driving an optocoupler with an ...optocoupler... - Page 1
Forum section: electronics / projects

author: Gibson486
author_rank: Frequent Contributor
timestamp: July 24, 2018, 07:35:54 pm
content: I guess the reason to do this would between two devices on separate PCBs: 1. get level shifting for free, and 2. you do not have to worry about ESD as much. Seems like waste of space, but also seems pretty robust and future proof (if you add the correct cuts [0 ohm resistors] in there). What do you think?

author: DC1MC
author_rank: Super Contributor
timestamp: July 24, 2018, 07:42:31 pm
content: Why do you need TWO optocouplers, they do produce troubles and degrade your signal ?Cheers,DC1MC

author: Gibson486
author_rank: Frequent Contributor
timestamp: July 24, 2018, 07:57:08 pm
content: Quote from: DC1MC on July 24, 2018, 07:42:31 pmI saw it in a design review today. The person had to talk between two boards. One was an off the shelf board, the other a custom board and it was just simple on/off logic. I originally had your thought, but then I thought about it more and kind of saw why they did it, however, I am assuming they did not do it out of ignorance.

author: DC1MC
author_rank: Super Contributor
timestamp: July 24, 2018, 08:07:38 pm
content: I could not think of any reason why would this be a feasible solution, in between the optocpuplers you'll still have the signal (seriously degraded) in between the two optos and now you need another power supply to power the intermediate circuit.If somebody knows a reason for this I'm really curious to find out, I have myself an excellent signal generator that isolates the analogue domain form the digital domain with nice small pieces of low speed fiber optic (similar with the SPDIF) where one can find bidirectional hi-quality transceivers for 3$ a pair and there you can go with meters of fiber all around your chassis.I also have an old DMM (Datron 1071 with something like 18 optos, that were proving to be a pain after 25-30yrs, I had to replace all.So, caveat emptor.Cheers,DC1MC

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2018, 08:51:06 pm
content: Ah, that can be fine I suppose. Obviously, make sure the interface isn't going to fail in terms of saturation voltage and current (over CTR variation), leakage current and switching speed.The head-scratchers I've seen are, using optos on a common-ground circuit, more or less for level translation. About the worst possible way to do it. That customer had some technical debt accumulated over the years, it seems. It was probably reasonable in their first version (almost two decades ago!), where the optos were useful. They just never cared enough about signal quality or function or cost, to stop and ponder if they should change it or not -- "hey, if it works" is a very dangerous phrase. (As it turns out, the design I received was not adequately guard-banded around CTR, so I pushed to change the circuit to a much simpler 'HC14 logic gate and pull-up resistor. Yup, that's all they really needed!)Oh, hey, actually, that example does go one level closer to on-topic. Because, some of the signals received were from optointerrupters. So they were in fact second-degree opto-coupled.Which is a dangerous thing to do: interrupters tend to have quite poor CTR (which was the case here), which was the motive for change.Tim

author: JS
author_rank: Frequent Contributor
timestamp: July 25, 2018, 06:41:34 am
content: I made that myself, they were both in the same pcb so come at me one by one.My design already had dual insulation for driving a load with optos and relays, 12V intermediate rail, 5V for the micro, mains on the other side. I used an AC optocoupler H11AA1 to sense the load and a 4N35 to send the signal to the micro. Not using that I wouldn't be able to route the sensing part of the circuit without decreasing clearance between the three parts of the circuit. Or I had the 12V crossing with mains or the 5V crossing with 12V in closer proximity than they should. I didn't want that as it's an industrial enviroment, spikes from mains are huge and relays and micro controllers doesn't hang out too well, the thing has been working for a couple of years with no problem and I didn't care for the cost of the 20 extra 4N35 in the whole installation while it made things a lot easier. Also it let me smooth the sense signal in between as it wouldn't be as nice with a single optocoupler, as I was using the optocopulers to multiplex the signals from and to the micro, so they were floating and with much less headroom than the 12V circuit signal.Now, there might be other reasons to do this, let's say capacitive coupling between sides is too bad, so you prefer to have capacitive coupling from each side to a common node. Or you could make them able to protect to whatever is in the middle, like not always connected to each other, being both ends sensitive circuits. It's likely to be avoidable but might not be desirable or trivial, and cheap enough not to bother with it.JS

author: capt bullshot
author_rank: Super Contributor
timestamp: July 25, 2018, 06:49:03 am
content: Quote from: T3sl4co1l on July 24, 2018, 08:51:06 pmI've got a similar example here: A circuit that uses optos to isolate "small signal" digital GND from "power electronics" GND, usually they both are at the same potential, but are allowed to float within SELV range. So there's a valid reason to use optos.Now, we've got a customer demanding tying these GNDs hard together (pour GND plane all over), so the isolation is obviously no more necessary. But I can't replace these optos anyway because they act as "fail safe three input AND gates" additionally to the isolation feature - so replacing them by logic gates would remove the "fail safe" feature that is required by safety reasons. For someone who doesn't know, that circuit would look quite silly, I must admit.We've also got optocouplers driving simple indicator LEDs here in some other design - If you don't know why they are there (SELV isolation, and minimum clearance requirements), you'd think that is silly also.

author: Gibson486
author_rank: Frequent Contributor
timestamp: July 25, 2018, 01:17:10 pm
content: I bit and I asked....the person who did the design was not there, but his co worker said he did it just in case the i/o on the other side did not have an opto.As for level translation, I am guilty of using an opto in a design. My coworker speced out the wrong part and got one with this weird sinking output. The only way i could get it to work was to use an opto or a relay. It was an ugly design, but it suited the need well.

author: Seekonk
author_rank: Super Contributor
timestamp: July 25, 2018, 05:01:15 pm
content: They work quite well as level shifters when dealing with idiots.