Understanding the Hakko T30 series Tips

[seancsnm]

I'm in the process of designing a driver for a handpiece that uses these T30 series tips. I just received the handpiece and tip today to be able to pin them out, and have found a few things, which has left me a bit confused.
1) The handpiece has 4 connected pins: Green (earth ground), Red (heater+), Black (common), White (not sure what this is, other than that with nothing else connected the resistance to common is 10k or so (varies))
2) The temperature sensor is nowhere to be found.

The white-black connection clearly does *something*. When the tip is inserted and voltage is applied to the heater terminals, it outputs a voltage that seems to be proportional to the input voltage somewhere around 10 mV/V (but only when the tip is inserted). I suspected it may be a hall effect sensor and was picking up the heater current, but it does essentially nothing in the presence of a strong neodymium magnet.

For now, I am planning to use the heater element as the sensor, as it has a resistance in the range of 12 ohms (room temp) to 55 ohms (estimated 800 F but that's just a guess based on keeping the iron powered for a while after it began to melt solder).

Does anyone know how this type of iron works? I'm sure the information is out there, I'm just not sure how to make my search terms specific enough to find what I'm looking for rather than 9000 generic product info pages. I've added a few photos for reference.

[seancsnm]

I stared at this some more. Played around with a few things. It seems to me that there is a thermocouple embedded in the heating element (maybe they are one in the same?), as when the element is externally heated, the black and red wire pins put out a voltage going up to a couple mV. The resistance reading also changes significantly.

Additionally, I went ahead and heated the iron handle with some warm air and the resistance between black and white dropped to about 6k pretty quickly, pretty much confirming that there is a NTC thermistor in the handle (not the tip). I guess it just happens to be affected by current flowing through the heating element as well.

So for driving this thing, it seems to me that the intended approach is to cycle between measure and heat. If you limit the duty cycle to 90% or so and keep the PWM frequency low enough, this is doable, but it's giving me some trouble to come up with a topology that does this effectively and without too much expense. So far, the only design I've come up with is the following:

Use an analog switch to switch between a measure circuit and heating circuit. The PWM signal would drive this to ensure that the heater and measurement circuit aren't on at the same time. Getting the timing right is critical, as the heating circuit voltage would be up to 30 V (just a rectifier slapped onto the power transformer so pretty ripply) while the measurement circuit would be designed to measure a couple mV full scale for the thermocouple. I'm not great with transistors, be they mosfets or bjts, so I'll have to look up ways to make sure circuit 2 doesn't turn on until circuit 1 is completely off. Then I'd need to trigger the ADC so that it only measures during the PWM low window.

The holy grail would be finding a way to measure the temperature regardless of the PWM state, meaning heating element on or off, and don't worry about getting the timing right. It might be possible to do that by treating the sensor/heating element as a resistor you're trying to measure, then measuring V(heater) / I(heater) = R(heater). But the filtering required with a simple 1st order RC filter would have a cutoff frequency way too low to measure and control the temperature effectively (we're talking sub-Hz) when considering ADC delay between samples. I think a higher 3rd or 4th order 10 Hz LP filter would do it. Or you could get an ADC that samples both simultaneously. Or build a sample & hold circuit that samples both voltages and allows the ADC to take its time with the measurement.

I don't know what would be cheapest/easiest and I don't have the equipment to effectively get a temperature-resistance curve for it to see if it has a consistent response when treated as a resistor. If I simply knew for sure that it was a thermocouple and what type it was, that'd make things a good bit easier. So I guess the next step is probably coming up with a semi-accurate measurement rig to give me some data.

But I've already gone pretty far out of the scope of this thread. So I'll stop my rambling for now. Maybe throw some schematics in to illustrate wtf I'm talking about tomorrow.

Edit: Just adding a few search results that might be relevant:
https://electronics.stackexchange.com/questions/534554/how-to-amplify-a-thermocouple-that-shares-a-wire-with-a-heating-element-solderi

https://forum.allaboutcircuits.com/threads/identification-of-heating-element-leads.110970/

https://www.eevblog.com/forum/chat/soldering-station-thermocouple-vs-heating-element-resistance-sensing/

Last one pretty much confirms that the heating element and thermocouple are one and the same. Also, other than the ADC timing I think it's gonna be pretty simple to measure from the thermocouple. No switching required, just a resistor and clamping diode to keep the amp from getting over-volted. And an amp/amp design that recovers from railing fast enough to get a good ADC reading once the heater is in the off cycle.

[Alti]

This T30 looks like has same electrical parameters as a well known T12.

