Repair - Hot Box Temperature Controller Circuit - Page 1
Forum section: electronics / repair

author: kolbep
author_rank: Frequent Contributor
timestamp: October 18, 2018, 08:22:06 pm
content: Good day all.I am busy working on a hot box for transporting food (like they have for Railways, etc).The element wire has gone open circuit in several places, and the Thermistor does not seem to be operating (it is at a steady 41.7 ohms in Ambient temperature, and even with the box full of steam).I have ordered replacement plate elements, so that is not the problem, I just need to get the Temperature Controller working again.Anyway, I have reverse engineered the basic Temperature Controller, and it is as follows (I have Omitted the BTA10 Triac that is on the output of the MOC3041:It is run on 230vac with a capacitive dropper.I was just wondering a few things based on this schematic, and I have double and triple checked the thing.1) Why does the Thermistors 1 Leg (t1) run from the point between R1 and ZD1, but the rest of the LV Circuit picks up its +ve and negative from the bridge rectifier itself? I know the ZD is clamping the Rectifier output to a certain voltage, but I was just wondering why the thermistor is not connected directly to the +ve2) Is this a Bang Bang circuit, where as the temperature rises, the resistance of the Thermistor Increases (PTC), causing the Optotriac, Triac, and Element to shut off when the PTC resistance is high enough to cause the Base to be biased past the threshold of the Transistor itself?orDoes it maybe phase angle control the Element based on how much the transistor is switched on (I doubt it)3) Do you think the thermistor is really pooched, or is it just too deep in the Foam Insulation for it to read the change (i had a kettle steaming the inside of the box for 10 minutes, with no change on the reading.4) I will probably need to change the thermistor itself, to have a new one in a better position in the cabinet. Is my understanding that it is a PTC Thermistor correct, and if so, any specific range.5) Bonus Question : Why is there a resistor in parallel with the Yellow LED?6) Bonus Bonus Question : Where is the Current Limiting for the Transistor/OptoTriac and LED? or is it just relying on the current limit caused by the capacitive dropper supply.Thanks for the tipsP

author: kolbep
author_rank: Frequent Contributor
timestamp: October 21, 2018, 08:24:10 pm
content: Just Bumping this post up, in case anybody has any info to help me.THanks

author: ace1903
author_rank: Regular Contributor
timestamp: October 21, 2018, 08:41:59 pm
content: Analog electronics is not my field but will try to give you some ideas to try it.Zener diode is to provide constant voltage for divider between temperature sensor and R4 resistor.This is bang bang circuit.Measure voltage on zener diode. Replace temperature sensor with potentiometer and observe how circuit works.Probably 4k7 potentiometer will be fine. Find resistance when heater turns off.Then find sensor with that resistance at desired temperature.For test you can use lamp to see when heater is on instead of relaying on temperature measurement.

author: oventech
author_rank: Contributor
timestamp: October 22, 2018, 12:44:04 am
content: Hello saw this thread. Have a web site you could check out. Oven Industries. AC, DC temp controllers. Temp sensors with different resistance curves.https://www.ovenind.com/Also have explanations on operation