Magnetic stirrer with terrible design, advice for fixing it. - Page 1
Forum section: electronics / projects

author: David_
author_rank: Contributor
timestamp: November 17, 2024, 07:57:01 pm
content: Hello.I bought acheap Chinese magnetic stirrer with hot-plate from Vevor, as always when buying such devices from those parts of the world at those prices I opened it to inspect it before use. Long story short, I was not at all happy with the construction.The documentation that came with it make it clear that it should not be used for long periods of time nor on high speed for too long without breaks to let it cool down. Which simply isn't acceptable so I am in the process of rebuilding it(in the end the functionality will be greatly enhanced.)But about the warning to not let it run for too long, I am unsure of which components that would be most likely to die first.I apologize, I haven't been able to figure out how to include the pictures in amongst the text, but in the attachment I have put several pictures trying to show the inside in its entirety. Note that some picture may show partly upgraded things.There is no heatsink on the LM317 running the motor, there is no fan whatsoever, all cables are really thin gauge.From what you can see, where is the most likely weak point/points for system failure, in your opinion?Would it be the LM317?Capacitor next to LM317?The motor(which is very close to the heater)?I am:Replacing all cables with thicker gauge wires(including adding thick heat-insulated wires to the heater).Fixing the PE-Earth connection so that the case is actually electrically connected to it.Putting a heatsink on the LM317Cutting vent ports and installing a fan.Replacing the mains power and sensor cable which was "permanently attached" to the enclosure with connectors.Replacing all rusty hardware and adding lock-nuts.That covers the worst design aspects(I think) but in the end I am also:Adding a microcontroller to regulate the temperature with a proper PID controller(the original temp controller is really really bad and what the manual says about adjusting settings isn't true)µC will also run a display with simple GUI.Adding temp sensor(s) to monitor the LM317 heatsink and motor case.Adding a relay to control the heating element(which originally is always on)The fan will be speed controlled according to the measured temperatureI feel that I have forgotten something, I'll edit in that caseIf you gotten this far, thank you.I feel that with all this done this unit will be a pretty good unit, the fan is strong enough to push a lot of air through the case if needed.Do you have any suggestions or critiques about this whole thing?I must say, I'm really entertained by the markings on the enclosure which says "85-2 Numerical Show Constant Temperature Magnetism Mixer".RegardsDavid

author: xrunner
author_rank: Super Contributor
timestamp: November 17, 2024, 08:04:23 pm
content: Quote from: David_ on November 17, 2024, 07:57:01 pmDefine "long periods of time" and "too long".It's completely subjective so how do the customers know what it means?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2024, 09:08:12 am
content: I don't think one would need a speed controlled fan. With the motor running the noise is likely not an issue. If a fan is used, one may want to nearly close the holes besides the motor. One would likely not want the full air flow under the hot plate, but separate entry and exit.Ideally a LM317 also wants a small capacitor on the output side.Depending on the temperature sensor, one could consider an alternative 2nd sensor directly fixed to the hot plate. This way one could run the hot plate alone without the external sensor

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 18, 2024, 09:22:38 am
content: After seeing the review Old Tony made of a 150mm wide vise I also decided to buy one from vevor. What I received was a quite different type of vice and of much lower quality. It is in essence barely usable without modification.Your stirrer has a very small motor. It's quite possible it's current draw is low enough for the LM317 to not overheat, even without a heatsink. (That is, if it's a "real" LM317). I won't be surprised if it's the motor itself which will have the shortest lifetime.For the rest, what is your worst case scenario if this thing stops halfway through an experiment? If it's just a bit of inconvenience, then meh. If it wastes an expensive batch of chemicals, then you are probably better off with buying a better quality stirrer, or at least put better quality parts in this one. One of the things I don't like is the big hole under the heater element. That will radiate a lot of heat back into the electronic box

author: Siwastaja
author_rank: Super Contributor
timestamp: November 18, 2024, 09:42:34 am
content: You can add a heatsink to the LM317, but it's likely the cheap DC motor, specifically brushes, dies first. Very cheap low-quality DC brushed motors have operating time in some hours to tens of hours, so are fine for actuators that do their job in seconds, but not very good for continuous operation. It is possible to build brushed motors which operate for thousands of hours but they are not cheap.

author: ftg
author_rank: Frequent Contributor
timestamp: November 18, 2024, 12:26:28 pm
content: The lack of ventialtion holes is potentially for avoiding ingress by chemicals that boil over or get spilled.The sparking brushed motors is an ignition source to also consider during experiments.

author: mikerj
author_rank: Super Contributor
timestamp: November 18, 2024, 12:57:06 pm
content: Quote from: ftg on November 18, 2024, 12:26:28 pmThat would be an excellent reason for not having ventilation holes in the upper parts, apart from the huge hole directly underneath the heating plate which is exactly where any spills from boiling over will go...Without fixing that massive oversight this will need to be supervised at all times to avoid any ingress into the internals. Presumably those heater connections that look very exposed are at mains potential?

author: Haenk
author_rank: Super Contributor
timestamp: November 18, 2024, 02:58:55 pm
content: I would expect the motor to be the weakest point (besides the regulator), that's a simple brushed motor, and likely a cheap one.I have never opened a stirrer (they just ran & ran, for hours and days), but I assume they had a brushless motor, which will last much longer. Brushed motors have a limited life time.

