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Will this capacitive discharge welder design work? Answered

I am planning to build a capacitve discharge welder so that I can tab weld my battery packs, and I have no intention of paying 2K plus.  I recently came across a fairly simple design, and was using it to design my own.  If anyone that has built one of these, or knows how to, can look at this design in detail, and inform me if I have the right componants, and in the right order, it would be greatly appreciated 

*** Please, I know the dangers and risks to what I plan on building so please do not express your concerns here.  Everything has risks, but without risks their can be no reward.***

I added a new diagrahm with the opto-iso and relay that you two recommended, not sure if its right yet, but its a work in progress




2 years ago

i think it does not work propely..
<a href="http://www.welderreferer.com/2015/05/alpha-tig-200x-tig-welding-review.html">alphaTIG 200x review</a>

This guy used Mosfets on his capacitor discharge welder http://frikkieg.blogspot.com


6 years ago

+1 Steve

I would also keep the pulse current lines short.
This type of welding depends on resistance between the the target metals
to create the weld heat.

You want the resistance at the probes to be less then the target.
I would look into silver probes to help.
Switch mfg use silver buttons on high current contacts to avoid bounce arc welding for the same reason you want to remove your probes afterward.

I would also look into using am opto-isolator to fire the SCR rather
then risk body capacitance pre-triggering while you set your spot.


If the SCR is fully Insulated, would it still be susceptible to pre-triggering due to body capacitance. I only ask because im not familiar with otoisolator's and how they work, nor would I have a clue how to add one into this design.

SCRs are VERY easy to false trigger - I used to play with one when I was a kid that I rigged as a touch switch.

You can safe if by putting a 1K resistor to ground, but you'll need to really whack the gate hard to trigger it - the pros usually discharge a capacitor into it.

what are the battery sources for that schematic, is it just like it looks, 2 sets of two AA in series. the other thing is the scr, I have little experience with them, and the one that seemed large enough for what i am doing wasnt well described in the datasheet. i know the small red wire is the "gate" or trigger, and the small white wire is the cathode, but what side is the anode, the side with the nut or the long side large gauge wire side.

You are correct about the Gate and Cathode !
I thought you understood it represents your
120BPF SCR with the stud Anode.
The Red wires are the Cathode and the White wire is the Gate.

The way the SCR works  like atoilet tank,  once you pass a small current
through the Gate - out the Cathode  work the flush lever the SCR turns ON
and stays ON conducting until current is interrupted  like aflushed tank. That
is why Steve had you disconnect the Power Supply when you fire the SCR.

A stud SCR ( Silicon Controlled Rectifier ) can usually handle a one time Huge current 50 Amps to several hundred. ( One Time ) means every few
seconds or as long as it is cool to the touch.

Did you know this same circuit with a coil of wire in place of the probes is
used to magnetize iron into Magnets or crush Al soda cans or make coins
flip over,  Neat  Ckt :-)


....and like the flushing toilet, you get a bit of water flowing, which makes a bit more water flow which makes even more water flow - its a POSITVE feedback loop.....

that makes sense. it kinda sounds like an SCR is similar to a relay. adding a small charge to the gate through the cathode opens the "flood gate" allowing the power to pass through the SCR or allowing the circuit to complete. If I was to use the optoisolator to make the SCR less sensitive, what iso would i use. im not sure which one would be good for my design. and it seems that you have it designed to run off 4AA batteries?

this iso wont put out more than 2V or 150mA because thats the max the SCR can handle at the gate. only asking because it looks like it runs at 3V

The forward drop of the ISO and the series resistor Limit the output
current and voltage.  The  4N33  costs  25¢....

All  4Nxx  I know are optical isolators or optocouplers.

I'm pleased you liked my Flush SCR analogy.  The point being once fired,
you loose control of it until it has emptied your tank Capacitor.

but you do have it powering off of 4AA's??

Heres what I have so far, as for the opto and relay. Im a little confused on what to use to power the coil in the relay. from what i can find, all the relay's that can handle 5A through have to have at min. 12V to activate the coil and switch over, but the opto has the footswitch powered by 3V, so how do we make them work symbiotically. heres what i have so far


DON'T charge the capacitor straight off the supply - its likely to trip it, since its a short circuit when its empty.


Because his supply has an intrinsic current limit, since it has to charge flat batteries.


Ok, that makes alot of sense. I should have known that. so what di i do then?

Add a 10 Ohm resistor from the supply to the cap. It will take about 10x 3.3 seconds to charge to around 25 Volts.


thats all I have to do, add a resistor? i would assume a large watt resister. now if i have the PSU set to 14.4v, it will only charge the cap to 14.4 right?

Yep, and Yep. The "wattage" is highest for a fewseconds after switch on = V^2/R. Try a 20 Watt-er.


