High power transistors in disk head positioners

[Ionforbes]

I noticed that a lot of old TO-3 power BJTs, especially those from Motorola/onsemi, specify in their datasheets they're useful for 'disk head positioners' and other linear applications. I'm guessing it's referring to hard drives, but I don't know much about how hard drives work and wondered if anyone knows the details of how exactly they do (or did?) position the disk head and the circuitry behind it, and why they'd need such a high power linear transistor? They were all released decades ago so no doubt hard disks were bigger then but still, some of these are rated at 300W 50A+!

[Andy Chee]

I'm thinking the TO-3 power transistors were used in hard drives like this!

[T3sl4co1l]

Linear motor, voice coil, basically an overgrown speaker.  Can indeed get quite big, but, that shouldn't be relevant for anything that's been used in computing for quite a long time indeed.  Other applications abound, of course, for longer stroke and lower frequency response than say piezo actuators offer, and where the speed/linearity/repeatability is better than a (rotational) motor / geartrain setup (which would be more amenable to a class D amplifier).

To be fair, most voice coil apps are class-D-able too, but there can be some edge cases where linear just beats it out.

Tim

[indeterminate]

the first image is of a baby head and voice coil assembly for a 3 platter drive
second image is of a large disk pack  CDC 9766  250MB 12 platter 14" disk pack

The voice coil and head assembly was proportionally larger than the above image
surrounding  the voice coil was a large rectangular permanent magnet

The head assembly traveled 4">5" from the outer track to the inner track in a fraction of a second
apart from the occasional head crash they where very reliable.
and a bit scary with the covers off when you where doing a head alignment.

[fourtytwo42]

A long time ago I used to repair drives like that, the controlling VDU screen would jump about during a continuous random seek test (from a few feet away) and you had to leave your watches away in your desk (good practice when working on electronics anyway). One of the not so fun tricks of these things was an "emergency retract" while doing head alignment with a tool inserted from the rear. One of the causes of said "emergency retract" was VCM (Voice Coil Motor) overcurrent that could be caused by pushing on the tool slightly to hard upsetting the fine positioning loop. "emergency retract" consisted of dumping the VCM directly across the main power supply smoothing capacitors via a contactor thus retracting the heads from the disk with astonishing power and woe betide anyone with there hands in the way!! The tool was always held gingerly from the side by the initiated