Some telecoms opto isolators have dual (bidirectional) IRLED.

Yeah, I've found a good variety of AC input optos to choose from. I just didn't have a spice model to sim it that way, and I figured it wouldn't make much difference if I just simmed with the parallel standard optos, then used the AC input opto in the final design.

 

Looks like a good setup to me.

That said, I don't see any particular advantage in using a bridge rectifier feeding a standard opto vs. just using an AC opto instead.

I agree. It is as variant, when you have one channel optocoupler only.
Edit:
But DickCappels said: Some LEDs cannot tolerate any reverse bias without suffering long term degredation.
https://forum.allaboutcircuits.com/...ifferent-color-leds-with-2-wires-only.143034/

 

I agree, your design looks like a great alternative for different parts or different situations. I like that it doesn't require caps on mains or high wattage resistors like mine does - the tradeoff there being that you've got op amps on the output to provide current gain, where mine can more easily switch higher current directly. Not really right or wrong in my eyes, just different flavors with different benefits.

But DickCappels said: Some LEDs cannot tolerate any reverse bias without suffering long term degredation.

Yes, but I'm not suggesting using two separate optos where one will be reverse-biased. I'm planning to use a single opto which is setup for AC input. I think it's a safe bet that they've done whatever they need to internally to make sure the emitter elements are safe and happy, or they wouldn't sell them as AC input!

 

I agree, your design looks like a great alternative for different parts or different situations. I like that it doesn't require caps on mains or high wattage resistors like mine does - the tradeoff there being that you've got op amps on the output to provide current gain, where mine can more easily switch higher current directly. Not really right or wrong in my eyes, just different flavors with different benefits.

Yes, but I'm not suggesting using two separate optos where one will be reverse-biased. I'm planning to use a single opto which is setup for AC input. I think it's a safe bet that they've done whatever they need to internally to make sure the emitter elements are safe and happy, or they wouldn't sell them as AC input!

OK, I fully agree with you, you are free in your choice.
And they are not op amps simple logic gates only.
Edit: Six gates in one package.

 

OK, I fully agree with you, you are free in your choice.
And they are not op amps simple logic gates only.
Edit: Six gates in one package.

Oops! I should've looked more closely at that. Sorry - that makes more sense! Thanks for your insights and explanations.

 

Oops! I should've looked more closely at that. Sorry - that makes more sense! Thanks for your insights and explanations.

I’m happy to help!

 

0.1uF seems to be quite a stretch I use 1.8nF for neon bulbs at 230 volts of course you need more at 110v but can get away with 400 volts rated

You could use a zener diode + small capacitor to stabilize your low voltage rail + the small 1/4W carbon resistors arent any good for line voltage you should use 1W or 2W kinds, mandatory fuses always

 

0.1uF seems to be quite a stretch I use 1.8nF for neon bulbs at 230 volts of course you need more at 110v but can get away with 400 volts rated

You could use a zener diode + small capacitor to stabilize your low voltage rail + the small 1/4W carbon resistors arent any good for line voltage you should use 1W or 2W kinds, mandatory fuses always

Thanks! Lots of info to digest there.

1.8nF at 60Hz works out to 1.47M ohm. That's a lot of impedance! I was trying to run the opto emitters at 10-20mA, depending on voltage and chose accordingly. Maybe the situation is different because you're working with neon bulbs?

I'm not sure what you mean about the zener. Wouldn't a zener mostly be useful for clamping a voltage down below a certain threshold? I don't want to reduce the 5V any lower, I just want to filter out enough of the ripple for it to always register as a logic HIGH level. I think the capacitor setup I've got does that (as long as it's feeding a somewhat high impedance input,) but I'm open to other ideas. How would you incorporate the zener?

As for small carbon resistors, I was definitely planning for a large (2W or more) resistor at R2, but I had thought I could use a 1/8 or 1/4W resistor for R1, which dissipates less than 60mW. Is there a reason I need to use a really large resistor even though it's dissipating very little power?

And yes, the fuse didn't make it into the schematic, but I'll be sure to include a fuse before the caps.

 

0.1uF seems to be quite a stretch I use 1.8nF for neon bulbs at 230 volts of course you need more at 110v but can get away with 400 volts rated

You could use a zener diode + small capacitor to stabilize your low voltage rail + the small 1/4W carbon resistors arent any good for line voltage you should use 1W or 2W kinds, mandatory fuses always

Sorry to keep pestering you, but could you explain your concerns about capacitor size and resistor wattage a little more? I was thinking this design was ok, and I'm about ready to order parts (including the fuse that doesn't show in my schematic.) If there are problems with this design, I'd like to get them sorted out before I order anything. Any help would be appreciated. Thanks.

