by Ralph » Thu Feb 12, 2009 3:37 am
Graham,
As Klaas noted, the sole purpose of the diode is to protect the FET from the usual inductive "spikes". Without an external resistive load the braking effect is nil. I was quite amazed, given the low inertial load of the disc, how long it takes the system to spin down after power is removed!
With respect to the motor drive, there are almost certainly other engineering solutions, but my focus has been on the utter simplicity of the MUTR variations on other people's circuits. A recurring theme on this forum has been the need for a simple electronics package, suitable for novices, that will run a mechanical televisor with a satisfactory level of performance. Thus was born the idea of a simple, single circuit board solution. The engineering changes I am making are trivial:
(1) All operating voltages are regulated and all bias levels are referenced to a regulated 5V bus.
(2) The LM324N will be operated off a regulated 12V supply instead of the 6V used in the MUTR version. The 324 is a rugged little package, but the op-amps are a bit sluggish at that voltage in single-ended service. There is no potential for limited bandwidth at 12V!
I am also experimenting with a variable phase-delay that would let one "zero-out" any phase errors due to the placement of the chopper graphic or the optical sensors. There is room on the board for this function, I just don't know if is worth the effort.
Ralph