It is a thermocouple. It generates voltage on temperature gradient.
I suggest driving it with P-MOS transistor from DC and sensing it near ground with difference amplifier.

Where did you get the T30 handpiece from?

[seancsnm]

That's exactly the plan. The tip and handpiece were direct from the Amazon Hakko store. So both should be legitimate (though in this case I sort of regret not getting a clone for the handpiece - $70 is quite a bit for some plastic and a NTC thermistor). The discoloration in the tip is due to me not paying attention for a couple seconds when testing things.

[Alti]

The discoloration in the tip is due to me not paying attention for a couple seconds when testing things.

Definitely I suggest you should get a set of guinea pigs squad of T12 tips and a handle, Hakko clones. You'll ruin your OE tip and holder at first software glitch.
I did feed Hakko T12 tip with 24V rms voltage and at just 3% PWM it reached 230oC in free air. If you mess up the control loop with 100% drive, the tip is going to get cherry red and strained/damaged in just seconds.

[seancsnm]

Probably a good idea to use a sacrificial clone in the prototyping stage. I'm not known for following good advice though, so we'll see. I did some very rudimentary characterization of the tip thermocouple. I didn't compensate for the cold junction at all, but the room temperature was about 19.4 C. I'm not sure how the compensation is done (I think you just add the cold junction temperature to the calculated thermocouple temperature?) so I'll have to read up on it. For this data, I basically jammed a heating iron, a crusty old thermocouple and ancient thermocouple reader with a  room temperature error of about 10 degrees C, and the Hakko tip into a tube, filled it with molten solder, and started varying temperature. Let the temperature settle a bit, and write down the voltage vs. temperature reading. I used a TLC 279 op amp with a 165 gain. Took the average value using an oscilloscope (60 Hz noise was enormous - I didn't bother trying to do any filtering). This is the result. Seems like the sensitivity is half that of a K-type thermocouple. The accuracy of these results is probably horrendous. Actually, I know it's horrendous, but it should be within an order of magnitude and enough to build a better measurement circuit at least. Getting the "ground truth" temperature is another story. I think I should just be able to order some fresh thermocouples and build and verify my own amp circuit if the thermocouple reader is still putting out a very inaccurate value at room temp. There are tables that relate voltage to temperature for different thermocouple types so hopefully I can just look those up. And add in the ice water and boiling water temperature points to ensure accuracy.

The biggest issue is getting good enough thermal coupling between the thermocouple and soldering iron tip to get an accurate tip measurement. I think I'll build a better solder cup and see how that works out. If it's a small enough cup and bot the iron tip and the thermocouple are submerged very close to each other, I think that'd be accurate to within a couple degrees, especially if I wait for things to hit steady state before recording the measurement.

As far as driving the iron tip, I'm starting to wonder if I should change my approach to using a triac. The reason is that when I take my thermocouple measurement, it may take considerable effort to filter out the 60 Hz emf it picks up. But I could ignore that signal if I lock the phase at which the TC is measured to some position on that 60 Hz line. And if I do that, I may as well do the measurement at the zero-crossing. I could follow the same approach using a rectified signal, but I'd have to lock the PWM frequency to a multiple of the mains power to avoid trying to sample the TC during the PWM on portion of the period.

In case it wasn't clear, the chart attached shows the temperature vs. back-calculated thermocouple voltage. I realize it's not a linear relationship and doesn't account for the offset created by the cold junction, but it's a ball park estimate.

EDIT: The thermocouple voltage is in mV, not V

[Alti]

As far as driving the iron tip, I'm starting to wonder if I should change my approach to using a triac. The reason is that when I take my thermocouple measurement, it may take considerable effort to filter out the 60 Hz emf it picks up.

AFAIK Hakko uses DC. I think I have seen FX-950 PCB photos somewhere here.
I also have Bakon station for T12, P-MOS drives it.
BTW, those T12 stations are really cheap so maybe get one before you make your own. Or you can clone a clone.

Anyway, of course you can drive heater via triac but I am afraid you would have to trigger triac with opto-triac because MT2 needs to go to GND point. Only then common mode of a sensing voltage is small. If you want to tie MT1 to GND point and drive triac with uC directly, you need to fight with higher common mode which might be tough.



Also, consider that a heater or wiring might fail and go short-circuit.

