Hi guys, I've been tearing apart a Brother PT-1000 label printer with the eventua= l = goal being to connect it to a desktop PC to print labels for my various = component storage boxes. I've figured out how the print head communicates w= ith = the controller board, which just leaves the motor drive circuitry. Naturally, being a low cost device (=A315), the PT-1000 doesn't use anyt= hing = remotely stepper-motor-like for the label feed. Instead, it uses a cheap = Mabuchi DC pancake motor (an RF-300C-11440, for which I have yet to find a = datasheet) and a speed reduction gearbox. The speed control is performed by= a = ROHM BA6220 chip. Ideally I'd like to eliminate the ROHM chip (seeing as it's basically = unobtainium) and use a PIC of some description (probably a 12F675) to do PW= M = speed control of the motor. But first I'd like to understand how the ROHM c= hip = manages to do what it does. I found a datasheet for the '6220 here: = .. but like most ROHM datasheets, it doesn't say much about the chip, ot= her = than that it uses back-EMF sensing and how to determine one of the two = external resistor values. What I don't get is that there's no obvious way for the chip to sense = back-EMF. Everything I've been able to find about bEMF sensing suggests tha= t = it's normally used with PWM control -- the motor is powered up for a short = period of time, then in the off period the voltage across the motor is samp= led = and used to (roughly) determine the motor speed. Unless it's sensing curren= t, = but if it is, the "application circuit" (BA6220 datasheet, Fig. 2, page 2) = doesn't look like any current sensing circuit I've ever seen. In fact, it = looks like a voltage comparator, but the polarity of the voltage reference = doesn't look quite right... I've hooked the scope up to the driver IC's pins (and the motor itself) = and = didn't see anything that suggested the driver IC was switching the power on = and off. In fact, the voltage remained more or less constant, excepting the = ~50mV sine wave (plus one ~200mV spike per cycle) modulation that I suspect= is = being caused by the motion of the commutator relative to the brushes. Can anyone shed some light on this? Like I said, I'm getting rid of this thing anyway, but I'd rather like t= o = understand how the existing circuit works first, if at all possible... Thanks, -- = Phil. piclist@philpem.me.uk http://www.philpem.me.uk/ -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist