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标题: the solder box [打印本页]
作者: liyf    时间: 2013-9-29 23:56
标题: the solder box
Cookworks Mini Oven – White will be used for the first prototype
Published May 14, 2013 | By admin
                                                                                This is a inexpensive and fairly well built toaster oven, bought from Argos for £29.99, item number 423/5390.
The main consideration of this oven is that it is 1300 watts which will enable it to reach the desired maximum tempeture of around 250*C once the thermostat has been removed from it, on first observations I can see that the oven also has a lot of free space behind the knobs enabling electronics to be fitted in, with the consideration of any insulation if needed.
This oven has 4 equally spaced heating elements which will enable even heating within it, equaling even heating of the PCBs. The oven comes with a nice grill and trays which could also be useful in further development. Considering the power rating 1300 watts, using some simple maths of power is equal to voltage times current (p = I * V), 1300/230 we can see that a healthy 5.7 Amps will be drawn at the wall. This also means that we will require a large relay or even a solid state relay which would also give benefits of liking PWM a bit more than a normal relay.
This oven looks very promising and will most likely be my next purchase.
Solid state relays came in
Published May 14, 2013 | By admin
                                                                                So a few weeks ago I had ordered some solid state relays and heatsinks in from China. The relays I ordered were Fotek SSR-25 DAs which are 25 Amp high switching speed solid state relays. The advantage of using a solid state relay is the increased switching speed which enables PWM (pulse width modulation) to be used to control it.
The solid state relay works with a 3 – 32V DC input enabling it to be controlled with an arduino, and it is able to switch AC voltages of 24 – 380V which means it’ll be able to be used across the world at different line voltages. I purchased along side heatsinks as the switching of the AC requires some power dissipation in turn creating heat.
The idea is to keep all mains power within the toaster oven so it is critical the relay stays as cool as possible and we have the additional heat from the oven in contact with it. The heatsink is approx 50mm high by 80mm wide, and will provide adequate cooling for the relay, in addition a 50mm fan will be used in conjunction in order to help dissipate as much heat as possible from the heatsink.

                                                    
                        
Thermocouples came in
Published May 14, 2013 | By admin
                                                                                Unlike normal thermometer silicone chips such as the DHT11, K type thermocouples can measure temperature over a very large range of around -200 – 1300 *C accurately. This is thanks to the Seebeck effect where two different connected metals both at a different temperature will create a small voltage. More can be read of Wikipedia.
This voltage can be then be amplified and referenced against a known temperature by a discrete IC in order to find the temperature in the probe I have been looking at the maxim 31855 IC for this purpose in this project. The different letters in the thermocouple’s names only refers to the different combinations of metals used, in our case K refers to a combination of chromel {90% nickel and 10% chromium}—alumel {95% nickel, 2% manganese, 2% aluminium and 1% silicon}  (from wikipedia).
The thermocouples I purchased were inexpensive from China, they are 1 meter long, with a 15mm probe which has a nice thread on the probe meaning it could be screwed into the toaster oven securely. The thermocouples have full fiber glass insulation along with a rather nice metal weaved sleeve on the wire along with nice spade connectors on the end. At the moment I am aiming to keep costs of the final product down by only using one temperature sensor in the center of the oven.

                                                    
                        
Thermocouple Amplifier – MAX6675K / MAX31855K
Published May 14, 2013 | By admin
                                                                                As previously explained thermocouples use the Seebeck effect to generate a voltage through the temperature difference between 2 different metal elements. This voltage must be amplified and compared against a known reference voltage. This could be done by using an opamp to amplify the voltage which could be read by the ADC (analogue to digital converter) of a microcontroller, then compared against known voltages and temperatures to work out the final voltage of the thermocouple.
This method would work, but would be a very long winded and difficult method, as the temperature of the microcontroller and surroundings would have to be considered to compensate for different readings. Also doing this method would required more processing cycles of the microcontroller which would in turn increase the time to take a reading.
A method of improving this is to consider using a dedicated thermocouple chip such as the MAX6675. This chip is from Maxim, and offers full temperature compensation allowing a microcontroller to read the temperature directly from the dedicated chip using a SPI 3 wire serial connection. There are many already written libraries around which allow microcontrollers such as the arduino to easily read the current temperature off of the chip.
The MAX6675 is a 5 voltage chip meaning that it uses a 5V source voltage, and uses 5V logic in its serial connection. This chip would be very suitable for our purpose, though Maxim have discontinued this chip so it is very expensive to source at around £10 a chip, and is becoming very hard to find. Though Maxim have released its newer successor the MAX31855, which has a very similar spec as the MAX6675 though runs at 3.3V source and serial logic. This is rather annoying as it’ll require an additional source 3.3 voltage regulator such as the LD1117V33 (around 50p), and a couple of resistors for a voltage divider for the serial connection from the microcontroller 5v to 3.3v thermocouple chip. Fortunately most microcontrollers such as the Arduino will accept a 3.3V signal as a 5V high signal (the cut off is around 2.5V for a high signal) so this will negate the need for additional transistors to step a 3.3V signal to 5V.
This solution will run in much less than the MAX6675 even in considering the additional voltage regulator and resistors.
From Maxim;

