Archive for Reflow

Temperature Controller Update

Posted in PCB Manufacturer, PIC Micro, Uncategorized with tags , , , , , , , , , , , on September 18, 2011 by Karel M

No video today but  I have been working on the code and have added some new features.

It is now possible to install a jumper across pins 19 and 20 on the I/O header (with the display removed) to enable a serial terminal mode. When the terminal mode is enabled the display and push buttons get turned off, and pins 10 and 11 on the header function as RX and TX lines. There is one small issue that I will talk about later in this post. Here is a quick screenshot:

Now what can the terminal mode do?  It’s still in the early stages but it already supports several commands. Currently the commands include reading the current temperature (“t”), setting desired temperature (“T”), reading the current temperature set point (“s”), reading the current PID constants (“p”,”i”,”d”), setting the constants (“P”,”I”,”D”), set display to Celsius (“C”), set display to Fahrenheit (“F”), and print all values (“v”). These commands are all working and allow using the controller without the LED display board.

The one letter commands were chosen for a couple reasons. The major reason is adding lots of strings to be displayed on screen takes up memory fast and I want the space free to include additional features in the future. The other reason is the one letter commands should be easy to remember and use.  With my use of the terminal so far this has been holding true. In the future the simple serial terminal could allow a nice PC based application to be written to allow tuning the temperature controller. That’s beyond the scope of the project right now, but the serial option now makes that possible.

I spent time this past week updating the board schematics and layout to fix a few minor issues and add a few things. Mostly, I added vias that would allow 3.3V, GND, and some of the more important signals to be tied into.  I also added a place for a small transistor to replace the opto-isolater on the board.  This will allow the Triac to be removed and an external relay to be switched. There is no space for the relay on the board, but I added solder points to make it easy to wire one off board. These changes will make it easier for myself and others to modify the board for other uses. One that was mentioned to me was controlling a mini-fridge to use in home brewing, especially lagering. This is something I would actually like to try since I do brew my own beer. I won’t have time to try this for a while, but the temperature controller will be a good fit for controlling the mini-fridge.

Coming up, I want to add functions and commands to set the temperature to Fahrenheit or Celsius, save the new settings to the internal EEPROM, and add code to the LED display to allow the variables to the changed and saved. Now that the code for the serial communication is working the other features should progress nicely.

Now the small issue I mentioned before with the serial communication, the PIC24F08KA101 the project is based on doesn’t have 5v tolerant inputs and there isn’t going to be voltage level shifting included on the board (no space for it currently). This means that the serial port on the PC side must use 3.3V signals to safely communicate with the temperature controller. Thankfully there are several options that make this easily possible.

The one I use and recommend is a Bus Pirate. The Bus Pirate is a small board that makes working with different serial communications protocols much easier. The price is very reasonable and maybe even cheaper than a “name brand” serial port adapter.  For the serial terminal I’m using the Bus Pirate in the USB-to-Serial bridge mode so it acts like a USB serial adapter.

Another option is a FT232 chip setup to work at 3.3V. Sparkfun has a breakout board that it set to work at 3.3 volt and would be perfect to use with the temperature controller. This is a good option if the board is going to be left inside a project. If the terminal is only going to be used to setup the controller the Bus Pirate mentioned above is a much more useful tool but either option will work just fine.

I also wanted to mention my kickstarter page again.  If you are interested in this project and would like to get your own board(s) please take a look.  With enough support I will able to do a production run of the boards and get them to someone like you.

Thanks for following the project and enjoy your day.

Karel

Toaster Oven to Reflow Oven – Part 2

Posted in PCB Manufacturer, PIC Micro with tags , , , , , , , on September 8, 2011 by Karel M

Last time I left you with a video showing the control board running a toaster oven. Today, I’m going to start with a video showing a PCB getting reflowed in the oven. I will cover the changes I made after the video.

The first and most important of the changes, a new temperature sensor. The diodes used in the original design are only rated to 175c  and wouldn’t work at the temperatures needed to reflow solder. After doing some searching online, I found what looked like a good and cheap replacement. It’s a 50K Ohm NTC thermistor part number 135-503LAF-J01.  It’s in a glass package like a small through hole diode which worked well because it made mounting the thermistor  much easier.

Speaking of mounting thermistor, it isn’t as easy as running some wires and soldering it in place. Wire that will work up to 300c is expensive and the solder will melt every time a board is reflowed. The solution I settled on, with some ideas from my roommate, was to drill a hole in the side of the oven, cover the hole with epoxy, and run some thin copper tubing through 2 holes drilled in the epoxy. The thermistor leads are then crimped into the copper tubing to make the final electrical connection.

