Resistor

A resistor limits how much current flows in a circuit. Its value, measured in ohms, sets how many volts it takes to push a given current through it. Engineers use resistors to set currents, divide voltages, sense current and hold signals at a known level. On modern boards, most are surface-mount chips: tiny black rectangles with metal ends.

A thick-film chip resistor starts as a thin plate of aluminum oxide ceramic, scored into a grid of hundreds of future resistors. Silver paste is printed where the ends will be and fired to form the electrodes. Between them, a paste of ruthenium dioxide mixed with glass is printed and fired at around 850 °C. The ratio of conductive ruthenium dioxide to insulating glass roughly sets the resistance. A laser then cuts a notch into each film, nudging its value up while it's measured, until it's exactly right. A glass or resin coat protects the film. The plate is snapped into strips, the ends are coated, then plated with a nickel barrier and tin for soldering.

Hundreds of resistors sit on the main board, timing controller board and power supply board. Their value is often printed on top as a short number code.