Asking for help for tinkering with a hotplate. - Page 1
Forum section: electronics / projects

author: Leland_Gaunt
author_rank: Contributor
timestamp: April 22, 2025, 04:31:59 pm
content: Hello gentlemen.In todays episode i would ask to use your expertise in order to help me "fix" my working equipment.I do a little amateur chemistry and am using a sandbath any now and then (For those who dont know, just a pot or a vessel full of hot sand, meant to heat up a boiling flask that you put in it. When water and 100 C isnt enough anymore, you use silicone oil, up to 200 C maybe, after that you need quatrzsand. There are several reasons why i cant use an open flame, a bunsenburner for example, to heat the flasks which i wont get into, not important...)i have cheap, regular hotplate here, that manages to heat the sand up to almost 300 C. Its not my first try, i bought an inductor hotplate that broke after 5 min, another reg. plate delivered the same temps, was hoping with another one i could reach my goal temp. I need a little bit more, 350-375 C in the sand.The plate has 1500W which is more than enough, the reason i cant get the sand hot enough in the pot is that the plate doesnt heat constantly, but the hotter it gets, the shorter the heating "pulses" get. When i turn the knob to 5, it takes about an hour for my sand to reach 280-290C, then the faqer only heats for 4-5 seconds and then pauses for 30-40 seconds. Its obviously a kind of protection, but its in the way.my 1st thought was to Dremel a little square in this thing and attach a little fan, to get some air exchange inside any maybe delay the temp sensor to react later...but i don t think its gonna do something, now that i looked into the plate. Its pretty empty actually. I don t even know how he does that regulating.I don t see any electronics part inside, but it must have something to do with the yellow-green cable. i know the colour is ground usually, but its the only thing in there besides pos and neg, i have no idea how it registers temp and manages the on/off pulsing without any electronics inside. Im sure one of you knows how this works. So what options do i have?

author: tunk
author_rank: Super Contributor
timestamp: April 22, 2025, 04:46:44 pm
content: Yellow-green cables usually are earth/ground and has nothingto do with this. Most likely this is a safety feature to preventoverheating. Maybe you shouldn't be playing around with this.

author: Leland_Gaunt
author_rank: Contributor
timestamp: April 22, 2025, 04:48:27 pm
content: i appriciate your concern, but no safety discussion please.

author: Ian.M
author_rank: Super Contributor
timestamp: April 22, 2025, 04:53:37 pm
content: The yellow/green wire is just a safety PE/ground conductor for the top and hotplate element.The controller is a simple electro-mechanical duty cycle control, with a resistively heated bimetallic strip arranged to oscillate with variable duty cycle. It does not measure the hotplate temperature but is influenced by ambient temperature, which decreases the on time as it increases. This may to some extent be by design as a safety feature.To increase the max. temperature you need to decrease the air temperature around the back of the controller. I suggest adding non-flammable thermal insulation covering the area with burnt paint (avoiding the terminals) It needs to be thin enough not to block the air flow through the base. You could hold it in place with some thin aluminum flashing bolted to the top in its corners.

author: themadhippy
author_rank: Super Contributor
timestamp: April 22, 2025, 05:35:08 pm
content: cut the 2 red wires going to the control stat on the right of your picture,join them together in a porcaline terminal block and then it will just continue to heat up until you unplug it

author: Leland_Gaunt
author_rank: Contributor
timestamp: April 24, 2025, 01:51:25 pm
content: thanks for the input.Yes, just "bridging" the controller would be the most radical way. I guess i would just use the on/of switch of the 3-way plug multiplier to turn it off an on. I have programable timerplug, that can be set to time cycles, for example x sec on/ y sec off.....However, if i could keep that on/off mechanism alive, just delay it 50-75 C., would be much more useful to me. So the "sensor" does in fact measure the air temperature in that casing, not the plate, right? SO maybe my idea with fan and airflow was not as wrong as i thought. Or is that sensor so close to the plate that it measures the radiated heat?@Ian: can you clarify which spot you mean? Im not really sure which spot youre talking about. You mean putting something, between the sensor and the plate? To shield the sensor from heat radiation of the plate?

