| ??? 01/21/05 14:40 Read: times |
#85509 - Heres Why Responding to: ???'s previous message |
If a pin is also an occasional output the microcontroller output driver can wire-OR with the external active low driver.
An active low driver can typically sink its output to very near GND. A pull down resistor that can overcome the onboard weak pullups of a typical 8051 style I/O pin needs to be a quite low valued resistor in comparison to the size of resistor to do an adequate job of pulling up an idle input. As such if a driver is brought to overcome the resistor it is far less current to overcome a 10K pullup resistor with an NPN type driver than it is to overcome a 470 ohm pulldown resistor with a PNP type driver. If someone desires to try to use an NPN type driver to overcome a pulldown resisor (which is the commonest type opcoupler output and which I would only do as a last resort) the current to drive the pulldown resistor makes a much more serious type driver be required. In an opto coupler for instance this can demand a CTR that is at least 10 times higher or a 10 times higher current requirement in the input diode of the opto coupler. Michael Karas |
| Topic | Author | Date |
| Inputs | 01/01/70 00:00 | |
| Opto | 01/01/70 00:00 | |
| Opto input | 01/01/70 00:00 | |
| Optocoupler Designer’s Guide | 01/01/70 00:00 | |
| What type of opto? | 01/01/70 00:00 | |
| pulling high | 01/01/70 00:00 | |
| Why??? | 01/01/70 00:00 | |
| Misleading.... | 01/01/70 00:00 | |
| OK, clearer | 01/01/70 00:00 | |
| I must be stupid... | 01/01/70 00:00 | |
| you are not | 01/01/70 00:00 | |
| Heres Why | 01/01/70 00:00 | |
| Not quite so. | 01/01/70 00:00 | |
| Pull-downs must be much smaller! | 01/01/70 00:00 | |
| OK you are right. | 01/01/70 00:00 | |
A little history | 01/01/70 00:00 | |
| Link... | 01/01/70 00:00 | |
| use opto-coupler as NPN | 01/01/70 00:00 | |
| Thank you All, & a question for Kai | 01/01/70 00:00 | |
| Opto to pull-down signal | 01/01/70 00:00 |