[seancsnm]

I've not used a triac before - it'll take a bit before I really wrap my head around how they work but I think I see what you're saying - if a gate current is required WRT MT1 which would require MT1 to be tied to ground, you need to the thermocouple voltage in a weird way rather than a simple single-ended voltage measurement. I'd be much more comfortable driving it with a mosfet. I just need to try something and see if I can get it to work. As for the 60 Hz noise issue I was complaining about earlier, it turned out to be an amp circuit issue. The 60 Hz overlay isn't bad at all now.

I tested a drive/measurement circuit and I think I have enough to get something implemented. See attached schematic sketches. The mosfet I had on hand was an IRF9640. Also, there should be a resistor between the FET gate and the BJT collector - I used 1k. The zener diode should be a value that keeps the mosfet gate well above the threshold voltage but below the device limit. I used 3V at first but found the FET got very hot so switched to 10V, after which it didn't heat up much. Note that I was using a 60 V pk transformer at a bit less than half voltage for this test, meaning the iron was receiving less than 1/8th of the total possible power. That way I could leave the setup on for a while and not have the iron heat up too much. I was also able to get away with lowering the amplifier input resistor to 10k which helped lower the pickup of 60 Hz noise. I just used an ill-suited rectifier diode for the over-voltage clamp and there was a noticeable effect on the amplifier output due to the high leakage current - something like 20 mV. I think my best bet to improve this is to use low leakage switching diode that have closer to nA instead of uA leakage current. I can further improve the leakage if I need to by adding a second clamping diode between that amp input and the -5V rail... but I'm not planning on adding a negative supply rail so I'm just going to use a lower leakage diode and hope I can calibrate any remaining error out. Lastly, I didn't add a filter to the op amp input but since I'm driving the PWM at 10 Hz, 90% max duty cycle, I probably will add a cap to filter out high frequency noise.

The last pieces of the puzzle are the thermistor measurement circuit and the microcontroller timing. IIRC, most microcontrollers have interrupts that can be masked in from a set of GPI pins. If I can feed the outgoing PWM signal back into one of these pins I should be able to use that interrupt to trigger an ADC read. A simpler method would be to trigger an interrupt when the PWM subsystem changes state. I know there are lots of conditions for the counters to trigger interrupts, I'm just foggy on the details as it's been several years since I've touched a microcontroller. A third possibility is to use an analog solution - something that causes a pulse at the negative PWM  signal edge. That pulse would be used to trigger a sample and hold circuit to grab the latest thermocouple amp value and hold it for whenever the ADC gets around to taking a sample. That third method is overkill I think but it might be useful if one were to implement an entirely analog controller.


 Ultimately this project is partly to save money, partly to learn/have a cool piece of equipment I can call my own. I'll be implementing a couple custom features that'll make it more "me" than an OEM or clone.

The next step is going to be to find a suitable microcontroller and lay out a  PCB. To keep the discussion of this thread specific to using/characterizing the T30 tips, I'll probably put specifics of the controller I'll be designing in a separate thread. The controller will also be driving a set of Weller hot tweezers.

[Alti]

Triac seems to be a tempting way of driving heater because it can be directly driven by transformer. No rectifying, no filtering, no big capacitors.

The amplification that you proposed, well this requires some careful layout. You'd better make proper difference amplifier and not the noninverting simplification where you assume both reference points are the same because in practical PCB layout it is going to be tough to put opamp next to connector. You need to sense voltage across T30, difference from both ends and amplify the difference. Your gain is 275x (33k/120).

Difference amplifier.

[seancsnm]

Triac seems to be a tempting way of driving heater because it can be directly driven by transformer. No rectifying, no filtering, no big capacitors.

The amplification that you proposed, well this requires some careful layout. You'd better make proper difference amplifier and not the noninverting simplification where you assume both reference points are the same because in practical PCB layout it is going to be tough to put opamp next to connector. You need to sense voltage across T30, difference from both ends and amplify the difference. Your gain is 275x (33k/120).

Difference amplifier.

I understand that most thermocouple measurement circuits use differential configurations. The main reason is to cancel out common-mode noise that appears on the signal lines. If the negative terminal of the thermocouple is connected directly to ground, wouldn't that effectively be nulling the induced voltage on the negative line, leaving only the induced voltage on the positive line, thus making a differential amplifier useless? Every differential config I've seen has either had the thermocouple terminal(s) biased in some way or the negative terminal grounded with a differential amplifier. I'm struggling to see how this second method is different than using a ground-coupled single-ended amplifier configuration. Is line noise/magnetically induced current on the thermocouple line really enough to move the ground voltage that much to where it's necessary to measure thermocouple voltage with respect to thermocouple ground at /before the point it connects to the pcb ground?