weird double optocoupler schematic - Page 1
Forum section: electronics / projects

author: Teun
author_rank: Regular Contributor
timestamp: October 06, 2020, 12:27:47 pm
content: Can somebody tell me why you would use the second optocoupler like this and not just leave it unconnected?

author: greenpossum
author_rank: Frequent Contributor
timestamp: October 06, 2020, 12:29:36 pm
content: So that there are no floating pins?

author: jfiresto
author_rank: Frequent Contributor
timestamp: October 06, 2020, 12:33:15 pm
content: To make fewer holes in a ground plane?

author: ArthurDent
author_rank: Super Contributor
timestamp: October 06, 2020, 01:14:33 pm
content: It's not unusual in a lot of commercially made products to use chips with multiple sections and not use them all because it allows a smaller inventory of parts. The dual part may be used elsewhere on this board or in other products and because of the large numbers used may actually cost less than a single optocoupler used in one location. This practice is common with digital circuits

author: gamalot
author_rank: Super Contributor
timestamp: October 06, 2020, 01:16:51 pm
content: Grounding the four pins instead of floating them can provide better mechanical strength, I guess.

author: Teun
author_rank: Regular Contributor
timestamp: October 06, 2020, 01:23:16 pm
content: The reason I asked is that I have a Meanwell converter and they used 2 single port optocouplers this way. Since they make a fair share of converters I really wondered if there was a reason for it.

author: Nusa
author_rank: Super Contributor
timestamp: October 06, 2020, 01:37:49 pm
content: It also means that when routing traces on the PCB, one doesn't have to use limited board space to go around the unconnected pins/pads. Especially when there are creepage/clearance requirements for the optocoupler application that have already made the space underneath the IC unavailable.

author: Teun
author_rank: Regular Contributor
timestamp: October 06, 2020, 02:27:55 pm
content: Quote from: Nusa on October 06, 2020, 01:37:49 pmBut then it still wouldn't make sense to use 2 single optocouplers.

author: wraper
author_rank: Unknown
timestamp: October 06, 2020, 02:38:07 pm
content: Quote from: Teun on October 06, 2020, 02:27:55 pmOn your picture it's not visible how optocouplers are actually connected. IMHO it's only your wrong assumption rather than how it's actually made. Most likely both optocouplers are actually used.

author: Nusa
author_rank: Super Contributor
timestamp: October 06, 2020, 02:56:20 pm
content: Quote from: Teun on October 06, 2020, 02:27:55 pmI was actually responding to your original question as to reasons that schematic might be that way. Apologizes if that confused you.I suspect the picture you showed of two singles is not an example of that schematic.

author: Teun
author_rank: Regular Contributor
timestamp: October 06, 2020, 03:23:02 pm
content: I looked specifically for that schematic because that is really the way they are connected. It already seemed strange to me.Don't know what else to say

author: wraper
author_rank: Unknown
timestamp: October 06, 2020, 03:25:11 pm
content: Quote from: Teun on October 06, 2020, 03:23:02 pmI don't see optocoupler terminals shorted, neither traces under optocouplers can be seen. So you are just speculating.

author: jkostb
author_rank: Regular Contributor
timestamp: October 06, 2020, 04:07:00 pm
content: Maybe the opto coupler are connected this way for complying with safety standards (SIL?). If connected like this, if one optocoupler fails to short circuit mode, then the other optocoupler is still there (single fault). These kind of redundancy are typically also found in circuiits for hazardous environments.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 06, 2020, 09:29:34 pm
content: Quote from: Teun on October 06, 2020, 12:27:47 pmI guess it's the result of a design rule "don't leave any pin unconnected". Sometimes such rules are set up for a reason, and later either strictly enforced or just mindlessly followed.QuoteMaybe someone replaced all dual optos with 2 single ones (maybe because of a temporary shortage) and didn't check the schematic ...Anyway, I don't know a valid technical reason to connect the second (unused) channel of an optocoupler this way, but I've seen that before. Just to keep the ERC / DRC quiet, for example, it's easier this way than to attach an exeption from a "all pins must connected" rule.

author: Teun
author_rank: Regular Contributor
timestamp: October 07, 2020, 06:45:27 am
content: I don't see optocoupler terminals shorted, neither traces under optocouplers can be seen. So you are just speculating.They are on the bottom side of the board.And I just figured out that the manufacturer is not "meanwell" but "maenwell". I think that might have something to do with itQuote from: jkostb on October 06, 2020, 04:07:00 pmThe second one would be bypassed if that where the case.

author: wraper
author_rank: Unknown
timestamp: October 07, 2020, 10:27:51 am
content: Quote from: Teun on October 07, 2020, 06:45:27 amBut did you actually measure shorts between the pins? Trace running on the other side of optocoupler may have different connection to the optocoupler pins than you think on the first glance.QuoteThis likely is related with traces running under optocouplers thus reducing creepage distance.

author: Teun
author_rank: Regular Contributor
timestamp: October 07, 2020, 03:34:33 pm
content: Quote from: wraper on October 07, 2020, 10:27:51 amYes shorts between the pins of the second opto.And "Maenwell" is probably a very bad immitation of actual meanwell converters. Which could explain a screw up like that.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 07, 2020, 07:58:50 pm
content: Quote from: Teun on October 07, 2020, 03:34:33 pmYes, some of the proper Mean Well use more than one optocoupler for a reason (one for the control loop, another for OVP, and maybe a third for remote on/off or other features). So I'd suppose this was done on purpose, for imitation or fake.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 07, 2020, 08:30:39 pm
content: - Easier to make the PCB- Easier to switch to the second optocoupler if the first one fails, by shorting the old and cutting the shorting traces from the new.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2020, 10:56:20 am
content: Shorting the inputs and outputs of the unused optocoupler to ground would reduce capacitive coupling from input to output, but most applications would never be affected by such a small improvement.