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from the Automation List department...
Information about output modules
PLCs and related questions. topic
Posted by R Herrmann on 8 February, 2004 - 11:42 am
I am trying to find information about AC digital outputs. I have seen information about relay outputs, and TRIAC outputs. First what exactly is a TRIAC output and under what situations would I use something like this? Also the general relay outputs, what are they really?

The outputs that I am refering to are Allen-Bradley
1746-OA16 - TRIAC output card
1746-OW16 - Relay output card

I appreciate any and all help


Posted by Meir Saggie on 8 February, 2004 - 2:29 pm
I am not familiar with the AB products in question but: Reliability (product life): The life of a Triac is measured in number of hours (hundreds of thousands of) at a certain current The life of a Relay is measured in number of operations (hundreds of thousands of) at a certain current So, depending on your application nature, you might be better off with one or the other.

Leakage current (when OFF): A Triac always has a leakage current when OFF (a few milliamps to tens of milliamps) A relay (assuming there is no snubber circuit across the contacts) has no leakage at all when OFF. A snubber (a resistor in series with a capacitor) is needed when switching inductive loads (like coils, solenoids), but not when the load is purely resistive (a heating element, an incandescent lamp)

Minimum current: Each of them has a minimum current: A Triac has a Holding current A relay has a Contact Wetting current (needed to break the natural metal oxide barrier - unless it is Mercury wetted or Gold plated)

dV/dT (Voltage rate of rise) A Triac has a maximum dV/dT rating, beyond which it may trigger (start conducting) on its own.

That's more or less the long and the short of it. Meir


Posted by Mike on 9 February, 2004 - 3:56 pm
A relay output is a mechanical device. It is a good choice for outputs that have a low-to-medium volume of on/off cycles, at low amperage. Examples: alarm outputs, interlocks, etc.

A triac is a solid state device. It has no moving parts so it is a good choice for applications that have a medium-to-high volume of on/off cycles, at low amperage. A quick tutorial is at: http://www.americanmicrosemi.com/tutorials/triac.htm

Triacs might be used to activate heater contactors (mechanical or mercury), solenoids, AC triggered solid state contactors, etc. Triacs don't mechanically wear out but warm temperatures or voltage surges can cause problems.

For loads that are greater than a few amps, it is common to use external solid state contactors. There are many people who make these including: www.ab.com , www.sprecherandschuh.com , www.power-io.com and others. These can be triac activated, but are more commonly activated by a PLC's DC output card.


Posted by Dave on 10 February, 2004 - 5:11 pm
There are a few things to consider when selecting output modules. A few of the earlier posts have touched on these as far as cycle life. The main difference in the two, however, is usually current loading and isolation.
Relay cards typically have a higher amperage rating than triac outputs. Isolation has to do with the common side of the card. Most triac cards have a single common providing power for groups of 4, 8 or even 16 outputs. Relay cards have a common for each point typically. This will allow easy control of mixed voltage signals if the need arises.

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