Build your own CD battery tab welder for about $100.00+-

[Maine_Guy]

SCR Trigger Subtleties?

OK - so I read Jeremy's post and diagram showing a 8.2 ohm resistor between the capacitor voltage and the input lead for the trigger. This seems to allow a lot of voltage to the trigger.

I don't know what Jemery's SCR is rated for - but mine is rated for 3v @ 150ma. I see most are. I also see he mentions that the SCR likes a fast voltage rise... I'm confused.

So my question is this: Can I harm my SCR by overpowering it? Or is it just "extra" to ensure it fires. I can't seem to find guidance on this from the web.

Thanks if you know the answer!

-jason

[hillzofvalp]

Update: Go to next post.

I guess I learned the hard way?

if you look at the Vgs vs. Current graph, a Vgs greater than about 9.5V would push the fet above its 1000A max pulse current. Am I correct in thinking that my Vgs of 15V blew my fets? Can't be.
http://www.irf.com/product-info/datashe ... 3pbf.pdf''

Here's another phenomenon. If I charge the caps up to 13.5V, say, and wait.. they will discharge maybe .2V in 5 min. Then! THen--when I discharge the caps to .8V, they start to charge up slowly to over 1.57V. Is this the capacitance/ leakage from the fets? When I first ran the welder with these 8 IRF3703s (8000A net pulse max), the first 4-5 welds became progressively worse. The first one was like "ka chunk" (the good kind of "ka chunk!"). Then they got poppy.. from incorrect electrode setup... or so I thought.

Update: According the the driver datasheet (http://ww1.microchip.com/downloads/en/D ... 21946a.pdf), I might've missed the cap bypass to the Vdd.. WAIT!

VGS decreases when I charge up the caps... so that's why when it did fire, it was less than 10V. The solution to this issue has got to be very obvious. Either increase driver output to 18V for Vgs of 3V or decrease driver output to 5-7V to make Vgs -8-10V. I'm losing track of which end is negative or positive, but I believe it does not matter for Vgs (as long as potential exists it will fire). Right, guys? Only problem now is that my driver will only have a 160ns rise time instead of 80ns.
Re: Jeremy
I would assume that when you connect to your trigger voltage, it will pull the correct current, like a relay's power requirement specification (just a guess). you may be able to over-volt it a little bit..

[hillzofvalp]

FOUND BIGGEST HINT:

Discharging the cap bank completely turns off fet bank. I have the IRF510 driver firing fine. Fets are not turning off unless I discharge the caps. Almost like the caps are supplying gate-source threshold.. maybe a diode from driver fet drain to fet bank gate?

[hillzofvalp]

FOUND BIGGEST HINT:

Discharging the cap bank completely turns off fet bank. I have the IRF510 driver firing fine. Fets are not turning off unless I discharge the caps. Almost like the caps are supplying gate-source threshold.. maybe a diode from driver fet drain to fet bank gate?

Seems like these FEts like to be high on the high side... I will spend another 5 hours figuring this out

[Maine_Guy]

re: SCR Trigger

I simply hooked up my system similarly to Jeremy's diagram - although I used a 10ohm resistor. All seems to work well for triggering so far.. although my relay only works at 6.25v and above. Utilizing the welder below 6v is not possible... but then maybe I don't need / want to anyway.

I will post photos shortly - my build came out nicely thanks to some powah' from southern end of the US (3f thanks to Mr. Texas). I utilized an old Gateway PC case to house everything and was able to reuse the LEDs, main power-switch. The trigger switch / foot-pedal utilized an existing hole for the PC "lock" - which accepted a 1/4" phono jack readily. Cleaning out the PC case was a PIA (ironically, lots of things spot-welded inside). They don't make PCs like that anymore.

Still waiting for my voltage controller do I can adjust output voltage.. and this project is ready for it's photo debut.

[texaspyro]

Take videos... we love the sights and sounds that SCRs make when they vaporize themselves...

[hillzofvalp]

don't doom his project..pyro

[Maine_Guy]

My Build

I started with my old gateway box - it was free, but difficult to clean out! They don't make computer cases like that anymore!

Gateway Welder 1.jpg

(224.47 KiB) Downloaded 3 times

+-3 Farads of rockford fosgate caps. Going through an ebay SCR and a lot of components. This took some time to build and troubleshoot. Weeks of effort. It easily welds / bonds .060" stainless wire, at approx 12-15V on the caps - depending upon the wire arrangement.

Gateway Welder 2.jpg

(103.18 KiB) Downloaded 3 times

I was able to reuse the power button, the LEDs light up when you turn it on, fans, the "reset switch" is repurposed for bleeding the capos through a resistor and the foot-switch connects through the removed lock (now a phono jack that goes to a music pedal).

I used Jeremy's circuit diagram as a base - with a few customizations.

This was fun (most of it)and I really appreciate the assistance along the way - esp. the caps from Texaspyro. They did the trick.

[texaspyro]

With an SCR welder, I would put a beeper across the probes. That way if you pull the probes off early (or blow out the weld) and the SCR never turns off, you won't be so surprised when you put the probes on the next joint.

[Maine_Guy]

My probes.

This is a re-purposed set of hose clamps from sears. The welder is only as good as the method used to apply the welds.

[pwbset]

So I haven't used my tab welder since the summer of '09. I left everything connected and just now hooked it back up to my 16v DC unit and... nothing. The cap is reading -0.82v across the poles. Hmm. Do caps go bad after sitting for a couple years or could it be my SCR? How to test? Thanks for any tips!