author: calzap
author_rank: Frequent Contributor
timestamp: November 18, 2024, 05:18:23 pm
content: That stirrer/heater is sold on eBay and elsewhere under various brand names. Unless you’re wanting the experience of upgrading, junk it and buy a used or new high-quality stirrer, like a Corning, CMS, Thermolyne, Cole-Parmer, etc.If you’re planning on stirring flammable liquids, use a water-driven stirrer. Heating flammable liquids or volatile corrosives safely requires a protocol that depends on the circumstances … lots of possibilities.Mike

author: Wollvieh
author_rank: Contributor
timestamp: November 19, 2024, 12:40:08 am
content: Over the years I tore down several professional magnetic stirrers from the 80ies to recent models. If you ask how to improve that piece of Chinesium I would suggest to drown it - with steady stirring - in a nearby river or lake. There is just no way to come close to the build quality and functional details of the real things.On the other hand it will get hot and be stirring. Which should fulfill most requirements in a home lab. You won't spoil a 3-months-study because the last 50 of 150 experiments where 25 degrees warmer or colder than indicated. Which you learn the very day you receive your mail-ordered 500 paper copies of it. Or ruin a $500 protein mixture because of a 50-degree-overshoot.What I would recommend most is an external thermometer that is measuring inside the liquid. Even on high quality stirrers you have to measure the actual temperature, as it is in no fixed correlation to the plate's temperature. From there it's only a small step to link the temperature control to that sensor. Professional stirrers have standardized inputs for that.I assume the heat transfer is poor on your device, as the professional devices use special alloys of aluminium and silicon to get the heat from the outside where the heating coil is located to the unheated center (because of the magnet) where the glass is sitting. The Chinese stirrer seams to use just a slightly modified bottom plate of an electric kettle. As it is stainless steel it won't block the field of the stirring magnet, but the coil on the outer rim will heat everything in the room except your experiment.And I would add some elastic sealing washers to the plate's supporting studs, so no liquids capillate inside along the threads.

author: amyk
author_rank: Super Contributor
timestamp: November 19, 2024, 01:41:33 am
content: Quote from: Siwastaja on November 18, 2024, 09:42:34 amThat motor looks like the type used for tape transports, which should last at least hundreds of hours, although without opening it up it's not possible to see what type of brushes it has - the very cheapest ones with stamped copper brushesdo indeed last only a few hours, but the ones withsintered or carbon block brushescan last much longer.Better and older versions of this appliance use a shaded-pole AC motor, where the only wear point is the bearings.

author: David_
author_rank: Contributor
timestamp: November 19, 2024, 05:09:14 am
content: Thank you for all the answers, many good points have been brought up.I would wholeheartedly agree that I should drown this unit in a river and get a used one but for the time being I am stuck with this one. I will be stirring mixtures containing ethanol for a couple of hours up to possibly a few hours, I felt that after these upgrades this unit will suffice(it kind of have to at the moment) but after having read all the answers I realize that the design of this stirrer is much worse than I thought it was.Now I don't perceive the failure of components to even be related to the weakest parts of the design, this unit has much bigger problems than cheap components that might give out quickly.The hole under the heater was mentioned(and yes the heater is being connected to the mains), I was quite unhappy with the lack of heat insulation and am going to cover the hole with kapton tape for now(yes I know that's a ridiculous "solution" but its better than nothing), but the temperature insulation that does exist is done in a terrible way as well.If I could afford it I would drown this one in the river and get another used one but for the moment I am stuck with this one.The following might partly be cope or trying my best to see things in a positive way but I find this rebuild to be thoroughly satisfying since it gives me an opportunity to make use of many things that I have learned about during my spare time the past 10 years.If that wasn't the case and if I didn't already own everything I need to upgrade the electronics it would make no sense what so ever to pay this unit any attention.I hadn't considered the situation of stirring and spilling flammable solutions, it will take me some time to ponder the consequences of this aspect but right now I am considering possible ways to cover and seal that hole under the heater, sealing it entirely(apart from allowing the motor shaft to poke through).I have also bought one of these4L water distillers from Vevor, and when I opened that one up I was pleasantly surprised. I am actually perfectly happy with it's build quality and feel that they have done a pretty good job with that one(incidentally, its the same heating element as is found in the stirrer).Assuming I can find a way to seal the hole under the heater, and assuming that the ventilation holes I make take the problem of spilling liquids into consideration(and finding adequate solutions) then I will be able to get some use out of this unit(with much closer supervision than I previously intended), but if I had more cash I would feel stupid to use this at all.This is a lot to take in, but I'll return after having consolidated all these view-points.I really appreciate all of your responses, thank you.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 19, 2024, 06:59:47 am
content: Quote from: amyk on November 19, 2024, 01:41:33 amIt sure looks that way. Maybe if the manufacturer has secured a supply of C cassette player motors (the use of which in the original purpose would have obviously dropped by 99.99%, so there could be a large stock of excess motors somewhere) which are at least half-decent, they could last in this use case almost "forever".On the other hand, if the permanent magnets attached are strong and viscous liquids are mixed that could put a lot more strain to the motor than running the tape.