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14 May 2010
Author: Giorgos Lazaridis

Worklog - The idea - some materials (May 14 2010)

The 30 Watt 13 Euro soldering iron that i hope i will transfer into an expensive soldering station

Soldering stations comes in a variety of prices and capabilities. The cheaper stations have a power controller, to control the power delivered to the soldering iron. More expensive stations will have a temperature controller near the tip of the soldering iron, and control the temperature using this feedback.

The quality of soldering with a station is much higher than with a simple soldering iron. Especially if the station has a temperature controller on the tip. That is because, the solder has a very specific working temperature. For example, the one i use (60% tin 40% lead), is liquefied at 190oC. Of course, you do not solder at 190oC! The soldering iron i use, exceeds by far this temperature. I measured it up to 410oC! This has negative effect on the soldering quality.

The idea

The idea was simple, yet the implementation according to the standards that i had set was much harder to follow as i originally thought. I will use a 30 Watt soldering iron for the job. Somehow, i will measure the temperature as close to the tip as possible. For temperature sensor, i will use a K-type thermocouple pair. I have a complete theory for thermocouples that you may be interested to read. A controller circuit will read the temperature. A potentiometer will be used to set the requested temperature, according to which the controller will turn on or off the soldering iron heater.

An unexpected surprise from Maxim

The chip soldered on the PCB

I got myself a couple of them

If you read the theory of thermocouple pairs, you will see that the controller must compensate the cold junction temperature. Moreover, the temperature acquisition from the thermocouple voltage, is a result of a long polynomial solution. I was just about ready to start the design using a thermistor for the cold junction compensation, when i accidentally came across this:

MAX6675 - Cold-Junction-Compensated K-Thermocouple-to-Digital Converter (0ï¿½C to +1024ï¿½C)

This chip will do all for you. It has a built-in temperature diode, used to do the cold junction compensation. The thermocouple pair is directly connected to it. It also carries an analog to digital converter, and produces a 12-bit number, with 1/4 of a degree precision. With the simplest protocol ever (3-Wire/SPI), the microcontroller can acquire directly the hot junction temperature in degrees Celsius. The chip comes in a tiny SO package.

There is only one drawback with this chip. It is somehow expensive and cannot be found everywhere... From where i get the PICs, i buy the 16F88 for 3.9 Euro. This chip is sold for over 8 Euro per piece. So, the question here is: Do i buy this expensive chip that will double the price of the controller and avoid all the hard work, or do i go the hard way and save 8 Euro? I was too curious to see how this chip works, therefore i had no time to answer the previous question. My finger followed the checkout procedure by reflects... I got myself a couple of these chips to check them.

And then i got the chips! There is only one word to say: "It works!". With minimum effort and even less programming effort, i got an accurate reading from the thermocouple! The connection and the communication protocol are ridiculously simple.

The display

I got 3 LCD modules, one is already on the workbench

The original idea was to use some 7-segment digits to display set temperature value and tip temperature. But then i decided to give it a more profesional look and feel, by using a 16x2 character LCD module. Surfing the ebay, i found a manufacturer that made a terrible mistake. A whole batch of these displays had a mistake: the backlit connections were not according to standard pinout! Pins 15 and 16 were in reverse order, and therefore he was selling them ridiculously cheap. I got 3 of them, one was for this project.

The first line, will have the set temperature value, and the real temperature value on the tip. The other line, will have a slick bar with the set value. That is subject to change of course.

The thermocouple pair

Some K-type thermocouple pairs
I will use the one size fits all K-type thermocouple pair. I already have some wires. Yet, i am not sure which to use. I still have to solve the problem of the thermocouple mounting. I need to install it as close to the tip as possible. So, i have a thermocouple pair from a multimeter, some thermocouple extension wires, and some pieces of thermocouple wires. If you are interested, i have run some interesting experiments with K-type thermocouple. You may also like to read the theory of thermocouples.

So, let's get to work...