                                                    
                        
MAX31855 came in.
Published May 14, 2013 | By admin
                                                                                After previous research I ordered some MAX81355 samples from Maxim to use on this project, they came in today and are pretty small! They are 8 Pin SOIC chips and are surface mounted, which I find quite ironic! I ordered the K versions which are calibrated for K type thermocouples.
A small description of the spec of the chips from MAXIM;
The MAX31855 performs cold-junction compensation and digitizes the signal from a K-, J-, N-, T-, S-, R-, or E-type thermocouple. The data is output in a signed 14-bit, SPI-compatible, read-only format. This converter resolves temperatures to 0.25°C, allows readings as high as +1800°C and as low as -270°C, and exhibits thermocouple accuracy of ±2°C for temperatures ranging from -200°C to +700°C for K-type thermocouples. For full range accuracies and other thermocouple types, see the Thermal Characteristics specifications in the full data sheet.


                                                    
                        
First circuit design
Published May 14, 2013 | By admin
                                                                                So had time to draw up the first circuit design of the prototype for the Solder Box, the main considerations in the design is the simplistic user interface consisting of a 16×2 I2C LCD display, an analogue dial (working on a variable resistor adjusting the voltage on a potential divider) and 2 buttons working on the both external interrupt on the ATMEGA328 (possibly PIC18F252).
As previously mentioned a MAX31855 will be using a K type thermocouple to read the temperature of the toaster oven (is read by microcontroller by a SPI bus).
To be noted as well is my consideration of  double isolating the microcontroller from the mains, by using an optoisolator (either 4N35 or 4N25) to control the solid state relay in turn to turn on the heating element within the toaster oven. Using PWM on the micrcontroller to control the optoisolators by the readings of the thermometer for the corresponding solder profile.
Apart from these elements there isn’t much else to write home about, the main idea is to try keep this as simple as possible to make TheSolderBox as affordable as possible to people. I will digitize the design on Eagle or similar after I finish my University Exams.

                                                    
                        
Microcontrollers
Published May 15, 2013 | By admin
                                                                                The idea of this project is to keep the whole system as open source and inexpensive as possible, to do this a microcontroller which is inexpensive, easily available and easy to code for is required. With my past knowledge I knew that there were two main contenders, and these were the Atmel ATMEGA and Microchip Picaxe series of 8-bit microcontrollers.
Looking at the two main contenders the Atmega328 and PIC18F252;

Atmega328PIC18F252
Flash (Kbytes)
32
32
Pins
32
28
Max operating Frequency
20
40
CPU8-bit AVR8-bit PIC
I/O Pins
23
23
EX Interupts
24
17
Both compare well and both offer enough speed and I/O for the purpose of a reflow oven, and in terms of coding, I would say they are more or less equal.
Though favoring the Atmega’s for programming, thanks to the Arduino project where you can burn an Arduino Uno bootloader onto the Atmega328, in turn allowing it to use the Arduino programming GUI which would allow for a easier and more open source platform to work on.
Also considering the large amount of similar open source projects based on the Arduino, and the many written libraries for many components which will help when it comes to coding.
With these considerations I think the Atmega328 will be used for TheSolderBox
Toaster oven and Atmega328P-PU Came in
Published May 15, 2013 | By admin
                                                                                Just a quick update, the Atmega328s came in from Atmel, these are the Atmega329P-Pus which are the PDIP package enabling them to be used with a IC socket, which will allow them to be programmed and exchanged on the fly between an Arduino and TheSolderBox motherboard. These do not have a boot loader on so I will have to burn a boot loader on them in order to be able to program them using the Arduino interface. My plan is to use a 16MHz crystal with these, and follow a very similar circuit as to the stock Arduino Uno.


Also the Cookworks white mini oven from Argos came in, on first looks it looks of good quality and is exactly from what I had seen on the website, it was 4 nice heating elements and has a 230°C thermostat which should be easy enough to override to get the oven up to the 250°C the maximum melting point temperature for near all solder paste on the market.

