How to limit current from spot welder?

It depends entirely on what's in the box that 'controls' it. It doesn't sound like there's much there. Conceivably your best bet would be to run it on 120V.

Graham

Tolerance of what? Why do you want to lengthen the time period? If four pulses weld .010" mild steel, then why change it? You put your parts in the jaws, clamp them down, and weld. Do you want to do .005" mild steel? Then try two pulses.

Or do you just want to slow it down so you can watch the weld taking place in slo-mo? ;-)

Good Luck! Rich

Greetings Rich, I want to slow it down because the timing relies on how fast I can turn it on and off. I'm the timer. When doing .005" sheet I melt through too often. ERS

Make and model of the welder?

get your self a high current mercury relay to run the xformer and get 2 adjustable delay on timers. the first timer gets started with your clamp switch to insure you have full compression. The first timer when it times out and comes on will then start the second timer. the Second timer will use a NC (normally Close) contact which will operate the main Mercury relay to turn on your welder xformer. when this second timer times out, the contacts will then open and wait for you to release the clamp to start the next cycle.. We did this once our self's for a shop, with the use of timers we removed the arching that takes place when you're clamping and releasing because you never know when the switch gets out of alignment with the electrodes and energize under light contact. Get your self short delay timers.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Jamie wrote in news:NYPyh.366$ snipped-for-privacy@newsfe05.lga:

The problem with electromechanical relays are the delays on make and break. In the case of the spot welder, "make" delay is no problem. But these delays are on the order of hundreds of milliseconds. Since one cycle is ~16 mSec, once the timer shuts-off the relay current, it would still power the welder primary for 16 or more cycles. What he needs is an electronic timer that can have a start-stop response time of

1 cycle @60Hz, and a solid state relay that can also respond in that short of time. The SSR should have a current rating twice that of the spot welder.

I built one of these several years ago, after trying to spot weld 0.010" stainless with a large Miller LMSW-52 (220vac/2.5KVA) spot welder, without a timer. And the timer that Miller sold cost more than the spot welder, and stand w/foot operated tongs, put together. And it was electromechanical, and was never ment to do really short cycle times.

The one I put together is archived in the Metalworking Dropbox 2001, under "Miller Time"

If I were doing a lot of light weight sheetmetal I would get a lower powered spot welder.

Ken

yeah, that's all well and fine how ever, you must pay close attention to the type of SSR you use to insure zero crossing control and make sure there is always a load with a HV suppresser on the electrodes. arching loves SSR's personally, unless micro spot welding is being done, i don't see why the dwell time is so short. I mean, it's a manual machine as it is now, how can he even be close to that ? I think some one with that kind of speed should be in track :)

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

"Jamie"

** Risky idea - ensures maximum possible " inrush surge " peak currents every time.

........ Phil

** Wrong to assume the secondary is always effectively shorted.

There will be very many occasions when the secondary is open.

Expect inrush surge peaks equal to the AC voltage peak / primary ohms.

My well exceed the SSR's repetitive I peak rating.

Splat.

........ Phil

Yep. A shorted secondary, current limited transformer (I think that is a pretty good description of a spot welder) is mostly inductor. If you could add logic to fire the SSR, on the first half cycle, near peak voltage, the transformer would get less hot and so would the SSR. But the shorted secondary also reduces the tendency of the core to saturate (except for the small magnetic shunt), so it may not be so important.

The problem i originally was stating is that Welders arc at various times. We have copper tape welders at work that one of our electricians tried to use a 3 phase SSR to drive the xformer. Not only was the idea bad to start with, the xformer was a single phase type and his thinking was that combining all three legs of this SSR would triple the duty of the SSR was idiotic. I gave up trying to explain to this individual the theory of diodes, SCR load sharing, arching, HV problems etc...

Needless to say, after shorting a few of these expensive SSR units, the problem was past to me. To solve this problem i simply used a large mercury relay. Problem solved.

We do have induction heaters at work that blow an SCR in the bridge once in a while from arching. i must say, you should hear and feel the HUM that comes out of that before it takes out the fuses!

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

"Phil Allison" wrote in news: snipped-for-privacy@mid.individual.net:

Phil,

In mine, I use the switch that is built into the lever mechanism for the jaws. In the original design it was just a power switch in the primary. Jaws close=power on...jaws open=power off. But the switch operates after the jaws make contact and before contact is lost. In my design it just triggers my timer. No timing or primary current unless the jaws are closed...and I don't open the jaws until the timer has cycled off. I suppose this where you could get powered open secondary...if you let up on the jaws on a long time cycle.

Ken

"Ken Moffett"

** If the jaws close onto an insulator ??

Even lightly rusted, paint coated or grime/oil contaminated surfaces are insulators, particularly at low voltages.

........ Phil

You might try making a gizmo to shunt some of the output to the weld contacts into another load. Maybe a carbon rod or some thing that can handle the short burst of current and reduce the amount being put into the weld itself.

You seem to imply you believe you can reduce the current and allow for longer weld times. That is doubtful. You would need a tapped weld transformer or at least a choke to do this.

A resistor in the primary is difficult. Possibly an electrical heater element could be used but you may find the welding performance falls off quickly.

Phase controlled SCR's can adjust the voltage to the primary side.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

How well will that work with such a device?

The following response assumes there are no control circuits that require the full 220v input voltage.

Try to find a large Variac, a voltage variable transformer, usually configured as an auto-transformer. See

for more information. This would give you fine control of your spot welding current.

Or, you could wire in a series opposing (bucking) transformer on the primary side. For example, If you had a transformer with a 40v 15a secondary, this could be wired in series (with opposite phase) with the primary of your spot welder. This would lower the spot welder input voltage to 180 volts and the output proportionately lower. But this only gives you one fixed voltage, and I don't know how do you would pick the right bucking transformer without testing.

Or, Since large Variacs can be expensive, find a 220v to 110v stepdown transformer with a 15a secondary rating. Wire this series opposing. Then connect a smaller Variac to the primary of the stepdown transformer. I think the Variac could be much smaller. (This off the top of my head) If you had the Variac at full voltage, your spot welder would be supplied

110 volts, the spot welder primary current would be 1/2 or about 7.5 amps. That means the current on the primary of the step-down transformer would be 3.75 amps, which is the same current supplied by the Variac. So a much smaller Variac. I have not crunched any other examples, so I'm not sure this is the worst case scenerio, I may have it backwards and this could be the best case. Group, HELP verify a couple more scenerios) Mike I got a brand new 240volt 9amp Variac at a hamfest for $15. The seller commented, "oh, I should have looked in the box before I gave you the price."

--
Hardly.

Since the resistance of the materials is causing heat to be
generated when charge flows through them, the rate of change of
temperature is going to depend on the quantity of charge flowing and
the amount of time that charge is allowed to flow.

In other words, it will take the metal longer to melt with 2000
coulombs per second flowing through it than it would with 4000
coulombs per second flowing through it.

If changing the current doesn\'t change the time, what do you suppose
does?