Arduino upgrade of a Weller VP800 desoldering station

[JacquesBBB]

Hi All,

After spending  a significant amount of time trying to fix the Weller VP800 desoldering station that
a friend gave me
https://www.eevblog.com/forum/repair/need-help-for-fixing-a-weller-vp800-desoldering-station/msg1086390/#msg1086390

It was working, but not in a very satisfying manner. It was not heating fast enough, and the examination  of the triac
output  with the scope did not gave very clean signal.

Some other problem is that  the triac driver, a SL443A, is no longer available, and the ones found on ebay are
refurbished and maybe not very reliable.

So I decided to completely change the  electronic part of the VP800.

The new design comprises an arduino micro pro (so that it fits on the board),
a zero crossing optocoupler (MOC3083),  a triac (L4008L6, but it could be another one), and a few resistors.

After looking for multiple designs, I end up with the most simple one.

The actual temperature of the iron (T) is measured  at an analog input of the arduino
with a voltage divisor with a 100 ohm resistor in series with the sensor  of the heating element (resistor from 20 to 60 ohm).

another  analog input pin is used for the potentiometer of the Weller to set the wanted temperature (T0).

The program on the arduino is

if (T < T0 )  turn the triac ON
else turn the triac OFF.

ON/OFF  is forwarded to an output pin that is connected to the optocoupler.

Thats all.

All this could fit on a stripboard  that can be put  in place of the previous board.
I just added an additional power supply to get DC 5 V from the AC 27 V provided by the transformer.

As  I had the arduino, I added a small oled display that I could fit in the Weller front panel to  show the values of T and T0.

The nice thing with MCU, is that you do not have to tune  precisely everything.
You make it.
Then you evaluate the output, and then fit the parameters with least square to get the desired output scale.
I  could measure the temperature of the iron with a cheap clone of Hakko temperature tester.

It took me a couple of hours and now everything is working, and  the heating is much faster than before, so I am quite
pleased by the result.
The regulation of the temperature is perfect for my needs.

[JacquesBBB]

This is the correspondence beetween the  temperature requested
with the original Weller pot knob and the T0 temperature displayed  (Celsius)

Pot     T0

400    400
350    365
300    295
250    242
200    181
150    124
100      75
50        50


Correspondence between the displayed actual temperature (T) and the measured temperature with
the clone of Hakko FG-100 (Tm), after verification of the behaviour of  the measure with an other soldering iron  (Celsius) .

 T            Tm
       
400        387
350        361
300        300
250        265
200        205
150        150
100          90
50            47

[Vtile]

So your control logic drives the Power/Triac to heater element when the sensor noticed that the tip of the iron is below set value. I don't see upper limit at all or any kind of heat control. Maybe I'm not just mis-understood the setup (I did not read the older thread, since I'm "closing the computer to get some sleep"  ).

If I'm understood your intentions correctly you do want to create bang-bang controller, you need to have also upper turn-off limit or ON have only certain time. Need some sleep now. 

PS.. I need that sleep... Arduino obviously do not have latched outputs.

PPS. On the otherhand it now might be a nice 100k..500kHz oscillator depending the Arduino program sweep time and the lag which the mass of the heated system creates.

[JacquesBBB]

This is  the main part of the code

void loop() {
  gettemp();   
  printterm();
  printoled();
  control();
}
void control(){
// regulation of the iron temperature
  if ( temp < temp0 ) {
      digitalWrite(opto, LOW);    // turn on the iron
    }else{
      digitalWrite(opto, HIGH);    // turn off the iron
   }     
}

There wont be any 100khz loop as the oled refresh is quite slow. So there is a natural delay
in the loop.
The mass of the desoldering iron tip provides also some inertia. 
It works well.
I dont see any need for more complicated.

[Vtile]

Oh, don't take me wrong I think this is cool repair/upgrade. 

Even better if it doesn't have problems and you were already aware of the possible shortcomings of the controller loop.

[Kjelt]

It was working, but not in a very satisfying manner. It was not heating fast enough, and the examination  of the triac
output  with the scope did not gave very clean signal. 

Are you sure that is not inherent on the design. I have the old Weller desolder iron and it is sluggish/slow as hell compared to my new WX desoldering iron.
Heating up to 350^oC will take 2 to 3  minutes with the old design (50W and a  lot of iron to heat) and 30 seconds with the new one (120W and lean design).

[JacquesBBB]

Are you sure that is not inherent on the design. I have the old Weller desolder iron and it is sluggish/slow as hell compared to my new WX desoldering iron.
Heating up to 350^oC will take 2 to 3  minutes with the old design (50W and a  lot of iron to heat) and 30 seconds with the new one (120W and lean design).

Maybe, but in my case it was much worse before the upgrade. I could never go to 300 C, and it was taking forever to reach usable temperature.
Did not measure, but was more 20 mn than 2 mn.
I will make the measure in the new setting.

[JacquesBBB]

@Kjelt,
This is what I find for the heating, starting with  room temperature

 Temp (C)      time (mn:s)

100               0:20
150               0:36
200               0:56
250               1:23
300               2:00
350               3:00

So quite in par with what  you had, but much better than what I had before this repair,
and very usable. Sure this does not change the max power of the iron, but  this will be perfect
for me until the next upgrade.

[Kjelt]

A job well done in repairing it 

[ctbully]

Hi, I am very interested in your design. I too got the VP801EC from a friend (without the desoldering gun) and I went ahead and ordered a Desoldering gun from a company in Poland (Still waiting for it to arrive). They were the cheapest. Today I opened up the station and lo and behold, the gun I ordered is 24V and the station provides 230V to the gun LOL. So I need to do exactly what you did except I need to provide an additional 24V 3A supply. There are two transformers inside mine and both output 25V so I might be in luck.

Would you be so kind as to share your circuit design, please?  I'm good with writing my own code but I battle a bit with the circuit design sometimes.

Thanks
John B.