Originally Posted by damon91
I can not understand how the negative side of the field circuit works on this welder. R1 is the fine current rheostat, R4 is a big resister. What confuses me is the ground right at the negative brush, it seems like it would bypass the rheostat.. If any of you understand this circuit and can explain it to me I would appreciate it, I love to learn.
When you switch the ignition on, dc from the positive terminal of the battery goes through the single diode above the diode bridge (4 diodes in a diamond shape), through the positive slip ring/brush, the revolving field, negative slip ring and brush, to ground, and back to the (grounded) battery negative. This current flow through the revolving field turns it into an electromagnet, so as the engine starts/runs, the rotating magnet going past the stationary windings produces ac voltage - some of which is fed through the diode bridge to become dc, at higher than battery voltage, going from the top of the diamond, through the positive brush/slip ring, rotating field, negative brush/slip ring, the rheostat and resistor, back to the bottom of the diamond. The rheostat allows you to adjust how much current is flowing in this path, which controls how strong the magnet is, which controls how much voltage is produced. Meanwhile, the single diode prevents this higher dc voltage from feeding back to and cooking the battery. Clear as mud?