Sciencemadness Discussion Board

Help with hotplate

Natures Natrium - 20-7-2005 at 20:37

It looks like all my expirements with chlorine gas have taken thier toll on my hotplate/stirrer. :(

Recently I found that the stirrer had froze up, but I spun it loose with a screwdriver and then set it to high speed...to break away all the rust that had formed on the metal components.

Then, the other day I was trying to heat a solution up, and I noticed the plate was dead cold even though the power LED indicated it was on! :mad:

So, doing what I do best, tinkering with shit and F'ing it up worse, I took the hotplate/stirrer apart. It was actually suprisingly simple. The only thing I messed up was when I pulled this sheet of mica (? pressed quartz chips? my hotplate goes to 550C) away from the other sheet that the element was wrapped around, I didn't notice that it was rivited at the corners, and broke it. (See pictures in attatchment.) This stuff is very thin and very brittle.

Anyways, I put the plate back together, turned it on, low and behold it works! ... almost. Now when I turn it on it always draws maximum power and heats up very quickly. So, I took it apart again (and took the pics) and messed with the control unit which I expected to have all kinds of chips and coils and whatnot inside. Instead it appeared to be a very simple mechanism which I think involves changing the distance on a spark gap to achieve varying power levels. Maybe. Despite being simple I still wasn't able to really figure out how it worked. Or how it turns on and off at regular intervals. See pics. Putting the entire thing back together did not yield a working unit. In fact, now the dial for the stirrer is acting a bit flaky. :o:(

So, anwyays, I need to know if the ceramic top is conductive (thermolyne's site doesn't even recognize my model number, SP46925, Cimerac 2), although if it was I would expect it to ground out (notice how the frame is grounded in the Innards pic) and throw the surge protector it's connected to. Something somewhere isn't right though, and I sure as hell don't have $400 to replace this thing.

Please oh Guru's of Electricity, help me to solve my problem!

Thanks,
Nature's Natrium

Edited for Grammer

[Edited on 21-7-2005 by Natures Natrium]

Attachment: Hotplate.rar (631kB)
This file has been downloaded 700 times

12AX7 - 20-7-2005 at 20:51

I'm guessing it is a bimetal strip thermostat. Is it? :D

Chlorine... that reminds me... I was boiling lead in HCl in my $10 microwave (so you can see, it is very precious to me ;) ). The digits were starting to go out and stuff so I let it dry off and hosed it down.. seems to work okay, except the bell doesn't sound when time runs out and most recently it blew its fuse on opening the door, apparently the door switch shorts something even with the power switch turned off??? :o

Oh, and yes, mica is used quite a bit. Every soldering iron I've taken apart has the nichrome coiled up with it.

Tim

Mr. Wizard - 20-7-2005 at 21:27

The way most heaters are varied in temperature is by turning the current on and off. While the thermostat is cold (temperature less than what it is set at), the contacts are together. When the thermostat reaches the desired temperature, the contacts open. This is usually accomplished by a bimetal strip which bends when heated. Sometimes the bimetal strip has it's own thin wire heating element wrapped around it. If this wire is damaged, the bimetal will never get hot enough to open the circuit, and the main heater will stay on forever, or until it damages itself. Often heating elements have a 'safety link' or fusible link that melts, shutting off the unit before damage is done.

I can't tell from the pictures, but is that orange part in the black box a fine wire heater? You may have experienced a thermostat failure, and then the heater element, or the fusible link failed.

In summary the heater has to be turned on and off through the thermostat. Leaving it out is not an option. If you had a Variac you could use the thing in series with the heating element, or even try a light dimmer in series with the heating element. Whatever you use has to be able to handle 15 amps or 15x120= 1800 Watts, a fairly large current.

Another possibility is you have shorted out the heating element to the frame by breaking the mica insulation. If you have done that, you might have an electric shock hazard as well as a fire hazard. To check, disconnect the heating elements from everything and see if you have a connection to the metal frame from ends of the heater element.

