spectacular failure of a triac

[made2hack]

Hello all,

I've previously posted regarding a capacitor discharge welder. After some research, it seemed a better way to approach the issue was via triac / thyristor (scr). So after figuring out how to pulse a triac circuit via 555 + optocoupler, I finally came around to trying it using some copper electrodes and some battery tabs + batteries to try it.

As a result of this test, a spectacular failure resulted pictured below. Leading me to believe that I exceeded the momentary pulse / peak current that the triac can handle. According to the BT136-600E Datasheet, it states non-repetitive peak on-state current full sine wave @ 25A / for t=20mS. I guess I exceeded this, but by how much? How do I calculate? At the time, my R2 + R3 resistance was set to 11kOhm so, tc = 121mS. Could this be the contributing factor? I'm inclined to think not. I'm thinking I exceeded the Peak current regardless if I had a shorter pulse (since just pressing the momentary switch might exceed this time constant).

So, how do I limit peak current? Can I ? What kind of current pulse do I need from my triac to have a spot weld? In my case, the current did not have time to reach the electrodes, it blew the triac before arriving @ the tab needed to solder.

Can I even use a triac? Or do I have to go with a thyristor / scr?

Here's the schematic and the corpse for you to judge.

Thanks,

[alsetalokin4017]

This may solve the problem of having the "long" button press keeping the output high for too long:
Edge-triggered monostable 555:
http://www.electroschematics.com/11032/edge-triggered-555-monostable-multivibrator/

This idea of sending the 240VAC through the triac and welding electrodes directly from the mains doesn't strike me as being very safe, though. You need high current to do the welding, but not high voltage.

[Gyro]

Err, so you effectively put a triac straight across the mains, pulsed it on and think you might have exceeded it's 25A peak current??? 

Time to go RIGHT back to the drawing board:

1. You're putting non isolated mains directly into whatever you're trying to weld!!!
2. Goto 1. 

You probably want to look at making a low voltage, high current welding transformer - something like 2V @100A (or 1V at 200A maybe), ie. resistance spot welding. Then you might stand a much better chance and actually live to tell the tale! 

Google, spot welding transformer.

Edit: Then you can probably use your pulsing circuit and a new triac to control the transformer.

[T3sl4co1l]

Domestic mains have a short circuit / fault capacity around 2kA (give or take a wide margin for wiring differences).  Unless those "welding rods" (??) were a lot longer than the diagram suggests, well... use your imagination.

Also... are we to understand this was an unfused test?  Uh... yeah...

Tim

[Brumby]

There's no easy way to put this, but ....

[Brumby]

Arc welders operate at lower voltages and much higher currents than you have used in your design.  The welding circuit also tends to be isolated from the mains.  While there is a possibility of electric shock using a commercially available welder, this is lessened to a degree by the welding circuit being isolated and therefore floating, but also by the lower voltages and the fact that the 'live' circuit is confined to the workpiece and the welding rod.

When I see full mains being applied directly to what is likely to be a large metallic shock hazard, an old saying comes to mind .... One flash and you're ash!

Aside from other issues, this idea is really dangerous.  Please don't pursue it.

[Aodhan145]

You were asking why a 555 monostable circuit was always high the other day and now you are f**king around with mains. Be careful.

[free_electron]

someone notify the darwin award committee to put one aside ...

[dom0]

Consider it done.

[alsetalokin4017]

Now that I think about it, the C1 capacitor should keep the circuit from re-triggering due to long button presses. The problem is that this capacitor will stay charged and you won't be able to trigger the circuit again until that charge is gone. So put a 10k resistor across the C1 capacitor to allow the charge to bleed off. This will allow you to retrigger the circuit by pressing the button repeatedly but still will only trigger once per button press.

But yes, the 240 v part is pretty scary. 

[alsetalokin4017]

Here are a couple of scopeshots. I'm just switching an LED through a transistor of course... no messing about with 240 volt mains for this bear!

These are with a 10k resistor across the C1 capacitor. The first shot is with a 3.3 uF cap and a 1k resistor for the timing components, and the second shot is with a 1 uF and 1k.

Yellow trace is the Pin 3 output and blue trace is the button press. I tried to press and release the button pretty fast. I'm using a standard PCB mount tactile momentary-on pushbutton.

As you can see there is no problem with re-triggering due to over-long button presses, and the timing components are doing the job accurately. The 10k resistor across C1 allows re-triggering with a new button press, without a long wait for the charge to disappear from the cap.

[Aodhan145]

Can we have a picture of how dangerous this set up you had was?

[made2hack]

  At first, I originally planned to use the triac on a secondary of an isolated transformer. I don't know how I got detracted to using the mains. I guess I had not explained myself when asking for advice so people assumed I wanted to turn on an AC circuit.

So, if I isolate using an isolated transformer and pulse the secondary via a TRIAC / SCR, how do I limit the said current pulse to 100A? And, will I need an SCR as opposed to a Triac?

Nobody mentioned how to calculate what kind of Pulse / Peak current I can expect. Do I just assume it will max out the current in the 100A on the secondary and prepare accordingly?

[Psi]

Put a dummy load in series, so the short circuit current can only be as high as whatever the dummy load draws.

