Developments in the computing industry progress rapidly, Last year's expensive acquisitions are already no longer usable for running modern soft- wore, However, old computers, or at least ports of them, need not always end up as electronic scrap, Let's look at the motors found in old hard disk drives, for example...

By H Neumann and E Moller

recycling hard disk drive motors

Hard disk drives with capacities of 20 to 40 MB can often be found in discarded computers [in the basement or at flea markets]. The data stored on these devices is surely of no interest to anyone, but each one contains two motors. for which some useful applications might still be found. In addition to the stepper motor, which moves the read/write heads the actual drive motor is an interesting item. In the case of the drives investigated by the authors, this is a d.c. motor with electronic commutation. An example is shown in Figure 1. Our objective here is to see how such a motor can be brought back to life.

Before you can do any- thing with the motor, you must first disassemble the drive. Start by removing the enclosure cover and the printed circuit board. The proper tool for removing the screws is a Torx screwdriver but a sturdy slot-type screwdriver can also be pressed into service if necessary. After the cover and the board have been removed, you will see the two motors [Figure 2] You can now dismount the head drive motor [Figure 3] and the patter assembly [Figure 4]. Finally, remove the fixing screws for the drive motor [Figure 5], and you can extract the desired object.

The motor has six leads, three of which are connected to its windings. These leads can be easily identified with the help of an ohmmeter. The winding resistance will measure around 3 or 6 ohms depending on whether you measure across one or both windings. You can thus directly figure out which leads are the winding leads. The three other leads are connected to a Hall-effect sensor IC, and the measured resistances between these leads will be significantly higher. With the test circuit shown in Figure 6, you can easily sort out which of these leads is connected to what. In the worst case you will have to connect each of the three leads to earth in turn while the other two are connected to + 12 V via series resistors and LEDs. Turn the rotor of the motor slowly. If the proper lead is connected to earth one of the LEDs will be constantly on; the associated lead is then connected to the power supply terminal of the IC. The other LED will blink as the rotor is turned; its lead is thus the signal output of the IC.

Now you have completed the necessary preparations in order to be able to use the motor. For this, you will need a circuit as shown in Figure 7, which can be quickly constructed on a bit of prototyping board. The Hall sensor built into the motor has an open-collector out- put. If the output transistor is switched on, transistor T1 is cut off. Resistors R2 and R3 then supply enough current to switch on T2. This in turn supplies base current to T3 via R4, and T3 is also switched on. A magnetic field is created in coil L1, and this causes the permanent-magnet rotor to rotate slightly. The Hall sensor is now exposed to the opposite magnetic polarity, so that its output transistor is switched off. The base at T1 now receives current via R1, and it switches on. Since the voltage at the collector of T1 is now very low, the current flowing through R3 is not sufficient to keep T2 switched on. Consequently, T3 is also no longer switched on, and current no longer flows through L1. However, since T1 is switched on, T4 is supplied with a base current via R2, and it switches on. L2 thus receives a current and produces a magnetic field, which in turn causes the rotor to rotate a bit further. This sequence repeats itself over and over again as long as an adequate supply voltage is present. Diodes D1 and D2 protect T3 and T4 against induced voltages. It you want to reverse the motor's direction of rotation, simply exchange the L1 and L2 leads.

Figure 7. Control circuit for an electronically commutated motor.