Its always nice being able to see an idea to start to come together! I will hopefully get TheSolderBox on Kickstarter by next week once I’ve done some more artwork, made a video and done some more on the prototype!
Opto-isolators Came in.
Published May 15, 2013 | By admin
                                                                                As mentioned in the initial circuit design the microcontroller will be double isolated from mains via both a smaller opto-isolator and the large solid state relay. Opto-isolators are basically a single chip package with both an LED input and a opto-transistor output, and are a way of controlling a high voltage device using a lower voltage device whilst at the same time isolating the low and high voltage devices from each other.
In our case this will be protecting the lower voltage microcontroller from the higher voltage mains. The additional opto-isolator between the SSR and microcontroller is not 100% needed, but is going to be implemented as an additional safety feature to the user.  Opto-isolators can offer up to around 10kV isolation voltage across them, protecting both the user and the microcontroller.
Two widely used opto-isolators are the 4N25 and the 4N35, both offering voltage isolation up to around 3.5kV (well over mains voltage). The main two differences between the two are the forward voltage of the light emitting diodes inside them and the current capacity of the opto-transistor of the output. The considerations in our case will be the possible current output/sinking of the Atmega328 for the input into the optoisolator, the forward current of the LED within the SSR required from the output of the opto-isolator.
In our case the atmega328 can supply up to 40mA for the input to the opto-isolator which has a forward current of around 40mA, and the SSR requires a 7.5mA forward current which the optoisolator can supply at least 4mA with the optimal around 10mA but can go higher which is more than enough.
The 4N35 is more efficient than the 4N25 and at only a few pence more will be the choice within this project. They came in this morning and look nice! I have everything bar the 16×2 LCD now to work on the prototype!

                                                    
                        
First Circuit Design Done.
Published May 16, 2013 | By admin
                                                                                So sitting down I have worked on a circuit and board designs for TheSolderBox’s main motherboard. As mentioned all features I wanted to, have been able to been put in it.
There are 3 main pin headers for breaking out to the toaster oven and I/O. One header consists of the thermocouple electronic contacts, along with the solid state relay control lines, and a power rail for the solid state relay’s heatsink fan. Another header consists of two I2C lines along with 5 volts and ground to power and drive the 16×2 LCD display, with the final header consisting of the button interface consisting of 2 digital buttons, 1 analogue pin and a 5 volt and ground lines.
All three headers consist of 1×6 pin breakout headers. Everything else is as previously explained, though not previously mentioned there will be power supplied by a micro USB power header, which has 2 potential dividers of 2.75v on the D- line, and 2.0v on the D+ line ensuring the maximum current draw of 900mA will be supplied if plugged into a computer to be powered.
The idea of the micro USB connector is to help drive down the cost of if mains was used to power logic as this would require an additional power supply consisting of transformers, full bridge rectifier and voltage regulators which would substantially increase the cost. Near everyone has now got a micro USB phone charger which can be used as a power supply, or the controller can be simply just plugged into a computer USB Port!
Its time for me to get on with the PCB design so I can get the first prototype boards manufactured!

                                                    
                        
Quick Update
Published May 17, 2013 | By admin
                                                                                So sat one of my University exams today, I have 3 more to sit and then I can really make a crack on with the prototype, I’ve had some time to go through and structure a little bit of code. I’ve also had a little time to double check the internals of the toaster oven which actually look quite nice and easy to modify which is awesome, as it will make it really easy for you to modify the controller in!
My last exam is on the 29th, but I am hoping to finish the Kickstarter video and artwork to hopefully put the project on for fundraising on the 22nd! Keep your eyes peeled!







                                                    
                        
Initial Kickstarter Donation Prices
Published May 19, 2013 | By admin
                                                                                So we are just finishing off the video to put on our Kickstarter campaign page, and we will then be on Kickstarter!
We have worked out the initial donation brackets and the prizes for each bracket!








So look out for us on Kickstarter in a few days hopefully! Let us know what you think of the donation tiers! Progress on this project is really moving forward!

Tested making our own PCBs for use with quick prototyping!
Published May 23, 2013 | By admin
                                                                                We had a go at etching our own single sided PCBs using acetate films and a laser printer!The PCB came out really good! Time to try a double sided one? This could be used to prototype designs. Here is the made test PCB, a twin T notch filter includes 2 TXInstruments 741 opamps & a NXP TDA1013B amplifier!

                                                    
                        
Fitted thermocouple in oven and mock fitted the relay heatsink!
Published May 23, 2013 | By admin
                                                                                After a little bit of fitting and drilling with the dremel, managed to mount thermcouple.
Managed to screw the thermocouple as made the hole the same size as the threading on the thermocouple. The lead of the thermocouple manages to fit through an old dial hole as well which is nice as it will give a nice finish. I fitted the thermocouple in the center in order to give the best average temperature within the oven and closest to the PCB and temperature.
Managed to mock fit and drill holes in order to fit the solid state relay heatsink, fits pretty nicely!


                                                    
                        
Breakout PCB For Thermocouple Amplifier
Published May 31, 2013 | By admin
                                                                                I made up a PCB design to print out and etch a breakout PCB for the prototype, this is needed as it is very hard to use a SMD device with a stripboard PCB otherwise, it’ll use 3mm pitch pin headers to solder it to the breadboard. Exams are finished so I will get the prototype finished for the end of this week!


                                                    
                        
LCD Is Working!
Published May 31, 2013 | By admin
                                                                                So as a quick update I have the LCD working all well via the I2C interface which is awesome! We are just about to etch the breakout PCB!


                                                    
                        

作者: robter    时间: 2015-10-21 22:12
感谢楼主提供这么好的交流平台



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