Hello. Sorry my english if I make mistakes.

I´m going to build a spotwelder. I have removed the secondary of a MOT wit 2 turns of thick copper wire and works great. I controll it with a foot pedal.

Now I want to put a timer to control the duration of the weld. So with a potentiometer I can activate the welder for example 0,1 sec no matter how long I press the pedal. I have though abput using a ponteiometer a capacitor and a transistor that control a small relay and that relay goes to a big power relay that sends the current to the primary of the MOT. But I have 3 questions:
a) I have no idea about the desing of the circuit of the timer hahaha.
b) What are the tipical times for a spotwelder? I mean if for example they are between 0,1 and 0,8 secs, to make the timer with that range.
c) Should I choose a special relay to feed the 220 AC to the transformer since is a coil? I remember taht coils give big sparks when you cut the current, so maybe I´ll nedd that.

Thank you

Best regards

 

One modern method is to use a Triac device to switch one side of the primary.
To isolate your foot-pedal, you could use a simple foot-pedal operated LV timer that would trigger the Triac. the timer could be DIY or purchased on line.

 

The AC switch Max mentioned could be a solid-state-relay (SSR) which uses a Triac to control the AC and includes an opto-isolator that requires just a few tens of milliamps for control.
Timer modules are readily available on ebay or Amazon.
They can be powered by a wall-wort 12V DC supply.

 

Thabks for your answers.

Could I use this circuit?



Decreasing the values of R1, the potentiometer and C1 i could get short pulses right? between 0,02 and 0,5 secons for example?

And could I use 12V instead 15V?

 

Looks about right, it should be OK with 12v, a Triac or SSR rather than relay would be a better choice IMO.

 

Looks about right, it should be OK with 12v, a Triac or SSR rather than relay would be a better choice IMO.

Thanks for your answer. My knowledge is very limitated. The TRIAC, once triggered keeps conducing the current. How can I keep the triac conducing only during the pulse?

 

The TRIAC, once triggered keeps conducing the current. How can I keep the triac conducing only during the pulse?

Only if it's controlling DC.
With AC it stops conducting when the AC current goes to zero (which happens twice per cycle).
So as soon as the pulse is removed, it will stop conducting when the nearest half cycle goes to zero (no more than 10ms for a 50Hz supply).

 

Thanks for your answer. My knowledge is very limitated. The TRIAC, once triggered keeps conducing the current. How can I keep the triac conducing only during the pulse?

Check out the APP notes by Fairchild, AN-3006 & AN-3003

 

b) What are the typical times for a spotwelder? I mean if for example they are between 0,1 and 0,8 secs, to make the timer with that range.

The time required all depends on the VA size of the welder and the size/dia of the electrodes, plus the type/size of the material/job being welded.
A few factors involved.

 

Ahh ok ok I see. Thanks both of you for helping me.

A 16A 800V should be ok? And the transformer would be conected to 220VAC through the main terminals, and what terminal go to the pin #3 of the 555 and wich one goes to the ground?​

​

Thank you!​

 

Hello Rufinus,

feel free to use my MOT spotwelder design. It works fine for me, to weld small things like battery tabs.

Take care, don't electrocute yourself. The entire circuit, except for the MOT secondary and electrodes, is live at the 220V line, so build your spot welder into a suitable enclose with safe insulation.

Manfred

 

Hello Rufinus,

feel free to use my MOT spotwelder design. It works fine for me, to weld small things like battery tabs.

Take care, don't electrocute yourself. The entire circuit, except for the MOT secondary and electrodes, is live at the 220V line, so build your spot welder into a suitable enclose with safe insulation.

Manfred

Thanks a lot!!!!

I have doing some research meanwhile and I have find that TRIACS are not recommended for inductive charges but you and Maxheadroom remmend it why?

And with that potentiometer and capacitor, which is the range beteewn the shortest and the longest pulse?

 

> I have doing some research meanwhile and I have find that TRIACS are not recommended for inductive charges but you and Maxheadroom remmend it why?

TRIACs tend to "get stuck" in ON state with inductive loads, due to turning themselves on immediately after turning off, when the inductive load and any stray capacitance ring. But a snubber in parallel with the TRIAC, in my circuit the 220nF capacitor and the 100Ω resistor, calm down this ringing enough to allow the TRIAC to work fine.

If you are worried about this, then replace the BTA16600 TRIAC by any 3-quadrant TRIAC of the proper rating. It will work fine in this circuit, since the 4th quadrant isn't used, and those 3-quadrant TRIACs are far less sensitive to sticking on with inductive loads.

> And with that potentiometer and capacitor, which is the range beteewn the shortest and the longest pulse?

From about a single cycle (20ms) to almost one second.

 

Ok thanks a lot. I'll use your circuit