At 10:24 PM 8/18/2005, Russell McMahon wrote: >>>Motor Brake ... > >>Just a silly thought - could you implement the motor brake by adding only >>2 extra PNP transistors? Collectors grounded on both, bases go to nodes >>D1 & D2 on the schematic, emitters to matching motor leads (D1:M1, >>D2:M2). Both transistors are ON when both inputs are LOW, the transistor >>connected to a motor terminal is turned OFF when that terminal is brought >>to Vbatt. > >It's a starter for 10 points, but the detail looks like it will make it >end up no better than the existing scheme. > >- The motor needs a current path on both sides so a PNP on one side alone >isn't enough. Diodes appear ... . Nope - try it. The reverse-biased PNP doesn't work all that well but it does work. But - a pair of diodes is pretty darned cheap. >- In the brake mode the transistor is turned on leading to the drear >quiescent current - and quite significant due to the current to be shunted >initially, were it not for the fact that ... Nope again - when both inputs are LOW, the quiescent current is no higher than your original circuit. While quiescent, both upper transistors are turned OFF. That means that both motor terminals are floating. Only while the motor is coasting and acting as a generator do the PNP brake transistors conduct. >- The bridge is depowered by the top transistors being off so once motor >energy vanishes the PNPs turn off. This may be OK but mat not - depends >on slowing motor characteristics at low voltage. Some brakes have allowed >the motor to slow rapidly and then coast slowly. That's pretty much how this one would work. It's intent is to allow quicker brake action than just that single low-value resistor in parallel with the motor. >The currently used FETs have the advantage of being actively turned hard >on for as long as needed and drawing essentially no power in standby mode, >even though still on. Exactly. No question that it is a better brake circuit. Question is: does the extra cost justify the extra performance? The real downside to what I suggested is that the brake transistors start to turn on when Vmotor exceeds Vprocessor by more than 1 Vbe drop. dwayne -- Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 21 years of Engineering Innovation (1984 - 2005) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist