The Tandon TM100 was among the first 5.25-inch floppy drives. It was included in the first IBM PCs and compatible clone computers. This drive model set many standards in design, and many subsequent drive designs featured stepper motors that conformed closely to the specifications of the originals.
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Unipolar and Bipolar Steppers
The Tandon steppers were unipolar. They featured a centre tap on each of the motor's two windings. The centre taps were wires to the positive power supply and the ends of the windings were alternately grounded to reverse the direction of each respective winding's field. Later designs featured other wiring schematics and bipolar steppers. Bipolar motors were similar to their unipolar predecessors, but lacked centre taps. This created a simpler motor in which the drive circuitry was more complex in order to reverse the polarity of the two motor poles. Unipolar steppers featured both five- and six-wire schematics, while bipolar schematics featured a four-wire set-up.
The stepper motors in 5.25 floppy drives feature four coils. This four-phase set-up allows each coil to generate electricity independently of the others. When one coil has no electricity flowing through it, the next coil in sequence is at its maximum charge. The four coils operate together in order to generate a near-constant voltage and current, keeping the motor spinning at a steady rate of rotation.
Degrees per Step
The steppers in 5.25-inch drives rotate at a rate of 3.6 degrees per pulse, or step. After 100 steps, the motor completes one full rotation. The optimal speed for these drives is 300 revolutions per minute.
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