 

For high voltage you can use two or more small wattage resistors in series.
----------------------------------------------------------------------------------------------------
Carbon Film Resistor
Maximum voltage rating
http://www.mgelectronic.rs/ProductFilesDownload?Id=2437

Metal Oxide Film Resistors
Maximum voltage rating
http://www.jauch.de/downloadfile/58afeb6b217fc_30b240076466a7e6e73b/nmo.pdf

 

For high voltage you can use two or more small wattage resistors in series.
----------------------------------------------------------------------------------------------------
Carbon Film Resistor
Maximum voltage rating
http://www.mgelectronic.rs/ProductFilesDownload?Id=2437

Metal Oxide Film Resistors
Maximum voltage rating
http://www.jauch.de/downloadfile/58afeb6b217fc_30b240076466a7e6e73b/nmo.pdf

Thanks! I see now that I would've been out of spec with my small R1 resistor. I'll probably do two in series like you said, or consider just stepping up to a higher wattage than I'd otherwise need.

Anyway, thanks to both you and @takao21203 for bringing that to my attention.

 

I got distracted for quite a while and I'm just now getting ready to physically build this circuit. Seeing it with fresh eyes after 6 months, I'm suddenly questioning some earlier assumptions.

I had ruled out mechanical relays for several reasons, but I can't remember if I had looked at step down transformers. I think I assumed they would be bulky expensive, but by the time I selected caps with the right safety ratings and resistors with suitable wattage and voltage ratings, those items were quite bulky and expensive too.

Seems like an appropriate transformer would be safer and perhaps easier, so I'm considering redesigning this around a transformer, maybe something along these lines:

https://www.digikey.com/product-detail/en/triad-magnetics/FS16-070-C2/237-1576-ND/3986440

http://catalog.triadmagnetics.com/Asset/FS16-070-C2.pdf

Any opinions on why redesigning around a transformer might be an especially good or bad idea?

 

I would not give up completely on using a capacitor as a means of current limiting. Below is a circuit I made about a year ago to generate interrupts for an AVR controller. It is strikingly similar to yours and worked well. I found only about 400 nanoamps or leakage current from the photo transistors to ground. Plenty safe in my book, and I am extremely averse to electrical shock. This circuit runs on 240 VAC.

The controller uses polling instead of interrupts so glitch filtering could be done in firmware if needed. It was not needed in my case.



The most expensive part was probably the 0.1/630 and I doubt it cost more than US$0.50.

 

See

 

I don't see any particular advantage in using a bridge rectifier feeding a standard opto vs. just using an AC opto instead.

On an AC opto, make sure the opto output is rated for both AC and DC.
If it's AC only, then it won't work with a DC supply on the output.

 

On an AC opto, make sure the opto output is rated for both AC and DC.
If it's AC only, then it won't work with a DC supply on the output.

Thanks. I'll double check that. Pretty sure it was transistor style output, not TRIAC output, so I thought it was good, but I'll definitely double check. That would've been a disappointing lesson to learn the hard way!

Planning to use FOD814:

http://www.onsemi.com/PowerSolutions/product.do?id=FOD814

 

On an AC opto, make sure the opto output is rated for both AC and DC.
If it's AC only, then it won't work with a DC supply on the output.

The triac type won't unlatch on DC till the circuit is broken, the dual MOSFET variety should be OK.

 

The IRED can be protected against overcurrent with a simple bipolar transistor & resistor. Sorry - words only at the moment:

NPN, collector to IRED anode, base to its cathode
resistor between cathode & negative of input supply
emitter also to negative of input supply
must have resistor between positive supply and anode/collector

B-E resistor sets threshold for BJT to begin to shunt current around the IRED.

This can be a reasonably convenient and inexpensive way of assuring the IRED isn't overdriven while allow the current at low input voltage to be adequate.

If you can get them, a flameproof fusible resistor is worth considering for the input. It does double duty as a fuse.

 

New update, finally. First off, thanks again to everyone who contributed to this thread. I learned a lot!

I'd originally planned to build this by hand on perf board or vero board, but I kept stalling and hesitating because I was going to have to do a LOT of scraping off copper pads between high voltage components in order to have good clearances... so I finally just designed a custom PCB with all the clearances I needed.

During the redesign I decided I wanted a few more features, so now it's got an indicator LED, much crisper output switching through BJTs, and a short-circuit-protected, current limited output.

I just built it a few days ago and everything is working perfectly: good current draws on high voltage side, perfectly smooth output, accurate current limit on DC output, etc. I'm a happy camper.

After incremental testing of 5V side, then high voltage side, I finally installed it in my espresso machine tonight, and everything's running great. Thanks so much for all the help.