K-Type Thermocouple PID Controller

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18 August 2011

Author: Giorgos Lazaridis

K-Type Thermocouple PID Controller

To read the complete worklog - click here (worklog)

To see the references for this project - click here (references)

The purpose of this project is to make a PID controller from a scratch, following the PID theory that i wrote some time ago. I plan to use this controller for a future project that i will make, a PCB preheater. Here is a video in which i demonstrate the controller:

I usually test the circuits on a breadboard first, but due to the fact that the MAX chip is an SMD and also the circuit is quite complicated, I decided to go directly on a prototype PCB instead. The whole idea worked perfect, except of a minor mistake that i did when transferring the circuit from schematic to PCB.

The bard has 2 potentiometers, one to control the set-temperature and one to control the speed of the fan with PWM pulses. There are 4 inputs for 4 switches/buttons, and 2 outputs for 2 LEDs. The controller can control both 3-wire and 4 wire fans with RPM feedback. When controlling 3-wire fans, the system uses the pulse stretching method to acquire the fan speed.

The controller reads the heater temperature with a K-type thermocouple, interfaced to the PIC with the MAX6675 (Cold-junction compensated K-Thermocouple to digital converter) through SPI protocol. The heater is controlled by a TRIAC (BT136D) and can handle up to 4A. A galvanic isolation is achieved by using an optocoupler to drive the TRIAC'S gate. I currently use the heater from a hacked hair dryer.

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Comments

At 27 January 2012, 7:26:09 user Giorgos Lazaridis wrote: [reply @ Giorgos Lazaridis]

@awan i've been trying to convert the old-style articles (like this one) with the new style articles. I have already upload the TCPDF library, but i cannot get it to work. I hope everything will work well in the end.

At 27 January 2012, 1:39:54 user awan wrote: [reply @ awan]

thanks bro for sharing ur project it will be very kind if u share pcb file for this project ....

At 7 November 2011, 21:17:21 user Brian S wrote: [reply @ Brian S]

Thanks for the details. It is a pleasure to read or listen to your instructive discussions regarding your interesting projects. I like your attention to intelligent detail.

At 22 August 2011, 7:59:14 user Kammenos wrote: [reply @ Kammenos]

@Drone i have both of them in stock ;) I think that MAX6675 is discontinued

At 22 August 2011, 7:51:32 user Drone wrote: [reply @ Drone]

Don't use the MAX6675. In late June this year Maxim came out with the MAX31855. It is a better part IMO.

At 22 August 2011, 7:25:35 user Kammenos wrote: [reply @ Kammenos]

@Fung they are different brands. Same type, same connection, different brand.

At 21 August 2011, 12:03:25 user Fung wrote: [reply @ Fung]

Why do 3 of the 7-segment displays have different surface color? Are they specified to use?

At 18 August 2011, 18:35:09 user _pike wrote: [reply @ _pike]

My opinion is to buy the same chip instead of trying to recompile the full programm also sometimes some fuctions are different from pic to pic....it is not worthy recompiling....

At 18 August 2011, 13:06:43 user Kammenos wrote: [reply @ Kammenos]

@Stefan i'm sorry but i do not do circuits (nor programs) on demand. You will need to do the change yourself, or get a same PIC. As for the ULN, it is a darlington pair array. Instead of using 6 transistors i use this.

At 18 August 2011, 12:42:49 user Stefan wrote: [reply @ Stefan]

Good job,I have a plans for this,but there not in function.So I would like you to finish this project and to build one for myself.But I don't have that PIC microcontroller that you're using I want to use PIC16F877A(887 etc.) can you recompile for it?And I want to know why there is ULN2023,what is his job? :D