Triac + Timer control of an Arc Welder - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: December 01, 2017, 05:05:11 am
content: I want to turn on my arc welder for short periods, typically few ac cycles. Basically I'm try to do spot welding with my arc welder.So I thought why not use a simple triac controlled by a 555 monostable. However when I went into the details it doesn't seem to be as simple as it looks since I am controlling an inductive load.The main principles that I came across for controlling inductive loads are as listed:1. Use a pulse train. (Not sure why this is the case ??)2. Use snubberless triac (Again not sure why this is the case ??)3. To limit inrush current trigger triac at peak mains voltage (possibly this is due to the phase shift in current and voltage for inductive loads). In my case though I would be triggering the triac while my arc welder secondary is in short circuit mode that is when my electrode is touching the workpiece. So not sure If this will work.4. For a transformer load remember to trigger the triac at a different polarity from what we had last turned the triac off at. (I guess this is to prevent the transformer saturating)There would be more principles but first up what i would like to ask is will using a arc welder in timing mode work because under normal conditions the arc welder is used in always on mode and the user first strikes an arc by touching the electrode to the workpiece and then slowly draws it upward to create the arc, right? So how would that work out here.I hope you'll can share your experiences.TIA

author: tautech
author_rank: Super Contributor
timestamp: December 01, 2017, 05:42:28 am
content: Special thyristors called Alternistors are sometimes used in welders, mainly MIG. Google them.Yes a snubberless Triac is what you need and preferably use a dedicated zero crossing Triac controller.What you are attempting is used in MIG for short timed and adjustable tack welds. When set correctly they immensely speed continuous start/stop tack welding.A controller I used some years back was U2008B, a DIP8 that was pretty easy to use. Might be EOL now but a little research of it will set you on the right path.You might have to add a 555 into the control path for weld time duration but it shouldn't be hard to suss out.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: December 01, 2017, 05:54:32 am
content: That some profound information you just shared, the U2008 seems to be a low cost phase control IC with soft start right? A quick search reveals that its no longer manufactured.OK, so I would need to sense the mains zero crossing right and then delay triggering pulse till peak mains is reached right? and then intiate the triggering for the triac.Quote from: tautech on December 01, 2017, 05:42:28 am

author: tautech
author_rank: Super Contributor
timestamp: December 01, 2017, 07:14:24 am
content: Look what I found and where it was from.http://www.eevblog.com/files/TDA2086A.pdfHave a good look at that little beasty.Then check what you might do with its Inhibit.

author: oldway
author_rank: Super Contributor
timestamp: December 01, 2017, 07:15:38 am
content: Arc welder ? What kind of arc welder ? MIG, MMA, TIG, ...?It seems to be MMA, but I am not sure.The principle of working you choosed seems not to be right.The welder must stay on all the time, but, when you detect a current, you must turn it off after an adjustable delay.So, there is no problem of turn on as you can use a soft start circuit.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: December 05, 2017, 10:18:52 am
content: Quote from: tautech on December 01, 2017, 07:14:24 amThanks a lot for that! I will place an order for that part.However I need to make this circuit as of tomorrow and that part might take a week to reach me.So I built a circuit 555 monoshot + MOC3041 and BTA25-800CW snubberless triac. And I set the monoshot to fire a 12ms pulse and when I triggered it with a push button with the primary of the arc welder transformer connected to the triac in series and the secondary I had already clamped to my plates that I want to spot weld. However the arc welder just stayed on and didn't go off. I hadn't connected any snubbers to the triac.Then I realised that probably I'm not going the right way primarily due to spot welding needs lower voltage and much higher current than arc welder can provide.So I scanvenged an old MOT from a microwave. And now I am trying build a spot welder by winding my own 2-3 turn secondary on it with 8-10 guage wire. But my problem remains the same how do I control the power of this MOT is a Triac a sensible way to do that. I would need snubbers still right across the triac?

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: December 05, 2017, 10:59:12 am
content: Quote from: oldway on December 01, 2017, 07:15:38 amIts an ordinary stick arc welder so it might be MMA / SMAW as you say. (it is not MIG / MAG neither TIG).No what I read was since its a inductive load it has to be switched on at peak mains voltage. I'm not sure I need to follow this rule since I will be starting the MOT when both current and voltage are zero. The triac will then go off at 0 current by the MOC3041. And to prevent retriggering of the triac due to dv/dt at turn off I'm using a snubberless triac and I will also add an external snubber if required. I think I will have to add it due to the experiment I did on the stick welder when the triac would just stay on and not go off.

author: oldway
author_rank: Super Contributor
timestamp: December 05, 2017, 01:02:38 pm
content: I thought this was your project:http://www.vintageprojects.com/metal-welding/welding-gun.htmlhttp://www.vintageprojects.com/metal-welding/WeldingGun.pdf