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This is 250A spot welder for welding 18650 Lithium ion cells........
The circuit consist of six 10F ultra capacitors having very low internal resistance. Choosing low internal resistance caps is key because they can dump huge currents in the load when needed.
The capacitors are rates for 5V, hence the weld current will be 5V to 4V, if you need higher voltages just add capacitance in series (but you will decrease in capacitance as per equation).
Mosfets are made parallel to distribute the current between each one. Each mosfets should handle 60A atleast. A 1k ohm resistor is connected in between gate and source, this is used to discharge the gate capacitance charge quickly.
For the timing part, we are using two 555 timers, one in monostable mode and other in astable mode of operation.
A typical weld will require 80 to 100ms, which we could not control by a regular switch so we need a delay circuit (mono stable mode)
The astable mode is used to add 3ms to 5ms pulse to the pulse made by the delay ckt.
The push button is connected to the 1st 555 timer
The output of the 1st 555 timer is fed into the reset pin of the 2nd 555 timer and the output from 2nd 555 timer is fed into a mosfet driver which will drive those parallel mosfets.
The mosfet's parameters are changed accordingly to conduct high currents.
The resistor between source of mosfet and ground represents the welding material (here nickel and steel)
Since i don't know the exact resistance, I give it a 1 milli ohm value.
The cap charge current is limited to 10A at 5V hence it may take 3 mins to fully charge it.
This is my 4th attempt making a proper ckt in every circuit and i deleted the previous design as it had some flaws in the timing ckt.
If you really need accurate timing and current control then use a microcontroller like Arduino, stm32 or even PIC.
If there's any fault in the circuit, please mention it on the comment section
Thank you
-ARKtech49
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