Oven Thermostat, using the BI - METAL switch at near room temp. Mystery behavior - Page 1
Forum section: electronics / projects

author: RJSV
author_rank: Super Contributor
timestamp: September 02, 2023, 06:21:50 pm
content: Hello:Trying to look, focus on the sort-of 'pre-contact' behavior, with dialed (way) down to what usually would have a TOASTER OVEN be switched off. It actually works somewhat, my ohmmeter doing a kind of march, up and down.That is, the ohms resistance when the two contacts are just in 'near touch', (an aircraft flight term, I guess).The ohmmeter readings could be adjusted, for a reading like '1320' ohms, in an extremely delicate play, with the salvaged thermostat switch. But then all a sudden, my readings became 'all or nothing'; reading either ' .000' or 'l . ' which is the 'open circuit' display on my (Velleman) meterPicture shows adjustable AC switch, for a 1980's style toaster oven.

author: RJSV
author_rank: Super Contributor
timestamp: September 02, 2023, 06:33:31 pm
content: This is the outdoor photo, the thermostat switch mounted in plastic box.Idea is to adjust the bi-metal thing to just switch, at my outdoor backyard temperature of approx. 90 degrees.For good luck, it just happened to be very changing, weather wise, with scattered clouds and bright sun causing constant heat and more cool cycles to exercise my thermostat switch!What I've seen is the settling being on the warm side, (of 92 degrees f), the switch should open, as seen by ohmmeter. Surprisingly, I could sit there and watch that ohms reading climb, from 000 and up to .020, .030, .040, etc. (30 ohms on the 2k meter scale). Then, quickly the reading of contact resistance climbed up past 2 K ohms and higher, before the meter just indicated infinite or off scale.Picture close up of the toaster oven setting switch.

author: Gyro
author_rank: Super Contributor
timestamp: September 02, 2023, 06:49:28 pm
content: Mains thermostats like that will have contacts resistant to making and breaking high currents. They will be made of something like Silver-Cadmium oxide or one of the other arcing resistant alloys. These contacts need a high voltage (probably 50V+) backed up with some current to break down surface oxide films and make full contact. Sometimes a little mechanical wiping action helps too (maybe what you are seeing).It's not fully clear from your description, but I suspect that this may be the cause of what you are seeing. Sometimes putting a capacitor (maybe 100nF to 1uF) will give enough of a current spike across a tiny initial contact spot to achieve full conduction. Really, the sort of voltage and current levels achieved by your DMM Ohms range or low voltage logic will never provide fully reliable switching.

author: RJSV
author_rank: Super Contributor
timestamp: September 02, 2023, 06:55:18 pm
content: So the mystery is, after taking apart my multimeter case and verifying battery good, now I cannot reproduce that fineness of mechanical adjustment: it either swings towards all the way gone (.000) or all the way on infinite.Even more tantalizing, I can sometimes do the tiny knob motion, (say 1/2 mm), causing the switch contacts to go to something like '.030', but the THAT intermediate fairly quickly will 'count down' to full 0.OR, the other way, similar thing: might adjust infinite down, with the most tiny knob rotation, down to 1200 ohms, but THAT reading some starts drifting upwards, to end up reading 'infinite' ohms, soon.Only wild guess I can make, is perhaps micro-arcs from the ohmmeter applied (tiny) voltagesThe contacts are approx. 3/16 inch diameter 'pellets' with rounded tops.This thing maybe needs a long lever, temporarily glued or taped to the knob, to do the very delicate task of tiny readjusting the switch point.

author: Gyro
author_rank: Super Contributor
timestamp: September 02, 2023, 07:15:31 pm
content: It is an interesting one, normally I would expect to only see the effect on closing contacts, with them remaining 'made' until opened. Maybe you are seeing the oxide layer actually starting to re-grow as the contact pressure drops. I don't know.

author: RJSV
author_rank: Super Contributor
timestamp: September 04, 2023, 02:35:57 am
content: Yes OXIDE layer explains. I actually got used to the case having old oxide (1982 toaster oven) to the point where it flipped me around, expecting that. So, when a half hour playing with the thing likely removed that interesting behaved layer. All flipped around, as the 'normal' case IS having all or nothing, meaning reading 'infinite', open circuit, or '.000' kohms, indicating zero resistance.Another stray dog, is the phenonima of mechanical hysteresis, often happens in done shaped things like cans. Was looking for something, not sure is there, in the bi-metal switch.

author: RJSV
author_rank: Super Contributor
timestamp: September 19, 2023, 01:09:00 am
content: I'm finding it difficult to establish a clear mechanical understanding, but it appears that there are TWO different types of bi-metal springs; one labeled 'B' in the picture, the other flat spring with contact is labeled 'C'. Looks like each tab would bend differently, according to current temperature.If I guessed correctly, the intended control, of resistance heater, is by way of the thermostat opening up, and that means flat spring-contact 'B.' will basically curve upwards away from 'C.' when a desired hot set point is reached.An insulating ceramic post does the set point adjust, as an increased ability for the 'B.' spring to 'pull away' at lower and lower temperatures. All very nice, but what's interesting is the qualities of having only a very tentative contact, virtually by way of very light weight forces pushing contacts together. Apparently, a whole phenomenon exists when dealing with (complex) surface oxides and various misc. impurities.That's where I've made some curiosity connection, with 'COHERERS' and the recent video by Medhi - - Electroboom (YouTube channel). He describes some of the early work, where a coherer can respond to EM signals such as 40 volts amplitude at 100 mhz, sourced out from an ordinary long gas lighter. The piezo crystal apparently generates 'enough' signal to burn through surface oxides (in the coherer).Another aspect that I don't have a good grasp of is if there is any mechanical hysteresis.

author: RJSV
author_rank: Super Contributor
timestamp: September 19, 2023, 01:21:06 am
content: Original thoughts are to use the switch for crude control of a 12 vdc computer fan, for use during bad heat waves, this summer. The control polarity is reverse connection, through a mid pwr transistor switch.The thermostat would go to open circuit, when warmer, say 98 degrees F. That would then allow base current through a mid size transistor, for having the fan run. The thermostat seems to be settable all the way down to almost 90 degrees F, although the knob gets a bit fuggidity below 100.Not a permanent project so far, but just an excuse to fiddle with thermostat switch and get a little relief from summer excess.

author: RJSV
author_rank: Super Contributor
timestamp: September 19, 2023, 10:44:04 pm
content: To view Medhi's video, please see 'Wireless Communication' which is way down on bottom of list of videos posted by ELECTROBOOM.Discusses the century old 'COHERER' used in original radio telegraph by Marconi Corp.The mechanical switching, is discussed, where there is OXIDE LAYERs that will respond to nearby EM field, say 40 volts, and act as a switch. The reverse action is obtained by tapping the contacts, disturbing any contacts made via an oxide layer, and returning the coherer to open circuit.