i've just finished building a microwave transformer based Spot welder. I had some issues with the welder at first which i managed to resolve;
-it wasnt welding! just heating the metal. i figured out that it was due to the current not being high enough. all resolved now, but i dont understand the reason.
-cant measure current since it i expect that it will blow my multimeter.
therefore tried to calculate as follows;
trasnformer rated 700W
Primary transformer coil: 240V , I=~3A
Secondary coil, initially with one turn of wire
assume 90% efficiency in transformer (dont know if this is reasonable??)
Power out is 630W
measure voltage of 1v
I= 600A ?? (p=i*v)
i understand that this current would be fine for welding, but it didnt, so i think actual current was much lower.
edit: increasing number of turns in secondary-> higher voltage, therefore lower current?! (i = p/v), assuming power out is the same
however, we also know ohms law (i=v/r) if i use the same length of long wire for secondary coil (no change in resistance), if i increase number of turns from 1 to 3 or 4, output voltage increases from 1v to 3v and current is also increased linearly. when i did this, the spot welder started to work and the more turns i added, the current visibly increased (i know since it burnt through metal much easier/quicker) **So, finally, my question is, what is wrong with my logic / Power based calculations for current???** **any other input/feedback/comments would be great** next step for me is to build a high current shunt so that i can measure the current output directly