of those roatating wind turbines located on the roof of your lab , it will probably get them spinning too 
[Edited on 23-12-2004 by Rosco Bodine]
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