This post is for anyone interested in owning or building a CD welder.
I wanted a reliable Resistive spot welder for lithium ion pack construction and or repair. I didn't want to spend 3k or buy from china. I decided to design and fabricate my own. Bellow is my first crack at a Mosfet Switched capacitive discharge mcu controlled spot welder.
This post is a work in progress. If you've stumbled across in hoping to build your own cd welder feel free to ask questions in the comments section.
I'll provide any help I can.
My CD Spot Welder
Here's pics of the first working version from 2018
What is a CD Welder?
It might be more accurate to say there are two power sources for spot Welding. Each has its own advantages and drawbacks. The selected one also heavily influences the design.
For instance my design is Mosfet Switched. If you tried to use it to switch a ac power source you would be greatly disappointed in its performance.
The long and short of it is mosfets are DC switches. You can use two of them in a special configuration to make an ac switch. That's not how mine are implemented.
The capacitive discharge part implies that the power source is a large Bank of capacitors.
I went with Maxwell d cell ultra capacitors.
Why Spot Welding ?
I also recently read that SpaceX is using spot welding to attach the hardware that allows the heat protective tiles on starship to stay put.
For a while Elon musk also owned Maxwell (actually I think Tesla bought it but that's similar to the same difference.)
CD Welder Design and Build
The Capacitor Bank
- 2.7 volts max
- Internal Resistance (IR) of 3.2 milliohms
capacitance of the series string:
Resistance of Capacitors in Series
Capacitance of S&P Capacitor Bank:
Internal resistance of S&P capacitor Bank:
(#of parallel strings)
if plugged back into I=E/R
2531.25 amps @16.2v as a theoretical Bank Max.
Aka: 41006.25 Watts or 41 KW
In practice it won't deliver this. For a cd welder this number is not a bad place to start.
I'm entering this on a phone or I plug in more equations for you. But when it comes time to charge your capacitors or ultra capacitors in series that R value per cell starts to become a pain as well.
More so with these ultra capacitors and their relatively high IR.
Especially so when you want to go towards their Max voltage.
Put simply the risk is some in the middle or on the far end will reach their full voltage before the others and at that point they're in an overvolt condition. If you like shrapnel don't worry about it.
It's also worth noting that the real life IR value of any given ultra capacitor is slightly different than the factory rating.
If you do a lot of plugging with the ohm's law and E equals IR you might see that in series there's a possibility that one of the cells reaches its maximum voltage before the bank is fully charged or the ones around it is. This can be a problem because all of a sudden you are over volting that capacitor.
To manage this risk you use what's called a balance board. This is the same with lithium ion batteries in series configuration.
A balance board is basically a mosfet a little bit of logic or clever use of diodes and some power resistors. When the cell in series reaches its maximum voltage during a charge the mosfet switches on a power resistor.
If you want to look it up more than that you're on your own. All the theory is out there but it's not worth going into here.
On a random note: the 3p6s config jump-started a minivan in 2021. Somebody had stopped to help before me and was trying to use their truck. with the cables attached for 10 minutes while I waited and watched it only produced a click on the van.
My wait was knowing I had the bank in my trunk and debating what are they going to think when this guy walks up with this random white box?
How do I explain it if they want to know what it is? I'm glad I thought about that because they did.
I said basically if you know what a car audio cap is this is 350 times more powerful.
I instructed them to disconnect the minivan and connected the cap bank to the truck (to charge it up). When the balance board LEDs lit up we disconnected the truck and connected it to the van.
It's sprang to life on the first try. I believe it was negative 10° f that night.
it put quite the load on the truck alternator as well. The engine actually revved to counter it. It took something like a minute to charge it. I wasn't actually counting though it just seemed inordinately long.
Low side mosfet switch
I needed this fast have no formal EE training. I was also relatively new to Circuit design in general. The easiest thing to implement was a low-sided mosfet topology. I went with six powder fets in parallel.
But here's what it looks like in this most basic implementation
I'll add this section soon. For now anyone pursuing this with atmel / arduino in mind should check here
If you want reliable timing you need to look into coding with port register manipulation
Fabrication of the CD welder
Control board PCB
Testing a Weld To Failure
Pulse Length Adjustment via rs232
In the tests above I only had one power supply and had not yet designed the circuit to monitor the bank voltage with the microcontroller or to adjust it. I was literally using the bench supply to charge the cap bank then disconnecting it resetting the voltage and powering the switching board. Luckily with the amount of charge the bank stored it was quite a few welds per volt dropped. Sweet spot was something like 13v