Relay vs SCR or TRIAC

[krypton_john]

Just wondering what the advantage of a relay is apart from perhaps isolation?

Why would you ever bother with a bulky relay that has mechanical wear and requires more power to activate?

I need to take a logic level output from a gate motor controller and use it to switch on some 12V AC garden lights when the gate opens. I initially thought I needed a relay but wonder why I wouldn't just use a TRIAC?

TIA,
JohnO

 

[Andrew Cowan]

With AC, a triac is good. With DC, a triac will latch so a relay is needed.

 

[MPep]

Hi KJ,

Power across a Triac is supposed to be 1A = 1W. If using lots of current, need a large heatsink.

With a relay, no need to worry.

An SCR is essentially a controlled diode, hence will rectify the AC, lowering the applied voltage to the motor.

Your choice really depends on how often the motor will be operated. If the answer is infrequent, 3 times a day, then a relay should be okay. If the answer is 10 times a day or more, then go for a triac. Mechanical wear and tear would cause the relay to fail, probably, too early.

Wonder if a DC motor could not be used. Then transistors are easy to use. Or MOSFETs.

Good luck.

 

[hax]

Your logic level output will need a circuit to drive the relay, or it will need a circuit to drive your triac. So either way you will have a PCB floating around.

I have seen triacs fail and I have seen relays fail. The relay will use very little power compared to the lights it is switching anyway.

The best of both worlds, but at a few more $$ you could use a solid state relay where everything is built in for you ready to go.

 

[krypton_john]

Thanks very much for the comments.

I agree that the SSR is probably the best option.

Note that I am not using the motor outputs. They are 12V DC and beefy, but I want the lights to stay on while the gates are open. Not just while the motors are running.

Reason for project: Wife backed down the driveway crooked in the dark and dinged one of the gates! :-(

 

[inglewoodpete]

My wife does that in broad daylight.

 

[Mycroft2152]

My daughter nailed the garage door when she was learning to drive.

 

[boriz]

http://www.wimp.com/parkingcar/

 

[BeanieBots]

Just wondering what the advantage of a relay is apart from perhaps isolation?

Why would you ever bother with a bulky relay that has mechanical wear and requires more power to activate?

1. No PCB required.
2. Smaller, neater and easier than any silicon solution for DC applications.
3. No degredation of the switched signal.
4. Isolation.
5. Lower impedance.
6. Easier to drive.
7. More robust electrically.
8. Copes with overloads much better.

I could probably think of quite a few more but bottom line, horses for courses.
High quality audio switches use relays in preference to silicon.
Precision instruments use relays to maintain signal integrity.
Safety circuits use relays for RELIABILITY.

Don't underestimate the humble relay. It is NOT a thing of the past.

 

[premelec]

I like being able to push the armature manually to close the circuit using a plastic stick [Bic...] whilest testing some power circuits - I've had both solid state and mechanical relay fail in a variety of ways... I do like the low power drive for the SSRs and there are some nice SSRs for DC low current ["Phtomos"]. Whatever works...

 

[nbw]

oh crumbs, it's this debate again LOL. I'm sure silicon must have SOMETHING going for it....... ;-)

 

[gengis]

I like and use both sometimes together - a SSR to do the grunt work of switching often without burning contacts, and a mechanical relay to absolutely remove power when necessary so there's no danger of shock or fire. I have a picaxe working my electric range controls using that setup - SSR modulates the power but a mech relay removes power when the control is turned to "Off." Contacts on both sides of the 240 V line.

More to the point of the garden light project, you have some other options.

Add a transformer to your lights and mount it with the gate power - then use full wave bridges to isolate the transformers on a multi-transformer one load scheme. You just don't add filter caps. The lights see pulsating DC but the RMS light and heat is the same.

Move your transformer to the gate supply if you don't need the flexibility of turning on the lights independently of the gates. Or separate some lights to only work with the gate.

Lastly . . . check out some zero crossing triac opto-isolators. They are intended to drive Triacs as a load, but can often supply one amp of mains current. If you have 120 mains that's ~120 watts of lighting or two ordinary car headlights - more than enough for a garden light system as a rule. 240 V mains; even better. That lowers your SSR cost to a whopping 30 cents plus the resistor/cap snubber.

That last idea is used - HP gas chromatograph I opened up had a half dozen working 120 V solenoid valves. "Rainbird" uses them to switch 24 VAC to operate remote solenoids to control plant watering. I've used it to turn on small transformers and relays from mains power.