Using an opto-isolator to control an AC-coil relay

I've mentioned this project previously, but I wanted to run a particular schematic by you all to see if there's anything severely wrong with it.

I want to actuate a 120VAC coil relay from HC logic. I'm not using a solid-state relay because the coil relay is far cheaper for the current rating, and I'm not using a 5VDC coil because, being connected to mains as it is, there's no reason the device should be drawing power from the logic circuit's supply.

This is to be accomplished using an opto-isolator, and a Fairchild MOC3022 400V triac opto-isolator happens to be handy.

I've never used a triac before, so I'm leery of my kneejerk idea for a schematic:

MOC3022 ............................. Vcc

Reply to
Peter S. May
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We've been using a somewhat similar but a bit more complicated circuit to control power contactors (in a charity project we've been working on for several years, involving load control for a small hydro power system, but that's another story). There are problems with your simple circuit. Note that the data sheet for the MOC3022 tells you, I'm pretty sure, that you should be using it to trigger bigger triacs; it's not designed to drive other loads directly. If the relay coil is low enough current you MIGHT be able to get by without the other external triac. We are after reliability and never wanted to push it, so haven't tried it--little TO-92 (or even TO-220) triacs are cheap enough it just wasn't worth worrying about. The power contactor coils are pushing the current rating of the MOC3022 in any event. But what you probably will NOT get by with is omitting a snubber network, to keep the high dv/dt that results when the triac turns off with an inductive load from re-triggering the triac. In our circuit, we have a series R-C in parallel with the triac (the external one), calculated for the worst-case expected inductance. You should be able to find ap notes on the web that will tell you how to design snubbers for triacs with inductive loads.

You can assume the relay is just an inductance; worst-case would be to assume it's pure inductance of a value to give you the rated coil current at the applied voltage and frequency. There's one little problem that could possibly bite you: when the armature actually releases (some milliseconds after you turn off the coil drive), the inductance changes rather suddenly since the magnetic circuit changes, and if there is residual magnetism, that will cause a voltage spike. The triac should have been off for long enough that it's much less dv/ dt sensitive by that point, but it's something to be aware of.

If you're doing this as a one-off project, you may find that surplus solid state relays from a place like

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are cheap enough and save enough hassle that they are worth considering. And if you are switching modest-current 120VAC loads, why not just use the MOC3022 to drive something like a 3A or 6A TO-220 triac? Of course, either the SSR or the triac will have quite a bit of power dissipation because of the voltage drop, and maybe you want to avoid that.

Cheers, Tom

Reply to
Tom Bruhns

What's the current rating to keep the relay on? Ken

Reply to
Ken S. Tucker

yes, that will work how ever, personnally, I would use a snubber on the load to help reduce HV flyback issues from the coil if you were for some reason to lose the 120 in mid stream. We have heat controllers (eurotherm) with the Triac output option that actually ties a MOV across the 2 terminals. My self, I think BiTVS diodes are better for this but that's just my opinion.

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Reply to
Jamie

That's not the right way (but you might sneak by). The MOC3022 output should drive the gate of a triac - the triac drives the relay.

--------- Signal ---------| MOC3022 |---/\\/\\/\\----+--- HOT | | | Signal_Ground---| |--->(gate)[Triac] --------- | +--[Load]--- Neutral

Above is the conceptual diagram - see the datasheet for a schematic.

Ed

Reply to
ehsjr

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