[Edited on 21-7-2005 by Mr. Wizard]

Rosco Bodine - 21-7-2005 at 06:34

If your stirplate has one of those percentage on/off type "rate controllers"
like on Thermolyne , Cimarec , and some others , maybe a Robertshaw controls
device , IIRC . They have a bimetal strip in the circuit which warms
up as it carries current and breaks the heating circuit through a pair of
silver button contacts , remaking the circuit on cooling just like the
flasher module in an automotive turn signal circuit . The control knob
simply increases the tension opposing the bimetal opening the contacts
so it has to get hotter , conducting current longer before it pops the
contacts open . Usually the problem is that the slight arcing of the
contacts erodes the contacts and widens the gap over time . Such
controls can be overhauled if you have the patience and skill .
Remove the knob and loosen the retaining nut underneath , disconnect
the wires and remove the control from the stirplate housing .
You can carefully pry apart the sheet metal prongs that retain the
bakelite control housing and get to the silver contacts inside to dress
them with a point file or a folded narrow strip of 220 grit sandpaper .
There should be a very light contact pressure when the control shaft
is rotated to the lowest setting of the "ON" range , and the contacts pop
open when cammed apart as the shaft turns to the "OFF" position .
You may have to slightly bend the bimetal strip to adjust for the new
gap after you clean the contacts , to get proper operation . It takes
maybe thirty minutes to overhaul one of these rate controllers if that
is the type you have .

P.S. There is a little spring inside IIRC , so you may want to put a baggie
around the switch to catch it when you slowly separate the housing ,
to spare yourself the proverbial "jack in the box" / now where did that spring
go sort of fiasco , that otherwise is guaranteed :D

Natures Natrium - 27-7-2005 at 11:16

Damn, I just don't seem to have much luck these days. :( Anybody know how I can synthesize some of that?

I want to thank all the informative replies, your help is appreciated as always.

The thermostat is indeed the bimetal strip type, and the silver contacts inside seem to be in pretty good condition. I therfor concluded that it must be the bimetallic strip that had died on me. It's not something I really wanted to mess with to get working, and doing a little shopping around I found an electronics store that sold the exact same contol box (albeit with a generic knob and mounting bracket) that even looks to have been manufactured by the same company.

So, I replaced the old one with the new one, and tested it breifly. I heard it go "click" after only a few secs on "1", indicating that the bimetallic strip had severed the connection. Awesome, I thought.

Well, I tried it again today, put it on "1", and left it on to see if there was large difference in temp as compared to the old box on the same setting. To my dismay, even though I heard the clearly audible "click" sound, the temperature of the plate just escalated continuously as long as the power was on.

After considering the situation, I am at a loss. I thought about the broken mica sheet, but if the switch isn't allowing power to pass through, then the plate's temp shouldn't just rise non-stop. Also, I think these ceramic tops are made of aluminum oxide, which has a very high electrical resistance. However, I guess the mica was on there originally for a reason, but I was thinking that reason was to prevent hot spots from occuring when the plate was first warming up.

I really am at a loss to explain what is going on. If anyone could post suggestions or ideas, I would really appreciate it.

Thanks,
Nature's Natrium

IrC - 27-7-2005 at 11:42

Draw a wiring diagram and post it so we can see the schematic, and tell you where the error is. Also measure the impedance between the element and the frame of the hotplate.

bio2 - 27-7-2005 at 12:21

These percentage timers on the Cimarec have 2 outputs. One is a pilot duty always on with the switch for the indicator light. The other is the switched contact. You may have a generic controller that has other contacts also. Make a sketch and post as was suggested.

So take a harder look at it. The thermolyne engineer sent me a schematic/ data sheet of the control when I modified mine. So send them an email to tech support.

Natures Natrium - 27-7-2005 at 13:45

Ok, well, something is most definetly not right. I attatched a MS paint work of art that is copied off of the piece of paper I used to write down how things were connected as I was dissassembling it. Together with the pics above, I hope this gives a somewhat clearer picture. I dont know how to read/write electrical schematics, nor do I have a voltometer unfortunately. :(

Just looking at what I wrote down, it is obvious that with this setup, the power to the element is being fed through what I think should be the LED feed, since this is setup to be either always on or off.

However, I am dead sure that this is the way the box was wired originally, and that makes absolutely no god damn sense at all. Even if I wrote all this down wrong/backwards or something, I am completely positive that the lead to the element from the control box was connected to "B" in the upper right hand corner. I disconnected it and only it twice when removing the top plate so I could look at the element to check for damage. It just doesn't make any sense. :(

In fact, it is so confusing that I am having to supress paranoid delusions of strangers sneaking into my lab just to randomly switch all the connections on the control box around! ;)

No matter what, the way I have it wired is clearly not correct. I need to change the way it is connected, but it is frusterating because I lack the knowledge to figure it out easily or communicate the situation to others who can figure it out. I know I have said this like five times, but any help is geniunely appreciated.

Oh, and the new control box is completely indentical to the old one. Made by the same company in the same country, everything the same.

Sincerely,
Nature's Natrium

EDIT: Bah, on the pic I uploaded the bottom picture, bottom left contact should be labeled "C" not "B".

