On Tue, Jun 02, 2009 at 07:19:25AM -0400, Jim Lynch wrote: > Ben Okopnik wrote: > > > > The only possible problem, oddly enough, is that the motors > > these days are of much better quality than they used to be: the original > > ones put out about 150gf/cm, which was about maximum for this size of > > motor back then, whereas the new ones put out about 3 times that much > > (finger-in-the-wind estimate based on perceived torque while holding > > the motor as it cranks up.) The run current on both is about the same, > > perhaps a little higher on the new one... but the starting current may > > be a problem. I can only hope that the driver transistors in the head > > unit survive the testing phase; I'm not a big fan of replacing those > > things. :) > > Ben would it be possible to add another heavier duty transistor > outboard? Let the one in the head act as a driver? There are some > really great MOSFETs out there.
I've thought about that; in fact, at one point, I thought of starting a business to build "output buffers" for autopilots, since most of the failures that I've ended up repairing for other cruisers happened at precisely this interface (either blown driver transistors or cooked motors, most likely due to low voltage.) The problem here, however, is not that I need to maximize the output current: if I drive the new motor at full blast, I suspect that I'll destroy the gearing in the drive mechanism. This is a pretty good design mechanically, but it does have its limits. > Also a resistor might limit the startup > current too, since you have much more torque than you need, it may not > cause any problems. This is the approach that I'll be taking (I like the way you think, Jim: it's brilliant, since it matches my own mental pretzel-bending... :) The only problem _there,_ as you can probably guess, is resistor wattage: the running current on this motor seems to be about 1.5A, I need to limit it to ~700mA (that being the specified running current of the old motor), so it seems like I need to experiment with 4-8 ohm resistors - which, according to Dr. Ohm, will require a _minimum_ wattage of 8-12W. Good engineering practice makes that 16-24W. In other words, *YIKES*. :) I could probably hail back to my ancient roots and design a transistor-based current limiter, but... we're now getting quite a bit more complex than I really wanted. It would be Very Nice to stick in an off-the-shelf solution, like a single resistor, and be done with the whole thing. I'll report any notable successes here; notable failures, by contrast, will be available in your local news media. "THE AUTOPILOT OF DEATH by Staph Writer, AP News Service Today, a horrendous accident in Charleston SC involving a unique combination of a car racing motor, a wire-wound resistor, an old autopilot, and a trained weasel on crack resulted in massive destruction of property and thousands of deaths..." I'll do my best to avoid that kind of notoriety. :) -- * Ben Okopnik * Editor-in-Chief, Linux Gazette * http://LinuxGazette.NET * _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://www.liveaboardnow.org/mailman/listinfo/liveaboard To subscribe send an email to [email protected] To unsubscribe send an email to [email protected] The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
