Mr Al:
OK, now back to a rather unfinished project. The micro/convection oven, First, it has to be fixed. A triac and opto+series resistor fried and I somewhat forget the circumstances, but the magnetron popped and wasn't replaced in time. The repair forum where someone posted a request for help on my very same micro/convection oven is apparently no more. I think I tried putting a small bulb for the magnetron and the other stuff popped.
I guess I'll have to take some pics. In any event that was a early Kenmore/sanyo convection/micro built around the mid 80's. Searspartsdirect did have a parts list and a stuffing diagram at one time that I nabbed, I finally found service manual, so I have a parts list which is not in the SM; the SM which does not have a schematic. It;s hard to figure why board level parts were available at one time and the SM doesn't contain either the parts list or the board schematic. It's still unfixed. Convection works. Could be unfixable if the processor port is damaged. The IC seems to be available if I could speak Chinese.
In any event, the oven is huge and it had three design faults. Sanyo fixed one and attempted a fix for the other which I corrected. It supported the fan with one bearing. It's now supported with two. They changed the shaft to be non heat conductive.The oven was like $950 in the 80's. Sanyo fixed the rack supports. Suffice to say, I really like it.
The third fault: What's peculiar, is how the temperature probe is designed. It uses a modified 1/4" phone plug with a large insulated area for the independent actuator contact that rests on and the plug is not compatible with normal phone plugs or high temperature ones. The high temp ones would be difficult to mount and don't include the switch. The probe operates from 105 F to about 180 F or so. 50K NTC at 25C.
I figured out what curve it is. It's a Y with 50K at room temp. Luck would have it, I managed to get a replacement probe with the IDENTICAL characteristics . The OEM probe has an intermittent contact at the probe. Both the new and OEM probe were characterized from about freezing to above boiling using a potato and (water or oil) and a stove. Try getting this info from the guys making the replacements. GE tried, but said take the data with a grain of salt which I did. It was wrong.
I managed to also get a 1/4 phone jack with an isolated contact because the original jack is damaged upon recent inspection. It happens to have a Nylon actuator which isn't a good choice because heat up to 450 F escapes through the 1/4 hole and passes the Nylon actuator. This isn't good long term.. So, a few possible fixes now presented itself. For all fixes, the thermal conduction to the jack phenolic will be reduced with Titanium washers.
Th easiest fix, is to use a piece of Macor (a machineable ceramic) to plug the hole when the probe isn't in use. The second and third options are dying fast because my access to a lathe could disappear at any time. A friend is in the process of dissolving his business. The lathe was used primarily to keep the old machinery running and to make jigs.
Other options are to replace the Nylon with Macor and either glue or try to use a plastic thread forming screw. All three should work. The latter two have convenience going for it. I think I have a suitable screw.
So, it looks like I have a fix.
So, I should have a working probe and a convection/microwave that's fixable. I have a new magnetron and HV capacitor.
The ORIGINAL intent was two fold:
1) Make a thermistor type changer with open thermistor detect.
2) and to TEMPORARILY change the range to 70 degrees less. e.g. 80F + 25 is 105F and 105+80 = 185F so I can include at least room temperature and yeast proofing temp which is like 95 to 100 F although these
**broken link removed** guys think differently than
https://en.wikipedia.org/wiki/Proofing_(baking_technique) A sloppy 105 F is too hot.
So, why can't I warm something from the fridge or freezer to room temperature?
The thermister curve is nuts.
Some numbers:
F C K-ohms
50 10 99.72
68 20 72.84
86 30 53.01
95 35 42.92
104 40 32.55
113 45 26.20
122 50 21.75
131 55 17.94
140 60 15.38
158 70 11.39
176 80 8.57
194 90 6.84
203 95 5.98
212 100 4.88
I need the simulation box to figure out what the 105 F to whatever in steps of 5 degrees is in terms of voltage across the thermister. I haven't reverse engineered the thermister input circuit yet. It is a thermister to ground, bit that's about all I know, I think the open circuit voltage is 5V.
Ultimately, I want to replace the thermister with an OPTO-FET and a totally different linearizion to accommodate room temp and the yeast proofing temperature. I envision something like push a button and the low temperature mode is entered. So, if I want 75 deg, I'll set the temperature to 75+80.
When the probe is pulled out, the OEM mode is reset. Readings would be off too.
Initially, I was thinking of a thermistor type changer with two ranges and open thermistor detect, but I don't really have to do that.
I think I got th MAX1452 and the corresponding development kit **broken link removed** but I don't think it's going to work out. This part the TI PGA309
https://www.ti.com/product/pga309 might be a much better choice. Maxim agrees that their documentation needs work, but it isn't going to happen.
So, I don't care if every temperature below room temperature is room temperature either. Right now, every temp below 105 F is 105 F.
In order to actively do anything, I need the voltage across the thermistor at 5 degree increments which is the resolution of the setpoint and the measured value. I will definitely include an indicator even if it's on the back of the microwave by the push button when the low temp mode is activated.
It gets messy especially with open thermistor detect. I don't know if there would be any other glitches. I can't use a 100 deg F offset.
The machining is conceptually easy. I need to machine a shoulder, but it's a small part and I've never machined Macor before. I found an epoxy that would work, but it's also way to expensive, thus the idea about the screws I already have. You can tap Macor, but this screw is tiny. I've procured the Macor for both options.