Also, the "white connecter to LED" also doubles as a neutral and so has a connection to the main neutral.

Edit2: I forgot to mention, the leads to the bimetallic strip connect to A and B.


[Edited on 27-7-2005 by Natures Natrium]

[Edited on 27-7-2005 by Natures Natrium]

[Edited on 27-7-2005 by Natures Natrium]

control box 3.JPG - 32kB

IrC - 27-7-2005 at 15:32

See if you can get a copy of the internal schematic of the controller, and hand draw a wiring diagram of the hotplate wiring as a whole.

Natures Natrium - 27-7-2005 at 15:43

As for a diagram of the internal controller, I haven't been able to get one. The Thermolyne website doesn't even recognize that this hotplate ever existed. I attempted to draw the whole thing previously, but it turned into a mess. I guess I can try again, although that wont change the internal control box.


I am sure of my connections (which is power, neutral, etc) because they connect on a simple strip of tabs, so it is easy to see what is what.

For myself, I just cannot figure out a way of wiring this thing that would feed power through the bimetallic switching part to the element, and still provide power to the LEDs. I also dont understand why the control box would need two neutral connections either. I have not messed with any of the connections to the tabs strip (viewable in the pics I posted above), except for the neutral lead to the hotplate, which is a cord so short it can only reach one place. So I know these are the set of connections used in the original design.

IrC - 27-7-2005 at 18:22

I thought you may not be able to get one, but the alternative is owning a meter and knowing how to use it. If I had the controller on my bench in a few minutes taking resistance readings and possibly an experimental hookup making voltage measurements, I could likely figure out it's internal circuitry, but I know this you cannot do. Long ago I learned that when in a circuit a problem exists that I cannot immediately see, drawing the circuit as a whole and looking at it usually makes the error show up quickly. In your case you may need to take it to some kind of appliance fix it person and pay them to wire it correctly. But depending upon their competancy they may or may not do it right either, not knowing the internal circuit in the controller. Some types use current flow through the strip to heat it, and some have a small heater that indirectly heats the strip, where the current flow through the heater coil actuates the bi-metallic strip.

Looking at the rar you put up previously, I would say the current flows directly through the strip. It appears that neutral should go to A, the main line to D, B outputs power to the heating element in the hotplate, E feeds power to a light, and C is the return for the light which would be the neutral connection. Point A should also connect to the other end of the heating element. If you have it wired like this then what is the chance that it is mechanically bound? I mean, is there some kind of rod that applies force against the strip, which is connected to a temperature set knob? It could merely be this is not mechanically positioned so the strip has the correct tension given a specific temperature setting.

I forgot to mention, but the line comes in at D, and when heating is going on power is applied to the light at E as the strip is touching E (low current contacts), while at the same time applying power to the heater through the high current contacts at B. A and C are merely the return for the heater and the light, which possibly are connected together internally.


[Edited on 28-7-2005 by IrC]

Natures Natrium - 27-7-2005 at 18:43

Quote:
Originally posted by IrC

Looking at the rar you put up previously, I would say the current flows directly through the strip. It appears that neutral should go to A, the main line to D, B outputs power to the heating element in the hotplate, E feeds power to a light, and C is the return for the light which would be the neutral connection. Point A should also connect to the other end of the heating element. If you have it wired like this then what is the chance that it is mechanically bound? I mean, is there some kind of rod that applies force against the strip, which is connected to a temperature set knob? It could merely be this is not mechanically positioned so the strip has the correct tension given a specific temperature setting.

[Edited on 28-7-2005 by IrC]


With one exception, this is in fact exactly how it is wired up. The one exception being that A does not connect to the other end of the element. Rather, the element plugs into that strip of metal tabs directly into the neutral connection.

I dont believe it is mechanical hinderance, because I can hear the audible "click" of the contacts seperating and coming back together. Even after the contacts seperate the temperature continues to escalate. This isnt too suprising since, with the neutral always connected, and the power running through contacts D to B, the plate is either on or off and the bimetallic strip assembly isn't doing anything at all.

I know the neutral is designed to always be connected to the element because the cord for it is simply too short except to reach one single tab (and this tab is the one I found it on when I opened the bottom up for the first time), and the control box connects the power on and off.

Can you see a way to completely rearrange all the wires so that the power is being turned on and off through A and C without shorting out the system and still supplying power to the LED? I certainly can't. :(

The sad thing is this really doesn't make any damn sense since I know this is how my hotplate was set up previously and it worked without error for three years.