[made2hack]

Ah, got it.
So like a lightbulb or a high power resistor?

[made2hack]

i'm guessing limiting the amps on the secondary.

Ie 230VAC -> isolation transformer -> 2.3VAC

and then, try to limit the amps on the 2.3VAC side

[Aodhan145]

Yeah I thought he was going to use a 1:1 transformer by the sound of it. So is he doing it properly then.

[MagicSmoker]

Like others have already commented, using the mains directly - ie, without a transformer for isolation - is a Really Bad Idea.

If you are going to spot weld with AC - like those dodgy Chinese battery tab welders you can buy for $150* - then you need a transformer to step down the voltage anyway. And you want to switch the low current primary on and off, not the high current secondary! You can still use a 555 timer and a triac for that, but you need to use an optically isolated triac driver IC (preferably at the zero-crossing [e.g. - MOC3041, MOC3063, etc.,] with extra bonus points for delaying the timer trigger until the AC line is at the zero crossing). In any event, don't drive the triac gate directly from the 555, and it's best to not drive it with DC when switching an AC load. And when switching an inductive load like a transformer you want to put an RC snubber across the transformer primary or across the triac MT1 and MT2; in either case, a 0.1uF/250VAC film capacitor in series with a 330 ohm 2W resistor should work.

For the transformer you might be able to repurpose a large soldering gun almost as-is, otherwise you might try your hand at rewinding the secondary of a "MOT" (microwave oven transformer) with as big a cable as you can cram through the window area to deliver ~4VAC open circuit.

All that said, understand that the typical on time for spot welding is in the 10ms to 100ms range, but a full cycle at 60Hz is 16.7ms, so AC output spot welders tend to be a bit sloppy with their energy delivery compared to capacitor discharge types.


* - Or less: http://www.aliexpress.com/item/DHL-FEDEX-FREESHIPPING-Mobile-Phone-Notebook-Battery-Microcomputer-Pulse-Spot-Welding-Machine-MCU-Pedal-Welder-Machine/2054203026.html?recommendVersion=1. Note that I am not recommending this product, just pointing out an example.

[made2hack]

Thanks MagicSmoker,

I already have an opto-coupler in my circuit that interfaces to the Triac. However, it is an MOC3052M (the random type) and not the zero-crossing type. So I guess I will pick up the zero-crossing version.

And I will implement the snubber portion as you mentioned. Now that I understand that I should be pulsing the primary yet using the secondary to weld, all is much clearer.

Now I'm @ 50Hz/230V, so, 20mS. But does that mean that I will only get 20mS delivery? Irrespective of my pulse? Or there will be a delay until the zero-crossing occurs?

[SeanB]

Zero cross switch means it will only trigger on when the mains drops to near zero at the crossing, and will conduct for every half cycle the trigger current flows in the LED of the opto. When current turns off the triac will stop at the next zero crossing of current and stay off. You can thus do a single 10ms half cycle or longer only, though you will need a TRIAC that is rated for use with an inductive load, with very hi dVdt ratings, and even then with your load you will still need to limit the dVdt with a snubber so it does actually switch off. You probably will need some large MOV devices across the transformer primary as well to handle the large inductive pulse it will generate with every pulse. Make them easy to replace, as they will fail after a few thousand shots.

[Gyro]

It's generally not a good idea to turn on a transformer at the zero crossing, without any reverse magnetization from the previous half cycle the core will tend to saturate. Turning on somewhere near mains peak avoids this.

I once had a graphic demonstration of this while working on traffic signal controllers, the signal heads have transformers to step down the triac switched mains feeds for the 12V halogen lamps. One test had a bunch of about 20 signal heads (60 transformers), cycling at speed had the signal heads resonating like bongo drums. Moving the triac switching point from zero-cross to mains peak silenced them completely and reduced the dI/dt by more than an order of magnitude, It also completely stopped Field Service reports of failed transformers, previously blamed on them just being cheap (which they were!).

Edit: typo

[made2hack]

You probably will need some large MOV devices across the transformer primary as well to handle the large inductive pulse it will generate with every pulse. Make them easy to replace, as they will fail after a few thousand shots.

Thousand? I have a total of 24 batteries I want to tab !! But I will MOV it as well!

[MagicSmoker]

It's generally not a good idea to turn on a transformer at the zero crossing, without any reverse magnetization from the previous half cycle the core will tend to saturate. Turning on somewhere near mains peak avoids this.
....

That's good advice for the usual case of switching an inductive load with a triac, but in this application the secondary will be shorted for most if not all the time the triac is on, so the triac will be looking into a very low impedance (primary winding resistance plus leakage inductance*) of a similar magnitude as if the core saturated (that is to say, in both cases the core effectively disappears). Thus the load the triac sees is predominantly resistive, not inductive.

* - plus, technically, the resistance of the secondary circuit reflected back to the primary by the square of the turns ratio, but that's still likely to be much less than the primary winding resistance so can be neglected.

[Gyro]

Yes that's true, this is certainly not a typical application, the transformer will most probably appear resistive in this case... So long as the secondary is shorted via the electrodes that is.