Thermostat design - Page 1
Forum section: electronics / projects

author: Rachie5272
author_rank: Regular Contributor
timestamp: December 15, 2014, 05:14:44 pm
content: This is my first attempt at making a prototype circuit for what will hopefully become a commercial product. The application is a small kitchen appliance, which is little more than a heating element with a thermostat and a graphical oLED display.The microcontroller is basically an arduino setup for ease of programming and testing. I'll probably switch to a cheaper micro eventually, but this is the easiest way to get the display running.For the power supply filtering capacitors, I'm using four 10 uF multilayer ceramics instead of the conventional electrolytic (heat sensitive) or tantalum (conflict mineral). Is this crazy?I'm more concerned about the high voltage side. I went with a TRIAC instead of a relay for silent operation, and the zero crossing optoisolator means the lights in the room won't flicker when the heater turns on. I'd very much like to move the bulky transformer off the PCB, but I have no idea how to source one with leads and a connector attached to it.The mains fuse, thermal fuse, and thermal circuit breaker aren't shown.Any comments on my design? Hopefully it's not too bad.

author: Seekonk
author_rank: Super Contributor
timestamp: December 15, 2014, 06:42:23 pm
content: Is your control wham bam like a comparator or do you cycle on off proportionally like 0-10 seconds on as you get closer to the temperature. What do you use as the temperature sensor. What exactly does the micro have to do? Seems like you could do the zero detecting there.

author: Rachie5272
author_rank: Regular Contributor
timestamp: December 15, 2014, 07:26:21 pm
content: I might do a PID control system, but I'm not entirely sure yet. I made the design somewhat robust so I can try different ideas, and only populate what I need. The goal is to keep the system at a fairly constant temperature, be user adjustable, and display the measured temperature. I'm planning on using a 10 k thermistor as a temperature sensor, placed next to the heating chamber.The microcontroller takes user input from the dial and buttons, displays graphics on the oLED, and turns on the optocoupler TRIAC. I may or may not use the piezo buzzer, and might use the diode bridge for zero cross detection instead of a slightly more expensive optoisolator.

author: firewalker
author_rank: Super Contributor
timestamp: December 15, 2014, 08:14:56 pm
content: What temperatures are we talking about?Alexander.

author: Rachie5272
author_rank: Regular Contributor
timestamp: December 15, 2014, 08:45:17 pm
content: Around 60-70 °C is the normal operating temperature. I chose a thermal circuit breaker rated for 85 °C, and a thermal fuse rated for 104 °C. Both are overspecced in current by a factor of five.

author: Seekonk
author_rank: Super Contributor
timestamp: December 15, 2014, 08:45:35 pm
content: I must be getting old. I've fallen in love with these $3.65 shipped temp controllers from China. They do temp, differential, delay, alarms and you get a display. They have taken a lot of fun out of building things. An arduino does run my solar powered camp. The chest fridge uses a stack of four diodes as the temp sensor. That was a pain to calibrate. One thing I should warn you of, plugging in you laptop will likely screw up your calibration. The board will take whichever 5V is higher and that will likely be the USB power and use that for the reference. I ended up cutting the 5V wire on the USB cable. I heat hot water with solar PV at the camp with a couple of tanks. The UNO keeps the panels at power point for maximum heating. I went to using DS18B20 temp sensors to get a digital reading of the tanks. I'm through with analog calibration!

author: tautech
author_rank: Super Contributor
timestamp: December 15, 2014, 08:54:42 pm
content: Quote from: Seekonk on December 15, 2014, 08:45:35 pmLink?

author: Seekonk
author_rank: Super Contributor
timestamp: December 16, 2014, 10:01:23 am
content: Here is the linkhttp://www.ebay.com/itm/141442045768?_trksid=p2060778.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AITMicro itself runs from onboard 5V regulator so it can operate at lower voltage. Cut the trace to the relay and you can drive a SSRY for lower current draw. Relay makes it hard to mount on a panel face. They do have newer versions where they mount switches and display on the backside.

author: SeanB
author_rank: Super Contributor
timestamp: December 16, 2014, 11:25:40 am
content: Quote from: Seekonk on December 16, 2014, 10:01:23 amInteresting shop there, especially the 99c stuff.................... Ordered some, but as well some more stuff. Interesting that there is a 99c breakout board with a 3 axis magnemometer there as well.

author: ptricks
author_rank: Frequent Contributor
timestamp: December 16, 2014, 11:56:38 am
content: For simplicity I would use a solid state relay instead of the TRIAC because the isolation can be easier to implement than placing components on the board and trying to make sure you have isolated the live AC side. SSR removes a lot of the circuitry you are currently using and while not as cheap as single parts on a board, going commercial will be a lot easier to pass the safety requirements.