Anyways, thanks IrC for your input on this matter, it's nice to have someone versed in electronics helping me out on this. :D

I will try and draw a full diagram, no promises on readability though.

Thanks again,
Nature's Natrium

IrC - 27-7-2005 at 19:36

The neutral should have nothing to do with the switching action, it should be always connected. In fact, I see no reason to have a neutral connection to the controller at all, so long as the return for the light goes to the neutral. In essence all the controller should be doing is taking the line in, and applying power to a high current contact and a low current contact, the heater and light respectively. Outside of the light return path not reaching your main neutral strip inside, why hook a neutral to the controller? I also see that from the way you are wording things you are not exactly seeing the true proper circuit as it was designed. No worry, this is actually hard to do unless you do this for years, which is why I wanted you to draw it, more for you to look at than for me to see. Picture this in your mind. You have a line cord coming in, the white being neutral so lets call it ground, and you have a black which is power so lets call it hot (the green if there is ground so it goes to the frame of the unit, and is connected to the neutral in your power box or grid itself). Now, you have a heater and a light. Each has one end grounded at all times, and each has a hot end which gets switched power from the controller.

Both can have their neutral returns connected to the white all the time, and no neutral goes to the controller. Now, the line goes into the controller, which has two power outputs, one for the heater and one for the light, which both are powered when the strip is against them. Now when the strip heats both hot connections break, so light and heater go out. I am trying to get you to seperate the light and heater and look at each circuit alone. It should be that simple, one power in, two power outs, one for heater and one for light. The only variable then is tension on the strip. Unless the controller is indirectly heated but I do not think so from the rar pics you upped. So the only conclusion I can come to is something is incorrect in the diagram you show for the terminal placement and function of each on the controller, and this systemic error is where you are going wrong. If you had a meter you could solve this in 3 minutes flat, rather than spend the hours of talking about it.

Doublecheck the controller, you need only be concerned with a main hot feed in and two switched power feeds out, the heavier one getting the heater.

So, whether or not you get this solved, buy a meter anyway, they are simply too cheap to not own one and you can bet you will need it again down the road for something. Buy one if for no other reason that to stop hearing me whine about it.

Natures Natrium - 27-7-2005 at 19:37

Sorry for the double post, but otherwise I could not attatch this "schematic". I hope it is at least halfway understandable. ;)

Edit:

I think I see what you are getting at. The only thing that does not yet make sense to me is that I only have one power feed in (as you suggested), but the inside of the box really has two seperate circuits. The first one is the auto-switching A <-> C, and the other one is the permanently on/off B<->D<->E.

So how can only one power wire power both circuits?

I have been hesitant to completely rewire the thing, for two reasons. One being my lack of knowledge which is being remedied as we speak, and the other being the fact that it worked before with these sets of wires on the previous, absolutely identical control box.

The most obvious thing I was tempted to do was just pulled the LED off the circuit totally, connect the power to A and the output to the element to C. I am sure it would work, but I cant count the number of times the LED light has reminded me to turn off the plate after I was done using it.

I suppose that if I absolutely must I can go buy some wire and crimps and rewire the whole damn thing so it makes sense again. It just seems to me that there ought to be a way to make it work with what is there, since it was working before.

[Edited on 28-7-2005 by Natures Natrium]

wire schematic.JPG - 80kB

IrC - 27-7-2005 at 19:53

It is not quite good enough to help me see it. Things like where is the neutral return on the heater, etc., and I wonder if what they call LEDS are not really neon but no matter. I get the feeling the controller may actually have two internal switches, one of which switches neutral but I cannot tell, and if it did likely it only is in the light circuit. Or one switch goes to heater and one goes to light but then we are back to a neutral connection not being really needed if the terminal strips have enough lugs for all neutral connections. You may hear a click, but does the strip move a contact arm that is welded, and the click you hear is merely the strip itself while the contacts stay engaged. Or are you running the hot feed to the heater from the wrong terminal and the heater is actually getting constant power whether or not the contacts are switching? Hard to say when I do not have it to look at. But, this is such a simple circuit you simply should not be having this much trouble hooking it up.

You are right it should go back the way it was, and it looks more and more like there are two switches in there. I think it is possible you should wire it all the way it used to be if the controller is identical, but this means there must be sticking contacts or something in there. Just a thought. At this point I would have used a meter, and rewired it so I had every function it used to have. If you really get desperate, buy a new one and give me that one. I keep getting hammered on ebay every single time I find a combo stirrer/hotplate and bid on it.