[amberwolf]

If the cap only has less than a volt on it, most likely it's not getting charged up, so you would wnat to look at it's supply and connections. If the main supply does not have any voltage when connected, but does when it isn't, maybe the SCR is shorted on (or being forced on by somethign shorting the trigger on)?

[pwbset]

If the cap only has less than a volt on it, most likely it's not getting charged up, so you would wnat to look at it's supply and connections. If the main supply does not have any voltage when connected, but does when it isn't, maybe the SCR is shorted on (or being forced on by somethign shorting the trigger on)?

Thanks Amberwolf. The DC unit is putting out 16.2v and the wiring is sound throughout. I'll check the SCR for a short.

[Andje]

http://search.digikey.com/us/en/product ... ND/2269426

Hello all. I am going to be building one of these thing soon. I am thinking of grabbing a couple of these caps; they seem to be the best I can find to make ~1F that I can buy new. I've been watching ebay but haven't found anything better. Im happy to spend about 250 on the caps to get quality. As far as I can tell these would be appropriate.
It will be a dual pulse micro controlled fet triggered welder. I am aiming to do 18650v cells or other spotwelding/plasma cutting in the future with it.

[pwbset]

If the cap only has less than a volt on it, most likely it's not getting charged up, so you would wnat to look at it's supply and connections.

Caps gone bad. I hooked it up directly to the 16.2v DC source and it couldn't make it past 5.9v and once disconnected drop <1v in seconds. Time to source a new cap. Hmmm.....

[texaspyro]

http://search.digikey.com/us/en/products/B41456B5470M/495-4217-ND/2269426

Save yourself a bundle 'o money and a lot of grief. Rockford Fosgate or Monster Cable 1 farad car audio caps are true 1F low ESR units. Schose is also OK. Almost every other car audio cap is truly a scam. I can hook you up with some measured/tested units.

[pwbset]

I got my tab welder working again, but I'm using a 4.2VDC supply to trigger the SCR. Is that okay or will that cause issues? I'm unable to find the specs for the thyristor I have, which looks like this:



And it rated for 600V 80A. Seems to be working okay, but I used to trigger the SCR with 2 AA in series so this is a little hotter. Thanks for any info.

[oldgrump]

Does anyone know what has become of Jeremy Harris? At one time he was a major contributor to this forum - - and a true gentleman. I used to have an email address that worked for him, but afraid that no longer works.

Jeremy always seemed to have excellent answers to all the questions.

[amberwolf]

There is a thread asking this question, and it also has the answer--he has other projects taking his time, and he does stop back in from time to time.

[Kin]

I am very amused that I was reading his page, thinking about putting one of these together, but about to check to see if anyone on ES had done this before. Right before I did that, I clicked the link on the bottom of this thread and found his page here.

[JOAT]

Hi, new member here, can someone help with finding all the parts to build one of these? Somewhere with hawaii friendly shipping ? I am having a hard time sourcing all the parts, I can get the caps for $35, they would be precision power 2 farad,would those work?
Thanks

[Kin]

I think buying the caps new tend to not be an option unless you're willing to pay quite a bit more. These are expensive parts new, but widely available surplus. You won't have as many electronic surplus stores in hawaii (i imagine), but you still might be able to source used car audio caps or from whatever electronics surplus areas there are. You will really have to look around. I'm still struggling to find enough caps around where I live for a cheap enough price.

Some options
1) Hope the 2 farad caps are enough. If they are real farad caps it could be enough (look at the link, he has some success with 1.5 farad but is much happier when he goes to 5 farad).

2) Look into the higher voltage version. 2F at 24v is the same energy as 8F at 12v. What are those caps you're using rated to? (don't use them at the limit of their voltage! Caps can blow quite quickly above their max; some conditions can cause such a voltage spike).

[JOAT]

I think buying the caps new tend to not be an option unless you're willing to pay quite a bit more. These are expensive parts new, but widely available surplus. You won't have as many electronic surplus stores in hawaii (i imagine), but you still might be able to source used car audio caps or from whatever electronics surplus areas there are. You will really have to look around. I'm still struggling to find enough caps around where I live for a cheap enough price.

Some options
1) Hope the 2 farad caps are enough. If they are real farad caps it could be enough (look at the link, he has some success with 1.5 farad but is much happier when he goes to 5 farad).

2) Look into the higher voltage version. 2F at 24v is the same energy as 8F at 12v. What are those caps you're using rated to? (don't use them at the limit of their voltage! Caps can blow quite quickly above their max; some conditions can cause such a voltage spike).

The caps I can get at my cost are only rated to 14v, 12-14v surge and working voltage, I dont mind getting better parts, I just want to do this right and do it right,No surplus places I know of in hawaii,and even if there was I wouldnt be shocked to pay more then 3-400% more of ebay prices sometimes

[texaspyro]

I have tested DOZENS of different car audio caps. The ONLY ones that met their rated specs are the 1F caps from Rockford Fosgate, Monster Cable, and Scosche. And ONLY their 1F caps. ALL the others that I tested were bogus. Many as low as 1/20th of their rated capacitance. BEWARE!

See: viewtopic.php?f=2&t=2633&start=810#p494786 for some of the testing results.

[JOAT]

I have tested DOZENS of different car audio caps. The ONLY ones that met their rated specs are the 1F caps from Rockford Fosgate, Monster Cable, and Scosche. And ONLY their 1F caps. ALL the others that I tested were bogus. Many as low as 1/20th of their rated capacitance. BEWARE!

See: viewtopic.php?f=2&t=2633&start=810#p494786 for some of the testing results.

Thanks, so will 3 x 1 farad caps be good ?