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relay vs Triac output

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#1 machinemaker



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Posted 16 April 2012 - 04:59 AM

I would like to know if there is a preference to using relay outputs or triac outputs. I have to drive 120vac contactors. Each coil draws about 100ma. Do the relay outputs last long?

#2 OkiePC


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Posted 16 April 2012 - 10:20 AM

Triacs will handle faster duty cycle and last longer than relays provided they are not overloaded.

A typical relay output will last from 100,000 to 500,000 cycles at or below its rated current.

If the contactors have surge suppression and a power requirement less than half the triac rating, I would use triacs for the long term reliability.

Triacs tend to die once and forever, and only after taking abuse. If treated properly they can last indefinitely.

Relays tend to start "acting up" sporadically at the end of their finite life, before they weld or stick open permanently.

#3 Gerry



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Posted 20 April 2012 - 11:58 PM

Be aware that triacs have a minimum load spec. as well as a maximum.
Check the minimum load specs for the module you intend to use - older A-B modules have higher "leakage" current than the new ones.
100 mA isn't much of a load.
You may experience contactors staying on when the output is off due to the leakage. Often, a R-C suppressor in parallel with the coil is sufficient to cure the problem.
In other cases, extra load needs to be added in parallel (e.g. resistor, lamp)

#4 Michael Lloyd

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Posted 22 April 2012 - 05:07 AM

I'm in the relay output camp. Primarily I use them to energize motor controllers, motor operated valves, and every now and then a solenoid valve.

To put things in perspective-

If you go with the 100,000 cycle per year value and start/stop a motor once an hour the relay out should last 11-ish years. Kick that up to 500,000 and it should last 55 years. In my world 2 or 3 starts a day is about the norm.

If you're planning to flash a lamp or something like that you probably want to consider a triac. Lets say you have a shutdown indicator lamp and it flashes once a second (on for 1 second / off for 1 second). Someone doesn't reset or maybe the shutdown self clears and stays active for an hour on a regular basis. That's 1800 operations an hour and your relay out could fail in 55 hours to 11-ish days. Not the best failure rate in that case.

I've never replaced relay output card or had one go bad in 20+ years of using them. One caveat to the last statement- If I worked for a design firm and said that it would mean just about nothing. I work in operations (which is why I am up typing at almost 5am). I see and use the stuff day in and day out. I started out, long ago, working for an engineering and construction company. I thought that we knew everything there was to know about building gas plants. Then I moved to an operating company and found out that while we built a decent gas plant we didn't have a clue about what it takes to build and operate one with high reliability in mind. No offense intended to designers. It's two different worlds. Designers are saddled with having to deliver the most they can for the lowest price or go out of business. Tough world...

Edited by Michael Lloyd, 22 April 2012 - 05:07 AM.

I can generate numbers randomly
I have gotten good at fixing my screwups
I am highly skilled at screwing up. See line 2.

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