The first step in mounting the thermistor was to drill a hole in the side of the toaster oven. I used a 1/2 inch drill bit but anything close will work. The only requirement is allowing enough room for the 2 small copper tubes to pass through without shorting. I left plenty of room on all sides and drilled a little above the top oven rack slot to allow the sensor to rest on top of the board that was being reflowed.  Here is a photo of the hole I drilled.

I placed a bit of duct tape on the inside of the oven to help keep the epoxy in place. I tried to put the tape on weakly and used the blunt end of a drill bit through the hole to push the tape away a little so the epoxy would be able to flow under the tape a little. That would help keep the epoxy from coming loose later. Here is what the tape looks like inside the oven. As you can see it doesn’t have to be pretty.

Next, I placed the oven on its side so gravity would help pull the epoxy through the hole onto the tape. After looking up several epoxies I settled on JB Weld.  It’s rated to work up to 300C and it’s easy to find. I wasn’t sparing and made sure to work the epoxy under the tape and completely cover the hole. Here is what the wet epoxy looks like.

Now, let the epoxy cure for 24 hours.  Once the epoxy is cured remove the tape from the inside and drilled 2 small holes. The holes should be just big enough to allow the small copper tubing to fit through. I got the tubing at Ace Hardware, but other hardware store should have it.  It came in a pack of 3 – one foot long pieces.  Here is a photo of the tubing with a small diode for size reference.

And a photo of the holes in the epoxy.

I then pushed the tubing in until it was about halfway into the oven, placed the leads of the thermistor into the tubing and crimped it down with pliers. I cut the excess tubing off with cutters and soldered wires from the tubing to the control board. Polarity doesn’t matter with a thermistor. That took care of the hardware mods, you can see the thermistor resting on a PCB in the following photo.

It’s nothing fancy, but it works quite well. The tubing has a bit of spring and gently holds the thermsitor on the board for better thermal contact.

With the hardware modified, I needed to update the software to compute the temperature from the thermistor reading. I settled on using the Steinhart–Hart equation to compute the temperature. This meant the first step was to convert the reading from the A/D converter into ohms. This was actually very easy since it was just ohms law, R = E/I.  E in this case is the A/D result times VCC (3.3 Volts) divided by the total counts of the A/D (1024 for the 10bit A/D). The current “I” in the equation is set at 55uA in the CTMU module. I had the PIC do the math using floating point since I was going to need floating point later for the Steinhart-Hart equation.

Now that the resistance of the thermistor is known, all I need to know was the A,B, and C constants to plug into the Steinhart-Hart equation. This is where I ran into a bit of a issue because there isn’t really a good datasheet for the thermistor I bought. If I was to do this again, I would make sure to buy a part with a good datasheet. Thankfully, the internet came to the rescue and I found an excel spreadsheet that calculates the three constants given temperature and resistance values at 3 points. If you do a search for Stein1.exe you will find the site that has the spreadsheet.  I had to heat the oven up and measure the temperature and resistance at a couple different temperatures. Once I plugged the data into the spreadsheet I had the constants I needed.

With the constants and thermistor resistance known, it was simple to have the PIC calculate the temperature. The PIC does all the math, including calculating the natural log of the resistance. The Steinhart-Hart equation provides temperature in Kelvin, so the program converts to Celsius and, if a flag is set, converts to Fahrenheit.  This number is fed into the PID  control loop and controls the Triac output level.  The floating point math uses about 10% of the code space in the pic, but there is still room left over for additional features.

While I was working on the code, I also added a new feature. The controller on power up checks to see what the line frequency is (50Hz or 60Hz)  and automatically adjusts timings for Triac control.  For use outside of the USA, the only change needed will be a transformer rated at the appropriate input voltage that provides around 9V AC out.

With the code updated, I had to give the oven a try and solder a part. I settled on soldering a 44 pin TQFP part, a CPLD in this case.  I will put pictures below, but it went very well. There were a couple solder bridges but that was caused by to much solder paste. This was my first time using this paste and the oven but it was nothing a little solder wick couldn’t fix. A little less paste next time and it would have been good right out of the oven. I used a ROHS solder paste from Digikey.

Solder paste on the board.

Board cooking in the oven.

Fresh out of the oven, you can see a couple of solder bridges.

After cleaning up with solder wick. It was a success!

Need to clean up the residual flux with some IPA, but the solder looks good.

Next, I want to add more options for setting up the PID control loop to the user interface. Stay tuned for that update.