On 23 June 2011 11:23, Erik Christiansen <[email protected]> wrote:
> The >3v dead-band now provided by the LEDs would allow quite small > current limiting resistors to be used in each opto's output H-bridge > leg, instead of the enormous 15k value, Something I forgot to say... That 15k was part of the only Opto-isolator I could find in the library of the software I found on this hard drive. I tried to modify it, but couldn't get it to stick, so meant to leave it in and give an explanatory note, which I forgot. >> As-drawn there is a problem that unconnected PWM turns on both optos... > > Well, not quite, as drawn. The opto-isolator LEDs are reverse biased > with no input, and so are as off as it's possible to be. If they were > connected the other way around (so that they are on), they'd need one of > the mentioned "critical resistors" in series, to avoid dead optos. (And > careful driving.) Yes, it appears that I put the optos upside down too. You can see why I always buy 3 of every component... I think that it needs a 330R on the high side of each LED. The output side should be OK I think (with the spurious 15k resistors omitted) The drawback of this circuit is that wherever it is put it needs both the logic positive supply and the controlled positive supply, but I can't see any way round that if you want to actively discharge the output cap on one part of PWM cycle. Having a passive discharge resistor leads to non-linearity and pwm frequency dependency. The "classic" one resistor / one cap converter doesn't have this problem, as 0v PWM discharges the cap and 5V charges it. With a single opto you lose that symmetry. -- atp "Torque wrenches are for the obedience of fools and the guidance of wise men" ------------------------------------------------------------------------------ Simplify data backup and recovery for your virtual environment with vRanger. Installation's a snap, and flexible recovery options mean your data is safe, secure and there when you need it. Data protection magic? Nope - It's vRanger. Get your free trial download today. http://p.sf.net/sfu/quest-sfdev2dev _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
