It's been a while but I've managed to run a couple boards with the design. They are eagle format so feel free to take a look. based around an ATMEGA168, there is a thermocouple amp, accelerometer, and a couple buttons and LEDs. Quite a small system so it can fit into the barrel of the pen. The MOSFET should be okay running 24V with a good few amps but I haven't tested it yet. I've got it all running except the micro. Can't seem to get it programmed using an arduino set up as a programmer. As a side note, the three pole connector at one end of the board is for a headphone jack, specifically this one LUMBERG - KLB 4: http://uk.farnell.com/lumberg/klb-4/soc ... dp/1200144 It's mounted by just sliding the terminals between the PCB and they line up perfect, just needs to be the right way around.
I haven't worked on it in a wile due to exams and such but might get back to it at some point. I've recently bought a "hakko-like" soldering station and hot air gun so got less motivation to finish this. If anyone is interested in finishing it, feel free, just remember that the amplifier only gives good readings where there is no power to the tip, i.e. between pwm bursts.
Right I've got it working! So forgetting what I wrote in the last post, it was late and the stuff I wrote about resistance was rubbish. Starting fresh... :P
I'm reading the thermocouple output (between SLEEVE and MIDDLE) using an AD8495 thermocouple amplifier. This reading is going into an AVR and I'm get a nice analogue value to work the control with. The amplifier goes to the high rail whenever the heater is active, then gives out a mV reading once the heater is turned off. Hence to keep from hurting the AVR, I run the amplifier off the 3.3V rail.
I'm now building a driver circuit with a couple MOSFETs and will feed some bursts of PWM into it. During the time between the bursts I can read the amplifier output voltage.
Hopefully that will allow me to create the control system to run the tip at a stable temperature. I believe a PID controller is what I should use. However I still have another year at university before they teach us that >.< anyone know any good sites I can learn about them from?
In other parts of the design I've got myself an ADXL335 3 Axis Accelerometer. This should give a good indication of then the iron is being used from movement. I'm trying to keep the devices analogue so I can use the internal clock of the AVR. I believe that I2C parts can't be used with an external oscillator or crystal. Am I right?
I plan to use two small tactile switches and eight 0604 blue LEDs to indicate temperature. The entire thing will be housed in a 10mm OD/8mm ID aluminium tube. The 3.5mm jack will solder directly to the board so the entire controller with switches and LEDs will be housed inside the barrel of the pen. I plan to use a 12V 50W laptop supply I have to power it.
Ok guys, got a new tip and I think I've cracked it. The idea seems to be that the temperature is read while the heater is inactive. Also I believe it could contain some kind of thermocouple because the tip seems to show a similar behaviour to all my other thermocouples. The MIDDLE to SLEEVE has a 1.1ohm resistance that quickly becomes zero and stays at zero once the tip heats up to soldering temperature (therefore can't be a PTC). I read 3.15mA between these points once the temperature reached the point to just melt some lead free silver solder. Then measure 0.07mV at room temperature. Remember these are values I see once power is disconnected, else I read 0V
I have also noticed the change in resistance as tip temperature rises. My previous experiment didn't give me this so I'm putting it down to having burnt something out. I get between TIP and MIDDLE 3.1ohm at room temp and 35ohms at soldering temperature. Again I have to remove power to get this reading, else it reads a short. It's also a pretty smooth curve all the way up.
So, I'm wondering how to go about making this controller. Do I make use of what seems to be a thermocouple, or just read the resistance between TIP and MIDDLE.
Note that this is not the heater resistance (TIP to SLEEVE), but rather something between the TIP and MIDDLE (looking at the above diagram from my first post, I haven't put anything there...). Could be a PTC, but can you get PTCs that are this low resistance? I thought they read in the kohm range
I had a feeling it could be a thermistor, it would explain why I get several kohm readings from it once it heats up, I think by having put 2V @ 2A into it, I may have toasted it. I'll try get hold of another another tip soon and start fresh.
That's the thing, it jumps around as I vary the input voltage, not if I change the temp of the iron by dunking it in water while it's hot to make it cold.
My meter has a temperature measurement function for use with k-type thermocouples, this is what I've been using so far both for using the probe to measure tip temperature and also taking the reading from the connector itself.
It's odd that I read voltage between the MIDDLE and SLEEVE connectors when it's powered, in the order of mV and V, this isn't thermocouple range. That's what is making my meter read in the order of 1000s of degC.
Hi all, I've already spoken to some of you on the blog about this idea but I'll repost everything here to make sure we're all on the same page.
So far I've gotten hold of an RT8 tip for the Weller WSM1 and similar controllers. I'm trying to build my own small form factor controller for these tips. A simple controller that can fit inside the barrel of the soldering pen handle is my aim and using a couple of buttons and some LEDs to show temperature in increments of 25degC. I plan to use a micro controller in order to control the thing and run it off a small laptop supply.
First of all I need to figure out how this tip works. There is a headphone style connector on it that I've read the following resistances:
The three conductors I'll refer to as TIP, MIDDLE and SLEEVE.
TIP conductor is not to be mistaken for the physical soldering tip of the iron, I can't help but use them interchangeably...
The SLEEVE conductor is a hard connection to the entire barrel of the soldering tip. The black rubber grip is also conductive.
So far I've determined this:
I have no idea what's between the MIDDLE and SLEEVE, I initially thought it was a thermocouple but I don't read anything useful off it.
In fact I read strange voltages and resistances out of it when I connect the supply to the heater. ranging from a few mV to a couple volts.
This leads me to believe there is some connection between the TIP and MIDDLE??
I used a DC bench supply to power the heater incrementing up in 1V steps and logging the temperature. I can get the soldering tip up to 300deC (as measured by holding a temp meter against the end of it) with 4V 2A (8W) input. I tried soldering with this set at 8W and saw no fluctuation in voltage or current as the iron cooled then heated again.
I put 4W into the MIDDLE conductor again referenced to the SLEEVE conductor and got nothing useful at the soldering tip in terms of heat. If anything, the barrel heated up...
So I cracked open the soldering tip and found this...
The SLEEVE is connected to that thin tube that I chopped off from the soldering tip to get the barrel off. Inside the thin tube were two wires:
Each is insulated in heat proof material however the conductors inside seem to be of different material. My meter didn't note anything very different about them in terms of resistance/length but then again, I have a cheap meter that barley measures down to 0.1ohm.
This tip is meant to be 12V and 40W rated but I haven't dared put 12V across it as it would exceed 40W unless I limited the current. I'm also guessing that the tip should only receive 40W for a short amount of time to crank it up to working temperature, I can solder fine with it running off 8W of power however takes about 30seconds to heat up.
This tip is also designed to be ESD safe where the SLEEVE conductor would be referenced to ground via a large resistor? If the heater is actually between the TIP and MIDDLE, surely there would be a voltage on the SLEEVE.
I've had another person who has these tip and the WSM1 station confirm they have the same resistances between conductors and their tip is working fine.
