stc 1000 temp limit issue (100 C limit)

[Owly055]

I went to a lot of trouble converting a cheap convection oven for kilning grain, never realizing that my STC 1000 is limited to 100C

While this perfect for base malts, I would like to be able to take the temps higher........ any suggestions for comparable temp controller that will run up to about 200C??

I love these STC 1000's........ I use them on lots of things, but never realized that there was a 100C limit.

It looks like PID controllers are the next option, but I have no desire to use a PID. They offer complex settings I don't need, and cannot be used with as simple contactor, but require an SSR. The simple rugged reliability of the STC 1000 with a mechanical contactor appeals to me.


H.W.

 

[day_trippr]

NTC thermistor resistance decreases with increased temperature.
I wager one could stick a 20 turn 1K micro pot in series with the thermistor and effectively shift the operating range upwards. Eg: adjust the pot so while the meter is reading 100C the actual temperature is 200C.

Remembering thermistors are typically non-linear without proper compensation circuitry you'd want to set the pot up with a known good thermometer for the range you want to use the probe. And of course you need to respect the operating range for the probe assembly (including the wire insulation used) which may be working against your intentions...

Cheers!

 

[Owly055]

NTC thermistor resistance decreases with increased temperature.
I wager one could stick a 20 turn 1K micro pot in series with the thermistor and effectively shift the operating range upwards. Eg: adjust the pot so while the meter is reading 100C the actual temperature is 200C.

Remembering thermistors are typically non-linear without proper compensation circuitry you'd want to set the pot up with a known good thermometer for the range you want to use the probe. And of course you need to respect the operating range for the probe assembly (including the wire insulation used) which may be working against your intentions...

Cheers!

Thanks......... I can do that!! I have a metallic probe ordered. By a 20 turn 1K pot, do you mean one with a gear head? I probably have one or more pots of that value lying around, but I'm not sure if I have a gear head that isn't in use.......

Is anything in the real world linear?

My inclination would be to experiment with fixed value resistors, rig a selector switch for low range and high range, and just print out a high range temp chart. Starting out with a pot would help hone in on the ideal resistor value.

H.W.

 

[Owly055]

Thermistor resistance / temp lookup charts seem to be readily available. Looks to me like all I need to do is select a suitable fixed value resistor based on the chart, and then work up a conversion chart using an oven thermometer.

H.W.


******** decided from the chart to go with a 10K fixed value resistor..........


H.W.

 

[Owly055]

...... well perhaps no so big. I should have realized from the parabolic curve on the NTC thermister that a resistor would NOT work. The curve flattens out at around 100C, which is the limit value for the STC 1000. Adding a resistor only changes the reading, but above 100C, the value of the thermister doesn't change at all......... or not significantly. What this boils down to is that the STC 1000 is for all intents and purposes limited to 100C, or actually 99C is where it cuts off. The only solution would be to find thermister where the curve is shifted toward the hot side, and I strongly doubt that this is realistic. I'm not sure where one would start on this, and the curve is likely to be wildly different. I really only need to be able to control temps in the 80 - 300F range

I shorted out the thermister I was using.....but I ave a few STC 1000's around

Actually I need ohm the thermister out at different temps. I can't make heads nor tails out of the chart Vishay has for their thermisters, but it does look like one similar to the one I was using may have an operating range close to one that will operate up to 150C.

A lot of messing about for what? I will have to live with the temp limits of the stock STC 1000 system for now, but it's clear that a PID and SSR is in my future ;-( I thought I had a PID somewhere, but I can't find it at the moment, and I never did have an SSR, though I have a bag of high current triacs in my collection. Wonderful little things. I love mosfets and triacs!!

The complexity of PIDs is enough to make my head spin. Their predictive ability and interpolative ability is amazing, but the set up is more than a little bit intimidating. Nothing else offers even remotely close the same level of precision once they are properly configured.... it's the once they are configured part that I find intimidating ;-)


H.W.