Electronics > Projects, Designs, and Technical Stuff
JBC-mini - a low cost T245 handle controller
<< < (3/12) > >>
poorchava:

--- Quote from: Rufus on May 29, 2014, 01:56:13 pm ---The most difficult part of a driver for JBC cartridges is the thermocouple amp.

In the configuration you have the amp has around +/- 1.5v on its input while you are driving the heater. If you don't turn off the MOSFET switch exactly at zero current you get an inductive spike from the leakage inductance of the transformer some of which the amp will get to see. The amp needs to recover from these overloads before you can measure temperature and you have to hope the gross overloads don't upset the internal auto-zeroing.

--- End quote ---

There is a clamping circuit on the input consisting of series resistor and schottky diodes to ground to shunt anything >0.3V or <-0.3. At those consitions the output will obviously saturate, but I think that the opamp will manage that. We'll see when I get the parts.


--- Quote from: Rufus on May 29, 2014, 01:56:13 pm ---For the best temperature control you need to measure on every mains zero crossing and quickly decide if the heater should be driven for the next half cycle or not.

--- End quote ---
I plan on synchronizing entire logic to mains crossings. uC will run from internal oscillator, but measurement and everything else will be done on mains crossings.


--- Quote from: Rufus on May 29, 2014, 01:56:13 pm ---The cold junction for the thermocouple is at the hand piece cartridge contacts. The hand piece warms significantly. A guess at hand piece temperature from a thermal model in software would do more than cold junction compensation on the control PCB. Taking a general ambient temperature into account in the model would help a bit.

--- End quote ---

The sensor on the board is indeed more for ambient temperature than anything else. I think it's possible to estimate temperature of the handpiece junction based on ambient temperature, time since the tool was picked up from the stand and tip temperature.

It think it may also be the case, that the contacts inside the handpiece are the same metals as used in the cartridge, in which case there would not be any cold junction inside the handpiece, and the real cold junction would be at the connector.
poorchava:
Interface board is ready.
Display is a common anode type. Drive circuit is a standard one: BSS84's for switching anodes, 74HC595 for driving segments. Current limiting resistors are 160R (entire thing runs at 3.3V). There is also a rotary quadrature encoder with a pushbutton and 5 LEDs for debugging. Now I will start wiring up the uC board. Despite STM32F0 being able to progrtam it's own flash I plan to have an external eeprom for calibration data and storing temperature.
amyk:

--- Quote ---yeah, it's similar, but the internal connections of the tip are different and thermocouple too. I don't remember specifics, but I believe the difference was, that the heater was actually grounded (i.e. connected to outer sleeve of the tip) and so was the thermocouple, which makes things easier.
--- End quote ---
Looking at the diagram again, it seems you have heater, thermocouple, and common connections to the tip, which means the controller design won't be much different from the 936 clones that use a thermocouple too. At first I thought it was more like Hakko's integrated tip which has only 2 terminals and the thermocouple and heater are wired in series.

I think a triac would be better for control than a MOSFET, since they turn off automatically at the zero crossing.
poorchava:
Another update: the amplifier arrived and I have put it all together. The circuit generally works, but my choice of BAT54 as input clamping diode seems to be poor. The one which is connected with anote to siangl line and cathode to ground is leaking quite a lot and it loads the thermocouple. I will have to find another diode or use much larger series resistor. Other than that the amplifier works so far. I will post pictures later.

Btw, I've made an error on the diagram in first post, inner sleeve and central rod ('core') are the other way around, so the thermocouple voltage must be measured between outer sleeve and the central rod.

Choice of mosfets instead of triac mau have been for two reasons: One - triacs have higher voltage drop than 2 mosfets in series do. Second - you cannot control triac turnoff time. You'd have to turn on somewhere in the middle of the cycle, and then triac would conduct until voltage falls below threshold. With mosfets you have complete control over turn-on and turn-off events.
C:
Just some Ideas

What if you added a balanced RFI/EMI filter. The ac switch and transformer winding on one side. The amp  and the leads to the tip on the other side.
Should remove some of the ringing and transient noise from the amp.

Next I see an AC signal that is offset from ground by a DC signal. The average of an AC signal is 0 so by averaging samples you would get the DC signal.

If you added some Active cancellation to the amp you could keep the signal in the input range of the amp. 

How much does the heater resistance change with temp? If you had some feedback the heater temp could help make fast temp changes better.

C

Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod