On Sat, 2010-11-13 at 14:38 +1100, Erik Christiansen wrote: > The easiest way to increase the high-state output voltage, when it's > only getting to three-and-a-bit volts, is to add a pull-up resistor.
Be careful: the parallel port's output transistor probably doesn't have a particularly high output voltage rating. It'll almost certainly work fine with +5 V, but don't connect +12 V or (shudder) +48 V motor power supplies. The problem is not the forward current conducted when the transistor is on (which will be limited by the external circuit), but the voltage it must withstand when turned off (which is set by the external supply). The port pin may also be connected to ESD protection diodes and suchlike that will react unkindly to higher voltages. The general idea of an opto-isolator is to turn on an LED with a current (in the neighborhood of 10 mA) that can come from a low-voltage source that's switched by a low-voltage transistor like the one in the parallel port. What happens on the other side of the isolator doesn't affect the LED side. A bare optoisolator IC generally contains just the LED, so you must choose an external series resistor to limit the current to whatever the LED requires. However, if you're using a packaged optoisolator module, it probably has an internal resistor that's specified to work with a particular supply voltage. If that supply is supposed to be +12 V, then the LED won't get enough current from a +5 V supply. On the other side of the isolator, the output transistor will be good for operation at the (much higher) currents and voltages required by the motor drivers. Hope that helps a bit... -- Ed ------------------------------------------------------------------------------ Centralized Desktop Delivery: Dell and VMware Reference Architecture Simplifying enterprise desktop deployment and management using Dell EqualLogic storage and VMware View: A highly scalable, end-to-end client virtualization framework. Read more! http://p.sf.net/sfu/dell-eql-dev2dev _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
