Guru
Joined: 30/03/2016
Location: United KingdomPosts: 1646
| Posted: 08:42am 10 May 2022 |
What is really required?
I am only driving a Pico's input from an external 24v. CTR is not an issue so I guess that the DS-stated LED If of 10mA is unimportant, correct? Really want to keep the resistor's power-rating to the minimum.
Craig
Guru
Joined: 05/10/2019
Location: United KingdomPosts: 9125
| Posted: 08:49am 10 May 2022 |
You usually only need a couple of mA through the LED to get good enough current transfer. With a 24v input, that's 22.2v / .002 = 11.1k. Try 10k as the series resistor. It'll dissipate about 0.05W.
The exact output current depends on the LED current, but it hardly matters here.
Guru
Joined: 30/06/2020
Location: GermanyPosts: 763
| Posted: 08:58am 10 May 2022 |
The resistor value is calculated like for any other LED; R= (Vin - ledVf) / Iled
Example: R= (24v-1,5v)/ 0,04A = 562 Ohm, for the Power rating of the resistor:
P= 0,04A^2*562 = 0,89W
Greetings
Daniel
Guru
Joined: 30/03/2016
Location: United KingdomPosts: 1646
| Posted: 09:00am 10 May 2022 |
Thanks Mick
I was looking at 3.3K but I guess, with this format, I can easily test a variety:
Craig
Guru
Joined: 11/02/2018
Location: AustraliaPosts: 3378
| Posted: 12:44pm 10 May 2022 |
The required LED current can be reduced by increasing the load resistance on the output side. Maximum speed may be reduced if you go too high. Experiment!
Guru
Joined: 05/10/2019
Location: United KingdomPosts: 9125
| Posted: 01:35pm 10 May 2022 |
You can get the "gain" of the opto from the DS. It's the current transfer ratio - the ratio of the LED current to the output current. There will be a graph as it's usually non-linear. It doesn't start at zero as you need a minimum current through the LED for anything to happen. Better allow a bit more than minimum though as your "24v" might be 19v in reality. :(
Guru
Joined: 05/03/2018
Location: NetherlandsPosts: 6103
| Posted: 05:14pm 10 May 2022 |
@Tinine
If you have 10k from 24V to the LED, and 10k pullup or pulldown at the optocoupler output that will work for all normal optocouplers. Assuming 3.3V.
In specific cases you can tune these values (speed / noise immunity / analog behaviour).
But if speed is an issue, please don't spend too much time tuning. Solve it by using a digital optocoupler. These are far superior in speed to anything you can achieve with normal optocouplers.
Volhout
Guru
Joined: 30/03/2016
Location: United KingdomPosts: 1646
| Posted: 05:04am 11 May 2022 |
Many thanks, guys
No speed concerns here. These are typical inputs from proximity-sensors and they all spend a chunk of time in software de-bounce.
I think I have another question but I'll start a new thread because I like to keep the topic in the main thread title.
Craig