Saturday, November 14, 2009

Just a peek at the Mosfet part of this welder. I am still waiting for parts


  1. Hi Fritz, with very much interest I follow your development and I'm still waiting for your new schemetic diagramms. But, what are the big heat sinks for? There will be nearly no power loss in the fets it's the current of some thousands of amperes you have to solve. Have a look to the link ( He used 5 Fets in parallel and a realy smale heatsink. I'm waiting for news. Gerhard

  2. Well Gerhard, it seems like he has 2x 80000 and one 150000 farad capacitors 35V. If you read my post below, you will see that having such low farad capacitors will discharge the full charge very fast and if you look at his welds you can see the black marks around them. Now 0.310 Farad will discharge in about 4.6ms. Within about 1ms his current would have dropped from about 11000 amp to about 4000 amp.You can put your finger on a red hot coal without getting burned if you do it fast and that's why he gets away with only a couple of mosfets.
    I over designed mine for a couple of reasons:
    1: Heat
    2: I can use higher voltage caps later on
    3: A lower on resistance with enough mosfets that will lower the heat and increase the current.
    This welders will get hot with repeated and non stop welds.
    The schematic below is the new one.

  3. Please note that with my answer above, I did not include all the resistances and just worked with a weld resistance of 3mOhm. I'm designing my welder to last and over designing is always better and cheaper than under designing.

  4. Fritz, I agree to all of your points of answer. A bigger heat sink will give you enough development potential for future considerations. In my thoughts I didn't include the modes of non stop welds. In any way being as low resistant as possible (paralling the fets) will reduce the overall resistant of the hole chain and will increase the max. amps.
    But in your schematics below (updated 10/25/09) I cannot find the weld capacitor and the "weld switch" (parallel mosfets including drivers and equalisung resistants).

  5. fritz, i really want to build me 1 of these myself, i have a complete parts list for your current schematic except for d9, not listed on your schematic, i have most of what i need due to being a repairman for 20 years, i have about $30,000 of inventory right now in all kinds of electronic parts, even some large mf caps, just not any farad caps, but i might have enough to make 1 fd. please let me know when you get this going full swing & by the way let me know if you need anything oddball, i may have it. i curtainly have a whole lot of parts to dig thru, & a whole lot of parts drawers, i have most of it on computer listed with locations. when you do electronics like i use to do, you aquire a lot of stuff, test equipment, meters, tools, etc. my work room is worth about $200,000. i want to build 1 of these for myself, i have some battery packs to fix, right now. it would be fun to have me a real challange.

  6. Gerhard, this blog work in a weird way, you have to read it from the bottom up and there is more on another page under "older posts". You will see that my version 1 design used a contactor to break the current(the video on youtube). Everyone was asking me for a dual pulse and I started to design this welder and believe it or not it takes money and lots of time so if you look at the date of the last post you will see that I posted the mosfet photo yesterday. If you send me lots of money I can do it faster for you :)

  7. Tommy, this design changed and 13 of the components used on this board for the SCR and contactor won't be necessary anymore. D9 was just put in for a place holder for a future capacitor protection circuit which I still have to make.
    I'm building this welder with a 3Farad cap so using a 1Farad is up to you.
    I always need parts Tommy, thanks for the offer

  8. hia fritz, i really like your new pcb, it looks nice, when are you gonna start selling the pcb's for us to build our own sets, i really want to have 1 for myself, i would love to have a working proto-type by the end of the month if possible, but i may be out in left field. i am just ancious to be able to build my battery packs back, brand new again, myself. please keep us informed, thanks fritz, i will be looking for a 3 farad cap. i do not know how your circuit is designed for the switch part of it, but i have found a small momentary switch that could be used to activate the discharge circuit, mouser has a 642-av0630c900 for $ 10.67. i cannot print the schematic to smoke it over & see how the circuit for the discharge works, i guess when i actually built the unit, then i would understand how the circuit works, it helps when you can actually put your hands on the actual piece. keep up the good work

  9. Overkill works great for me Fritz. -Mike

  10. There are so many variables when trying to design a welder like this, Ie. welding resistance, capacitor's ESR, Voltage, probe, wire and Mosfet ON resistance. This makes it very difficult not to over design it. If I had to build this welder just for myself it would have been so much easier and cheaper because I could calculate the variables and design it accordingly. Designing it so that people can use their own Wires, caps, probes etc. forces me to use max values.

  11. Hi Fritz , is there a components list already , complete or incomplete so i can start loocking for them ?


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