You are overlooking something and are trying to do it all in total blindness not having a meter to make some simple measurements. Oh shit I whined again.

[Edited on 28-7-2005 by IrC]

Natures Natrium - 27-7-2005 at 20:06

Quote:
Originally posted by IrC
You are overlooking something and are trying to do it all in total blindness not having a meter to make some simple measurements. Oh shit I whined again.


Lol, alright, I'll buy a damn meter when I have some spare cash. :)

I agree with what you say that I shouldn't have this much trouble with it. If I chose to rewire it all by hand I am sure I could get it running. I just don't understand how in the hell it was working before with this setup. :o I suppose I should just drop it, and fix the damn thing by any means necessary so I can get on with the fun stuff. However that's not to say I haven't learned quite a bit from this ordeal which would enable me to wire it correctly now, which I might not have done before.

Thanks again for all the help IrC, much appreciated.

Sincerely,
Nature's Natrium

PS This IS the way it was wired before. That is why I am so frusterated and confused, because there is no way in hell it could work...yet it did! :o:mad::o

Edit: for spelling

[Edited on 28-7-2005 by Natures Natrium]

Got it.

Natures Natrium - 28-7-2005 at 20:22

Alright!

I managed to get the thing working, after giving up on the original way it was wired.

I took the wire to the element that was hooked directly to the neutral and hooked it to the hot lead. I then hooked up a neutral to A, the lead to the other side of the element to C. Now the element is on the switching circuit as it should be.

I hooked the power to B, and the LED power lead to E. The LED neutral was connected directly to the neutral tab strip. C is unused and there doesn't appear to be a ...

I just figured out a another way it can be wired, where there is only power running through the control box, the right hand side element can stay attatched to neutral, and all the tabs are used. :o

I don't know why I am having so much trouble with this, for some reason I just miss the obvious. Ironically enough I just pounded down a few beers, and suddenly it all begins to make a lot more sense. ;) I have no intention of actually wiring it while buzzed, I will review the new setup in the morning whilst sober to make sure it actually works.

I just wanted to shout out to all those who made contributions to this thread, with a special thanks to IrC. :)

Thanks,
Nature's Natrium

IrC - 28-7-2005 at 23:51

The only other thing I would give thought to is if there are two switches in there, and a neutral connection does go to the controller, is it possible by design the neutral and hot are supposed to both be switched by the two seperate switches? This would make sense if they were worried about a contact welding, allowing a failsafe so the heater would not stay stuck on full heat without shutting down at over temperature? Then again if it were me I would find a thermal switch and add it to the heater hot feed as another fail safe. Similar to the switch inside a MR Coffee mounted on the heater frame, where above a certain temperature it opens even if the controller had welded contacts. Maybe not needed and the thermal switch would have to be rated slightly higher than the maximum temperature ever expected, so that it would only open in case of runaway heating. Just thinking out loud.

Rosco Bodine - 29-7-2005 at 11:24

I have a Cimarec stirrer hotplate , an old one which I have had apart for cleaning and sevicing , calibration of the controller ,
oiling the motor , ect . The pilot light for the hotplate is a simple orange neon connected with one lead to the line feed / unswitched end of the heating element ,
and having the other lead connected to
the *switched* input side of the rate controller reeds . The pilot neon stays on
all the time the dial is set to any position
other than OFF , so it only indicates power
is being fed to the rate controller reeds , and does not follow the clicking on and off of the rate controller as a " cycle light " would do . It could as easily be connected
to function as a cycle light if desired , simply by changing the connection to the
same point as the switched end of the heating element . Many mantles and hotplates will actually have two pilot lamps , with one acting as a " power on "
light , and the second pilot acting as a
" cycle " indicator . Two pilots is actually the preferred arrangement . And if only one pilot is used , it can be configured
either way is desired . I have seen it
both ways depending on the manufacturer .

BTW , a lubricating oil which I have found works well is actually formulated for a different application but does a great job anyway and is cheap .

The two-stroke oil that is TCW grade for
100:1 dilution use in outboard motors ,
evidently has a formulation and lubricity and non-gumming properties that also
make it great for electric motors . After
discovering years ago that blower motors
last longer by about double using outboard oil in their sleeve bearing cups ,
that's all I use anymore on electric motors . It probably wouldn't hurt a motor to actually immerse the entire motor
and soak it in the stuff , shake out the excess and return it to service .

IrC - 29-7-2005 at 15:00

Very good idea. I find that very high quality (and very expensive) silicone oil used for the fusor section's moving parts on dry toner copy machines is vastly superior to any other oil in high temperature areas.