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Help with a spot welder...

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LAN_403

Xray

Mon Apr 25 2011, 03:30AM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

I own this

spot welder that I've used numerous times over the past couple of years, and it has proven to be very handy when nothing else will do the job. The problem is the momentary contact switch failed after about a year of very little use, and now the replacement switch (that I got from Harbor Freight Tools) is showing signs that it will soon fail (sparking and smoking during on/off cycles). So, I was thinking that if I were to add a large triac to handle the high current switching, and let the mechanical switch turn the triac on and off, then the switch should last until our sun becomes a white dwarf. My problem is, I don't know how to spec the proper size triac, and I don't know what kind of circuitry thats needed to drive it (that is, the connection to the switch). So, I'm hoping that you guys can help me choose the right triac and other components.

I think I need a triac that fires during zero-crossing. Is that a thyristor? If not, please explain what type of part would do that.

Thanks!

klugesmith

Mon Apr 25 2011, 04:06AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Hi. Good idea.
Thyristor is another name for SCR, whose main terminals conduct in only one direction.
Triac main terminals conduct either way, and the device can be triggered either way
with gate current in either direction (though some quadrants need more gate current than others).
Neither device has any built-in zero crossing control, but some Solid State Relays do.

To size the triac or SSR we need to know the AC input current, its duration, and whether the instrument has any control of starting phase (zero voltage crossing is actually the worst time to energize a power transformer). PM me if you want to talk about this on the phone.

Does the spot-welder have a pulse duration timer (like an x-ray machine) or does it just depend on how long you push the button? In the latter case, there's no control over the AC phase at which the circuit closes.

ConKbot of Doom

Mon Apr 25 2011, 04:07AM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

Maybe Ive just been getting lazy lately, but I'd just go for a Solid state relay, plenty on ebay, and other sources.

You'll need something to bolt it to for heatsinking, but so would your triac circuit.
EDIT:
You could upgrade to a timer relay if the welder isnt already outfitted with one, and be able to set your on duration of the welder, to give consistent welds too. Mcmaster, grainger and the like would have those.

James

Mon Apr 25 2011, 04:11AM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

You could build a SSR with a triac and optocoupler. I've used the MOC3021 and 3031, random phase and zero cross trigger respectively.

No need to reinvent the wheel though. You can get SSRs with zero cross switching pretty easily. It says the welder draws 25A, so this 40A SSR ought to be plenty

Xray

Mon Apr 25 2011, 04:16AM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

According to the sales page on the Harbor Freight website, the specs are: 240 volts, 25 amps, 60 Hz. I do have a number of SS Relays, but they are rated at only 10 Amps. That's a great idea to use a SSR instead of a triac because they are easy to work with. All they need is a 10 to 30 volt DC input signal to turn them on. I just need to find one that will handle 25 amps or more.

To answer the question about weld time, it's simply a pushbuttin switch. You push the button for as long as you think is necessary to make a good weld.

Xray

Mon Apr 25 2011, 04:18AM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

James wrote ...

You could build a SSR with a triac and optocoupler. I've used the MOC3021 and 3031, random phase and zero cross trigger respectively.

No need to reinvent the wheel though. You can get SSRs with zero cross switching pretty easily. It says the welder draws 25A, so this 40A SSR ought to be plenty

Thanks for that link, James. That's exactly what I need!!!

)

UPDATE: Rather than buying that one from China, I ended up buying a similar 40A SSR from a U.S. seller on Ebay. It cost a little more, but I'll get it a lot quicker.

I'll update everyone again after I install the SSR and have it working. Maybe even show a picture of it.

Xray

Wed Apr 27 2011, 10:18PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

UPDATE: Here

is a picture of my modified spot welder. The 40 Amp SSR cost me only $16.00 plus shipping, and I needed to add a few parts that I mounted in the rear end of the spot welder that provided low voltage DC to drive the opto input of the SSR. I simply calculated the values of a resistor voltage divider that gives me 15 Volts at 10mA from the 240 volt AC input. A 1N4007 diode and a 47uF at 35V electrolytic cap gives me the DC voltage that the SSR needs. I tested the finished unit by spot welding a couple of steel straps, and it works great! Thank you guys for your suggestions!

James

Wed Apr 27 2011, 10:25PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Glad to hear it worked. Yikes, going from 240V to 15V with a resistor though... The usual approach is to use a capacitor feeding a bridge rectifier then put a zener and filter cap across the output of that. I use a film cap in the range of .68-3uF in series with the input of the bridge depending how much current I need, runs cool and reliable.

With the resistor approach you're burning up around 10x as much power as you are using, granted you're using very little.

Xray

Wed Apr 27 2011, 10:31PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

James wrote ...

Glad to hear it worked. Yikes, going from 240V to 15V with a resistor though... The usual approach is to use a capacitor feeding a bridge rectifier then put a zener and filter cap across the output of that. I use a film cap in the range of .68-3uF in series with the input of the bridge depending how much current I need, runs cool and reliable.

With the resistor approach you're burning up around 10x as much power as you are using, granted you're using very little.

Yes, I thought about all that, but there was barely enough room inside that rear cover to contain the few parts that I used. And since most spot welds take less than one second to complete, I wasn't at all concerned about how much energy was being wasted in my little resistor divider.

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