Arc welder triac controler

[oldway]

If the max rms current of your welding transformer is 26Arms, The max rms current of each scr is 26/1.41 = 18.44A (conducting only half wave)

TO220 pack is too weak for such a high rms current: terminals are 0.5mm thick and 0.75mm wide, this is only 0.375mm².
This mean a current density of 18.44/0.375 = 49.17A/mm²...This is very high...to compare, we use 4 to 5.5A/mm² in transformers.

Better to choose a TO247 or a TO3P pack.

Ta: must be at least 65° for convection cooling and 45° with force cooling because your heatsinks are connected with mains voltage and MUST be protected in a housing.

Tj = 120°: ok

Pg: ok...NB: with this duty cycle, it hardly happen to exceed the max gate power.

[aburastas]

Well.. After a few tries I return to "IXYS CS45-16IO1 Tiristor 1600V 100mA 48A 75A TO247 4`32euro" that give me the cheapest combo. it has:
Tjmax=125C; Vgt=1.6V; RTHj-s=0.82K/W; Vto=0.85v; Rts=11mR; Pgav=0.5W

The regulator must pass 26Arms so each scr must pass half a cycle each time so this give us 18.2A(rms) and 11.7A(av) so this give us P=Vto*Iav+Rts*Irms^2= 13.6W

With Ta=65C; and Ta=120C result in RTH<=3.22K/W and we can find the RA-T2X-64E with 3.1K/W by 5.37euro... Summing around 10euro each scr set. Not cheap but reasonable. Others scr with more consistents leads may pass more current but I don't usually put my welder to the max power. I think it will suffice.

Following the calculations as before with 1/3 duty cycle give an Igmax=0.9A so now begin the resistors calculations... Depreciating Rgk that only draws about 3ma If we put Ig=0.9A with Vsecondary=around 12V gives R2=R3= around 10R
And R1 must supply each half cycle the 0.9A+0.9A draws by the two scr plus the magnetizing current so perhaps around 2A that gives R1=12/2=6R. I know your values are calculated in a different manner so... What I'm missing??

[rodrigopires]

Hi guys! So since i had to open the controller i figured i would improve my fan power supply (12V) to feed the two small fans in the box and a big 36W fan for the transformers. I build this schematics in a breadboard and it seems to work good, but i want to know your opinion before i start soldering!

C0 and C1 = 1000uf each
C3 = 3.3uf
C4 = 1uf
R1 = 3ohm
Q1 = TIP32C
Regulator = LM7812

[aburastas]

Hi Rodrigopores! Nice to read you again!!! Really I hope you have any progress with your circuit because my brain is drying out with the design I'm planning. Finally ... Have you updated the triac by an 3 cuadrants? And the tvs by an bidirecctional type? And the snubber?

About your 12v regulator ... What I can tell you is that... R1 must be of more than 1w because being Vbe on the transistor about 1.8v... V^2/R1 is just a bit more than 1W so 2 or better 3W will be ok. About the transistor hfe=20; Vcemax=3A and 7812 has a max Iout=1A... You don't say what is your final load but being 36w/12v=3A of the big fan. Lets assume that transistor drive 3A this implies that Ireg=Vbe/R1 + Vcemax/hfe=0.75A. So your small fans can't sink more than 0.75A*12V=8W

To calculate the final thermal tolerances you must know the output voltage of transformer at C0. Anyway if you had made a long time run test with the fans on and T1, 7812 and R1 don't get very hot.... It will be ok.

[aburastas]

Well... Merry christmas for all!!! Although seem that not much people continue reading this. I know that some times my obsessive compulsive disorder can bore anyone so ... A toast for electrons that make life easier! 

[rodrigopires]

Well... Merry christmas for all!!! Although seem that not much people continue reading this. I know that some times my obsessive compulsive disorder can bore anyone so ... A toast for electrons that make life easier! 

Haha Aburatas... i'm still here! Merry christmas to you all! Working on the project and as soon as i have some news i let you guys know!
Sorry for the long wait! Here's an electron trying to reverse biased a diode!

[eneuro]

Welding machine built from a car alternator:

Really?
How is it possible that without synchronized rectifiers in DC stage of alternator one can have decent welding current ?  At  least 50A needed for thin 2mm welding electrodes? 
My MMA has  about 80V open circuit..at lower volatges spotwelder melts welding electrode - no arc no welding with electrodes