I am building a controller for my 110V Harbor Freight spotwelder and was looking for some advice. The controller will consist of a trigger/timing circuit, a current control circuit, and the spotwelder itself. I plan to put all the controller on the primary side of the spotwelder transformer because of the lower current values (110VAC, 15-20A house current) vice the higher current values on the secondary side. I have the timer circuit figured out but need advice on the best design for a dual opposing SCR or TRIAC controller circuit. I found this but the SCRs are rated at 16A.

http://www.electroschematics.com/wp-content/uploads/2013/02/SCR-Phase-Control-Dimmer-Schematic.jpg

I am concerned about the max current rating of the SCR, and would prefer a buffer for any transients and to lessen wear and tear on the components. Also, what kind of power values should I use for the components if I use this circuit?

I know that I could probably use a simple R/C circuit to control the gates of the SCRs with the resistor being variable, but am not sure how to design it myself. That why I am leaning towards the circuit I found. Any advice would be greatly appreciated.

 

Commercial systems have traditionally used either special purpose contactors or mercury vapour device.
I would tend to use a much higher rated SCR, you can still use the same circuit, just increase the current capability of the switching devices.
That would be my first choice.
Max.

 

Have a look at the 40A triac in the attachment.

 

I am hesitant using NTE due to the cost, they are traditionally a Substitute brand and usually charge at least 3x for the same product, maybe look for the OEM equivalent?
Max.

 

Here are some links that might help, I've built some of the other projects from 5Bears so he does know his stuff.
http://www.5bears.com/welder.htm

This guy also used some of the 5Bears circuit.
http://www.sm7ucz.se/Spotwelding/Spotwelding.htm

 

Thanks for the replies. I got the idea for the timing circuit from 5bears. I had to use another timing relay but it is pretty simple. The timing relay triggers an SSR to energize the circuit for the time that I set on the relay.

For the current controller, I would rather use two SCRs like in the schematic because of the symmetry in the AC waveform they provide over a TRIAC. If I replace the SCRs with higher power ones, wouldn't I have to change the R/C circuit values in order to get the trigger current correct for the device?

 

All of the spot welders I've seen only use time controls. When I took my current job, they had a 2.5KW/220VAC/60Hz Miller LMSP50 spot welder on a stand and foot control. Since I work with mostly thin stuff, I couldn't get my foot on and off the foot peddle fast enough. Miller wanted more for their timer than the cost of the welder and stand together. I looked at the 5bears timer, but desided to build my own from scratch. My timer has a minimum on-time of 16mS...one 60hZ cycle. The thumb wheels are 0-99. That gives me 16mS to 1.65 seconds range on on-time. Since I work with light weight steel and stainless, that and 2.5KW is enough. I drew up a design for a 0-999 (16mS to 59.9 seconds) timer, but never needed anything longer than one and a half seconds with that much power.

Ken

 

Don't know how I missed the repost to this thread. Like KMofet said, any I've used or seen only use a timer. No need to adjust current.