author: Leland_Gaunt
author_rank: Contributor
timestamp: April 24, 2025, 02:56:57 pm
content: Quote from: Leland_Gaunt on April 24, 2025, 01:51:25 pmi made another picture---the red spot i drew, is this where you say i should attach an insulator?What if i just drill severaly big holes in the top? Not through the plate obviously, but around the plate and right above that sensor? wouldnt that decrease temps inside?

author: Ian.M
author_rank: Super Contributor
timestamp: April 24, 2025, 03:05:52 pm
content: I propose adding a layer of non-flammable thermal insulation to significantly reduce the heat transfer to the interior, inside the top covering the large circle within which the paint has turned brown, with cutouts for the central mounting post, and the two electrical terminals. Hold it in place with a thin sheet of aluminum, with the same cutouts for the terminals, but with a minimum diameter center hole for the mounting post, and fit the ground tab and nut over it to hold it in place. It will probably also need fixing in the corners, though you may simply be able to extend it with tabs that are held up against the top by the edges of the casing base.The sensor doesn't really measure anything. Its an electro-thermal oscillator consisting of a bimetallic strip with attached resistive heater, arranged to have a bistable 'click' action. It heats up till it flicks to the other position, opening the contacts so it cuts power to its own heater and to the hotplate, and then cools down till it flicks back, closing the contacts and restarting the cycle. Although similar to a thermostat, its self-heating makes it a variable duty cycle oscillator. However high ambient temperatures slow down its cooldown, extending the off time so decreasing its duty cycle.For safety reasons, drilling extra vent holes in the top is not acceptable, due to the risk of conductive objects, conductive or flammable liquids, or debris entering them.

author: Leland_Gaunt
author_rank: Contributor
timestamp: April 24, 2025, 04:02:24 pm
content: i see. So you mean covering the complete broun circle? Thats a lost of surface area. I have ceramic fibre wool actually, flameresistant to over 1000 C. used that to insulate my pots. But i wonder why the area is brown around the plate, shouldnt the plate itself radiate most of the heat and the centre area be hottest? And also covered with insulation? Just wondering why it isnt as brown.

author: thephil
author_rank: Regular Contributor
timestamp: April 24, 2025, 07:13:10 pm
content: Well, it looks like the brown ring is where the metal under that heater is closest to the heating element. That's why you get the burn mark there.Adding more insulation while modifying the heater is a good idea. For better control, you could entirely bypass the the bi-metal regulation and add a digital temperature controller (typically PID controllers). they are cheap on ebay/amazon/aliexpress but would probably require even more modifications to the housing to fit. Or let it be an external control unit in its own case and only feed the thermocouple inside the heater or even your sand bath.

author: Leland_Gaunt
author_rank: Contributor
timestamp: April 27, 2025, 10:27:16 pm
content: attach a little fan, seems to work nicely, the off-intervalls are very short now, temps have increased severly. The insulationmaterial i had was to thick, so i went for this. Time will tell if the fan can handle the heat, for the first run it did, gets about 70 C warm

author: Someone
author_rank: Super Contributor
timestamp: April 27, 2025, 11:03:36 pm
content: Quote from: Leland_Gaunt on April 22, 2025, 04:48:27 pmOk, we'll just quote this so your insurance can deny their cover when your unsafe changes burn down the neighbourhood.Quote from: Leland_Gaunt on April 27, 2025, 10:27:16 pm

author: Someone
author_rank: Super Contributor
timestamp: April 27, 2025, 11:43:10 pm
content: Quote from: Leland_Gaunt on April 27, 2025, 11:31:24 pmIt is a significant enough risk that insurance companies in Australia openly warn customers about it:https://www.youi.com.au/you-connect/articles-and-guides/will-diy-electrical-work-affect-my-insuranceMy german language is not good enough to pick through your local details.