Look for a metal clad - or use a car light bulb !!! - something around 20W .....


I am so lost. I can use a bulb like this as a resistor. I thought it was supposed to be a 10-Ohm resistor? and with all this circuitry, will the cap still get charged enough, the smaller circuitry wont hinder the amount of power passing through???


Look for a 20W 12V bulb - a taillight bulb should be fine - ask at an auto store - and it will act as a perfect little power resistor for you for a few seconds.

No, it won't steal power, because no current will flow in it during the discharge cycle

A new day and a fresh mind. So heres what i found, can you tell me which one of these will work, if not all, too protect the psu from short circuiting. and does the diagrahm above look right, where i added it. also, if I use the bulb, am i right to assume, the bulb would only be lit while charging and the turn off once the cap is fully charged? like a charging indicator?

these are all rated 20W, the bulb is 12V, the resistors are 10 ohm

20W 12v.jpgA102450-ND.jpgFSOT20-10-ND.jpgPWR220T-20-10R0F-ND.jpg

Just use the bulb - as you surmise, it will act as a charging/charged indicator for you.


so how does a 5 farad stereo cap sound, too much. cant really have too much? as long as i keep the settings on the psu where i want them, right?

also, how does the G6L relay I mentioned below sound. will it work, or do you know of one that will fit my specs better. I wanna make sure I have all my Parts right before i order them. as always, thanks guys

Make sure the relay can handle the 5A charging current.

5 Farad sounds fine.

having a hard time finding a relay that will work. Heres what i think im looking for: a SPST-NC, that opens when you apply the coil current.  has to be able to handle a "Current Load" of up to 5A, and have a "coil voltage" of 3V or less. is this right? because I cant seem to find one. and im not sure what kind of relay im looking for. solid state, etc..

Just get an SPDT, or DPDT - 5A is pretty low for a relay. Why worry about the relay voltage - just trip it with your charging voltage, or run that through a regulator if you're worried.

All you need is a simple, mechanical relay - the kind of thing Radio Shack sells.


Im not sure what you mean?

Dont I need a NC relay for the diagram to work?

If its rated at 5V NOM will I have to adjust the schematic to incorporate more voltage to the coil, or will it avticate on 3V?

If its only rated at 10V and I try to put 14.4 volts through it, wont i damage it?
What if the relay is rated for ac, but not dc, can I still use that?

Nope, an SPDT or DPDT relay contains NO and NC contacts ! Wire the NC contact.....

Only use a DC relay. Running a 12 V relay on 14 v won't hurt it at all.

Ok, I wasnt sure about that because the datasheets I've looked at arent descriptive. I have noticed that the diagrahm of the internals will show only 4 or 5 pins, but the will be numbered like 1,3,4,5,and 8. atleast that explains whate pins 2,6 and 7 should do. So if "ALL" SPST are N.O. and N.C. then THIS one would be perfect for my set up.

so then i geuss SPST are not both N.O. and N.C., just SPDT and DPDT

OK, this ones DPDT, 3VDC, 5A.  sorry guys, new to relays, having a hard time getting it right. 

The biggest one I ever saw was used to ground a power stattion, and it was rated at several megawatts.....

new schematic with 10 ohm resister inline from relay to cap


are these labels correct


6 years ago

IF you don't like batteries here is a way to use the SCR anode.
Also as the SCR turns On, so the gate diminishes current.
The MOC3023 opto-triac available from Jameco.  .  .  .   A


I have no idea what you just wrote, i envy your understanding

I guess you've reached absorption limit for the day ( called Peter principle )

Read this later.

I show you how to turn ON your SCR without  two of the AA batteries.

The MOC3023 is a High voltage switch see the dotted line.

The SCR will turn On anytime current flows into the gate.

When your capacitor is charged and you are holding the probes against the target the SCR is holding back all the stored power.

IF that dashed switch gets closed,
a current will begin to flow Orange Path from the Anode through 470 ohm Resistor and ultimately through the Gate to the Cathode.

Of course the SCR turns On Red Path and rapidly discharges the main power capacitor.

When the Capacitor is discharged, both currents fall to zero
ie nothing even if you keep your foot on the switch.


I have the scr hooked up to the negative, so will that still work? I ask because you say that the current will flow from the anode? Its a nice idea, but i will probably just use 4 AA's, I keep lots of them around, and they should last a while

Personally I'd isolate the PSU during the discharge cycle. And nothing there limits the welding current AFAIR, you need a fat, non-inductive resistor to do that for you - or your weld quality may be poor.

steve, are you an engineer? It always seems like your the one to help me the most.

how would you recommend doing that, isolatethe psu during discharge, and the resistor?

For my sins, yes.

You could use a relay. The footswitch pushes off the relay then the relay fires the thyristor.