1
\$\begingroup\$

I'm beginning the design of a light control system based on a microcontroller.

This is to activate the lights, what is usually used are continuous voltage relays, in ranges of 5, 12 or up to 24 VDC.

These relays are activated by the microcontroller through a transistor.

Now, we have the option of acquiring 120 Vac relays similar to RTD34615.

To activate these AC relays with the microcontroller, I plan to use a circuit similar to the following, using an opto-triac to activate the relays.

schematic

simulate this circuit – Schematic created using CircuitLab

Can this electrical design have any problem or drawback?

Any comment or suggestion is welcome.

\$\endgroup\$
7
  • 1
    \$\begingroup\$ Or you could just drive the relay coil from DC. \$\endgroup\$
    – DKNguyen
    Dec 1, 2021 at 19:55
  • 1
    \$\begingroup\$ Well then you have little choice than to go with what you proposed. Note that the TRIAC will latch on and cannot be used to interrupt current. It will only turn off at the next AC zero cross after the gate signal is removed, but relays are so slow it won't matter. \$\endgroup\$
    – DKNguyen
    Dec 1, 2021 at 19:58
  • 1
    \$\begingroup\$ @FabiánRomo I use the MOC302x and MOC306x devices to drive 40 A relays just as you suggest, using shaded AC relays whose coils are designed to operate directly from mains AC voltages. Works fine. \$\endgroup\$
    – jonk
    Dec 1, 2021 at 22:50
  • 1
    \$\begingroup\$ @jonk Thank you very much, what you have answered may be the answer to my question. \$\endgroup\$
    – Fabián Romo
    Dec 2, 2021 at 0:54
  • 1
    \$\begingroup\$ @FabiánRomo Here is a photo of something I did a long while ago using the MOC series devices I mentioned with a 40 A relay, for validation and testing then. \$\endgroup\$
    – jonk
    Dec 4, 2021 at 22:32

2 Answers 2

Reset to default
2
\$\begingroup\$

It should work with the opto-triac, given a 120VAC coil, however I would use the 600V version of the opto-triac and add an RC snubber across the triac.

\$\endgroup\$
2
  • \$\begingroup\$ Hi Thanks for answering. Should a RC snubber also go in parallel to the inductor coil? \$\endgroup\$
    – Fabián Romo
    Dec 2, 2021 at 20:21
  • \$\begingroup\$ You only need one, and across the triac makes more sense to me. \$\endgroup\$
    – Spehro Pefhany
    Dec 2, 2021 at 20:21
1
\$\begingroup\$

Now, we have the option of acquiring 120 Vac relays similar to RTD34615.

I think you mean RT314615 (115 V AC coil with approximately 250 mH inductance).

Can this electrical design have any problem or drawback?

None that I can see other than a little nit picking...

If you can live with a several millisecond delay due to the opto-triac remaining on until the end of the AC mains cycle then it shouldn't be a big deal. Also, the triac in the opto-triac won't be as efficient as an opto MOSFET such as this (below) and, that inefficiency might cause the triac to get a little hot. But, on the other hand, using a triac means that the back emf on an inductive relay coil load will be smaller when the triac commutates into the off condition. Nit picking really!

enter image description here

Any comment or suggestion is welcome

I've tried!

\$\endgroup\$
6
  • \$\begingroup\$ The problem is that the end user can connect any type of lamp (incandescent, fluorescent, led lamp, etc) and I'm not sure if that device that you mention works correctly. An electromechanical relay is more reliable than a "solid" relay \$\endgroup\$
    – Fabián Romo
    Dec 1, 2021 at 22:13
  • \$\begingroup\$ @Andyaka - Hi, Please add the link to that datasheet. Thanks :-) \$\endgroup\$
    – SamGibson ♦
    Dec 2, 2021 at 0:04
  • \$\begingroup\$ @SamGibson the link has been added but, I thought we'd had this discussion before about data sheets; I'm doing Toshiba a favour by pointing their stuff out so they're highly unlikely to quibble over me extracting a section of their data sheet and posting it here. Isn't that the real point - it says "Toshiba" on the picture and the part numbers are clearly stated on the picture hence, anyone with only 25% of their brain currently active could easily track down the original data sheet from the info in the picture. \$\endgroup\$
    – Andy aka
    Dec 2, 2021 at 8:08
  • 1
    \$\begingroup\$ @FabiánRomo I'm not suggesting he gets rid of the mechanical relay. Having said that my answer is a little ambiguous so I'll fix that. \$\endgroup\$
    – Andy aka
    Dec 2, 2021 at 17:21
  • \$\begingroup\$ @Andyaka - I don't think we finished the previous discussion & I'm sorry if I gave you false hope of not having to follow that site rule. The requirement for a reference link to the original source for any resource used in an answer is a Stack Exchange-wide rule. There is no exemption for a claim of doing someone a favour and there is no discussion - it's an SE-wide rule requiring the author of an answer to supply a link to the source of any external resources incorporated therein. It doesn't matter how easy or hard it would be for someone to track down the source. Thanks. \$\endgroup\$
    – SamGibson ♦
    Dec 3, 2021 at 7:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.