On Saturday 27 September 2008, Ian Wright wrote: >Hi, > >I'm considering cnc'ing my Harrison Lathe and have a couple of steppers >which will do the job nicely. However, they take about 4.5AMPs and I'm >having difficulty finding a driver circuit to control them with. Due to >very limited funds I'd like to build the drivers myself and I'm thinking >that a PIC controlled microstepping driver would be nice. The primary >sticking point at the moment is that I can't find any decent information >on how I might incorporate current limiting and 'off-use current >setback' into a design. there are one or two circuits on the web for PIC >based microstepping drivers but they all seem to be limited to about >2Amp output with no indication that I could beef up the FETs without >everything toasting itself. I'm not too well clued up on this type of >electronics so any help you could give would be very much appreciated >or, if you could point me to a web resource which would help me, I would >be very grateful. Thanks..
Speaking very generally, yes the FET's can be beefed up. The problem in doing so is as much in the driver stage that drives the FET's as it is in the FET though. The larger FET also has a larger input capacitance, and in power FET's it is already considerable, and bigger ones can reach .5 uf dynamically. More typically, .05uf though. This is an unwanted function from the miller effect when the dv/dt is large. "miller effect" (google for it) is something we've fought with in triode vacuum tubes since before the 2A3 was king of audio outputs 80 years ago. Because of this, the driver has to grow in delivered current capabilities in order to keep the switching speeds up as slow switching leads to excessive junction heating during the transition time. The driver stage will need very large, 1 uf or more, of power supply bypassing directly at the point on the power rails that the driver is attached to, and they may also need to be replaced with bigger, faster devices too. Maybe I'm being cautious, but if I hooked my scope to the FET's gate, and saw transition times noticeably slower than my 100mhz dual trace's own rise times for it to make that about 20 volt swing, then I would be looking for ways to speed up the driver. Even 30ns rise & fall times will be reflected in the increased temps of the FET's as the current rises toward the max ratings. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) All men have the right to wait in line. ------------------------------------------------------------------------- This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
