I am waiting for parts to build I guess the part of this project that everyone is waiting for. I am trying to make this welder as cheap as possible and this weekend my calculator kept on spitting out data that I didn’t like. I have a circuit that turns my SCR’s off but to build it to work with all big SCR’s out there will be more difficult and expensive than just using Mosfets for the actual weld. I never wanted to use Mosfets but looking at the numbers made me change my mind.
Big SCR’s have an ON resistance of about 1.5mOhm (It differs). If we do a quick calculation you will see that if we make the resistance of the metal that we weld 0(dead short because we are trying to figure out the max current to protect our components) then we only need the Capacitor's ESR and the SCR’s ON resistance to work out the actual max current at the moment the switch is closed (we are not going to use the wire and weld probe resistance because they are variables)
Let’s say our Cap have an ESR of 1.6mOhm. If we use the above SCR and charge our cap to 20V (max) then 1.6 + 1.5 = 0.0031Ohm. I = V/R 20/0.0031 = 6452Amps.
The Mosfets in parallel that I’m going to use will have a much lower ON resistance of 0.0002Ohm which will give us about 11111Amps for a split second(much higher than the SCR). The current will be less if we add the wire and probe resistances and also the metal that we are going to weld. The number of Mosfets in parallel will also give us a much bigger surface area to work with.
I should receive the PCB’s by the 17Th. I am busy designing this new Mosfet circuit and will post pictures and more information
I'm willing to build and test a prototype of yours Fritz. We've talked about this before if you recall I'm highly skilled and can build and debug everything but the code itself. But I can make sure the code does produce the correct timing and waveforms. Glad to see you making great progress and keeping interested. I like the toner transfer method myself for years. Check your old emails from this summer for me at mikep_95133. Let me know when you want a set of heardware built or qualified. -Mike
ReplyDeleteHey, another vote for boards. I can make single-sided no problem at home but double-sided are not so easy. I'd certainly buy one!
ReplyDelete10-4 on the boards. I'll take one or two. Love the mosfet idea as well. IRF2907? Shawn
ReplyDeleteI'd definitely buy one of the boards. I've been waiting for you to announce when it will be available.
ReplyDeleteHello from Croatia.
ReplyDeleteI found this blog on the internet and presented to two of my colleges in company where we work.
We are very interested to build this welder and have lot of skill in electronic and lot of electronic parts and other garbage.
In company we also have small CNC machine for making pcb.
Primary interest is in welding battery cells for motorized bicycles and similar stuff.
Looking forward to see final working version of welder
Count me in as well on purchase of at least one PCB.
ReplyDeleteWell, thanks guys. It's going to be hard for me to buy any more PCB's if only 4 people are interested though. For my next order I have to buy 50 or more to get them at a cheaper price.
ReplyDeleteMike and Shawn, I still remember you guys and did not forget what we talked about. I have spent so much money and time on this welder so far that it is way passed the prototype stage. I will make the final stage PCB this weekend and will finish it when I received my components from Hong Kong (much cheaper than Mouser).
Thanks Fritz. Hope to hear that the fet version holds up under testing. -Mike
ReplyDeleteHi Fritz, you can count me in for 3 Just let me know when.
ReplyDeleteThanks.
Hi Fritz , you told me to watch your blog ok i'm in for two pcb's , knowing myself,,,one wil not servive :-)
ReplyDeletegrtz rik
Hi Fritz , i spoke about it in a german forum ,,anny idea about the cost ? ( estimatet)
ReplyDeletegrtz rik
At this moment about $60
ReplyDeleteI am interested in at least 2 maybe 3 of the PCB's when your final version is completed. Been reading your blog and watching the video on youtube. Curtis
ReplyDelete