JBC-mini - a low cost T245 handle controller

[poorchava]

Yes and no.

The tip contains a thermocouple between the heater element and the shield.Thermocouple as such allows to measure only difference of temperature between two ends, not the absolute temperature. You need to know the temperature of one end (cold junction), to calculate the temperature of the other end (hot junction). This is what MCP9700 is for.

And yeah, I'm reading the temperature by uC and then displaying it, but it's not enough to just turn on/off the heater. First of all you need to switch on slightly after mains zero and switch off slightly before it in order to measure the thermocouple voltage exactly in the zero. Also, the duty cycle once heated up is pretty low, as the JBC cartridges) are rated at 130W peak power (IIRC) and the transformer such as that used in original station would in this situation deliver over 300W if turned on permanently.

The switching circuit is a bit convoluted, because it uses MOSFETs to switch AC in order to reduce losses (as compared to switching rectified DC). Triacs/thyristors are not an option, because they cannot be turned off before zero in a controlled manner.

[cam]

I'm a bit confused.

All I would like to do is to read the temperature from its most suitable pin wherever it is on the JBC PCB and then convert it to binary in order to display it on three 7 segments digits. Even if this means doing math in the uC with this value then convert to binary.
I thought I would need one input for the analogue value (temperature of the pencil) and 21 digital outputs for the 7 segments displays and their shared ground. Isn't this correct?

[poorchava]

No that easy. Since temperature is measured by a thermocouple which has very low signal level,  no current may be flowing through the same wire during measurement. Since the cartridge has only 3 contacts,  then at least one of them is shared between the heater and the sensor. This means that you have to synchronize your heater drive with temperature measurements. Also,  since the heater is powered from unrectified AC, best moment to switch stuff and make sensitive measurements is near zero crossing.

Cold junction compensation is mandatory. That's just how thermocouples work.

BTW,  3 digital of 7seg display only need 10 digital pins if not using digital point. Multiplexing is the way to go. This can be bought down to 3 pins if need be (Multiplexing + shift registers).

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[mikeselectricstuff]

And yeah, I'm reading the temperature by uC and then displaying it, but it's not enough to just turn on/off the heater. First of all you need to switch on slightly after mains zero and switch off slightly before it in order to measure the thermocouple voltage exactly in the zero.

Triacs/thyristors are not an option, because they cannot be turned off before zero in a controlled manner.

or you could skip an entire mains half-cycle to do the measurement

Cold junction compensation is mandatory. That's just how thermocouples work.

Though for the accuracy needed in a soldering iron application, just assuming the CJ is always at about 20 deg.C, or using a very simple/cheap ambient temp sensor isn't going to affect things enough to be a problem in practice. 

[cam]

No that easy. Since temperature is measured by a thermocouple which has very low signal level,  no current may be flowing through the same wire during measurement. Since the cartridge has only 3 contacts,  then at least one of them is shared between the heater and the sensor. This means that you have to synchronize your heater drive with temperature measurements. Also,  since the heater is powered from unrectified AC, best moment to switch stuff and make sensitive measurements is near zero crossing.

As I understand, I need to know the heating signal and Vout of the MCP9700 as inputs. Would have been too easy if it was only a matter of convertion

BTW,  3 digital of 7seg display only need 10 digital pins if not using digital point. Multiplexing is the way to go. This can be bought down to 3 pins if need be (Multiplexing + shift registers).

I didn't got much into it and I assumed there is one input pin for each LED (I do not count Vcc and GND because they don't use uC outputs) but it may be different in practice. I'll check the datasheet later when the rest is clearer in my mind.

[poorchava]

Agreed, MCP9700 is just that, cheap and simple (and I already had it in my drawer). Any other popular temperature sensor could be used: LM35, DS18B20, thermistors, diode forward voltage  etc...

Assuming that CJ is always at some fixed temperature might introduce big errors. For example in my workshop the soldering station base unit is stadning in a shelf of sorts with lots of other gear,  so the temperature there is most definitely not the same as room temperature.

@cam:
You need to know when the mains zero crossings are, make sure that the heater drive is off, measure the thermocouple voltage and then drive the heater. Cold junction temperature can be sampled at any moment convenient.

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[timofonic]

Any progress on this?

[x_dadu]

What type of termocuple this JBC C245 has ?

[tcurdt]

Are the schematics online somewhere?

I am wondering about the AC vs DC choice.
Are the losses really that big at e.g. 120VA?

[Icchan]

Could you please move your images to imgur instead of photobucket? They're blurred out and one can't even visit the photobucket site with adblocker on :/
Best case would be you to upload them to the forum directly... but hmmh...