LED based optoisolator that can work from 5V 150uA 300Hz signal - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: November 22, 2020, 03:30:29 am
content: I'm trying to help someone remotely and it would be much easer if I could point them toan optoisolator that will accept a 5V, 150uA input signal directly to the LED. Data freq is 300hz.As you would expect all the normal optos just don't get bright enough at 150uA to switch NPN hard enough to generate a good output, even with 500k pullup on a darlington opto.Digital optos would work, but the person doesn't have easy access to 5V to power it on the input side.With such high efficiency leds available today you would think there must be something that can handle low currents.I've seen led's that are pretty bright on 150uADoes such a thing exist?

author: wraper
author_rank: Unknown
timestamp: November 22, 2020, 04:04:10 am
content: Say SFH608-5 should work IMHO. CTR is Rated at 0.5mA, so I guess it should still work at 0.15mA with significantly degraded CTR. There is also SFH618A-5 which has the same CTR specs, however it also has a graph showing that at 0.1mA it's still functional.

author: wraper
author_rank: Unknown
timestamp: November 22, 2020, 04:20:47 am
content: LDA111 Darlington should work as well according to the graph.https://www.ixysic.com/home/pdfs.nsf/0/66F34A78811BAA3985256A2C00693C54/$file/Lda111.pdf

author: Zero999
author_rank: Super Contributor
timestamp: November 22, 2020, 12:19:01 pm
content: Quote from: Psi on November 22, 2020, 03:30:29 amHow about a low current digital isolator, such as the ISO7021?https://www.ti.com/lit/ds/symlink/iso7021.pdf

author: Psi
author_rank: Super Contributor
timestamp: November 25, 2020, 11:14:03 am
content: Quote from: Zero999 on November 22, 2020, 12:19:01 pmThat would require a power connection on the input side, which is problematic.

author: Psi
author_rank: Super Contributor
timestamp: November 25, 2020, 11:21:46 am
content: Quote from: wraper on November 22, 2020, 04:20:47 amThanks!Looks promising.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 25, 2020, 12:04:01 pm
content: Another option may be to use a small pulse transformer.On the primary side, have the input switch a transistor to short the winding.On the secondary, feed it with a ~1-10MHz squarewave via a resistor, and measure the loading cause by the transistor being on or off.

author: BrianHG
author_rank: Super Contributor
timestamp: November 25, 2020, 08:02:35 pm
content: Quote from: Psi on November 22, 2020, 03:30:29 amDigital isolators exist without that 5v supply input. They are a little slower than the ones with the dedicated VCC pin, yet still at least 10x faster than the typical opto. Need to search TI.com. I also believe that some of the 1 channel TI parts allow the input VCC to be connected to the input data and power the 2 of them together to switch on, and 0v on both pins for off on the output side with some degradation on switch-on time.Here is a normal 40ua input normal LED opto, but slower than the data isolator and pricey at 3$:https://www.digikey.com/en/products/detail/broadcom-limited/ACPL-K70A-500E/11594302

author: BrianHG
author_rank: Super Contributor
timestamp: November 26, 2020, 03:55:19 am
content: I tried googling 'led simulated optoisolator'.Silicon Labs make some, but they require at least 1ma at the input to work, though they are fast.Check their 'Si873x' series. Though, I cannot tell is the 1ma requirement is the activation minimum current, or, if that figure is due to the simulated input current diode curve. Maybe it will switch on before the current reaches 1ma.

author: Psi
author_rank: Super Contributor
timestamp: November 26, 2020, 08:25:09 am
content: Quote from: BrianHG on November 25, 2020, 08:02:35 pmI think you have found the one.