| ??? 01/22/05 13:23 Modified: 01/22/05 13:24 Read: times |
#85576 - Opto to pull-down signal Responding to: ???'s previous message |
Sun said:
After reading your explaination, would I be right in saying.....
On the 1 to 0 transition, 650uA are sourced by the combination of the weak and very weak internal pull-up resistors, as a result the potential drop across the 1K pull down is greater than 2V. And due to the pin being at or greater than the 2V level the weak internal resistor is never deactivated, and hence the continued 650uA source. If however the potential was dropped to less than 2V on the 1 to 0 transition, this pull-up resistor will be de-activated hence, only allowing the IIL current of 50uA to be sourced by the pin. Yes, this is totally right and very well formulated! How would you suggest I do this? I would use a circuit providing a pull-up resistor and an opto which pulls the input low, like many other people recommended this here. Of course, with a very well designed circuit you could also succeed with a pull-down resistor arrangement, but keep in mind, that manufacturing tolerances of an opto can be rather high, and take care they show a relevant aging!!! Means, the current transfer ratio will undergo a change, making it rather difficult to keep your design properly work even after years. So, I would highly recommend to make a change in your design and to use the opto to pull-down the signal while having a pull-up resistor. Good luck, Kai |
| 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 |