Advantages of Controlling Solenoid Valves using Solid State outputs vs. Relay outputs

Entertron's solid state output design vs. Other PLC manufacturers

Controller used: Elite-2000

Why should an Original Equipment Manufacturer (OEM) consider controlling their solenoid valves of their application with a PLC using solid state triac outputs vs. Relay outputs?

Both offer advantages and disadvantages.

Relay Output - Advantages

Most OEM's use PLCs with relay outputs because they are less expensive and offer greater current capabilities, usually around 2 amps / output. The vast majority of PLCs offering triac outputs offer ratings of only about 500 mA / output. As a result of greater current handling capability, relay outputs offer the advantage of controlling a larger solenoid than do PLCs with triac outputs.

Below is a chart of several PLC manufacturers and their triac ratings. The information comes from current data sheets. Due to limited space for this article, we could not mention all manufacturers.

Relay Output - Disadvantages

Most OEMs rarely consider the ramifications of using relay outputs, mainly due to cost. However, there are several disadvantages. A PLC's relay, just as panel mount relays, have armatures, therefore they will ware out in a shorter period of time than a triac output, which has no moving parts. In order for a relay to operate, a certain amount of current must run through it, for the armature to move from an "off" position to an "on" position, most relays require between 8 - 10 milliseconds to turn on. Should your application require a fast response time, relays may not be adequate. A relay is rated to operate for approximately 1,000,000 mechanical cycles.

Example: To operate an output once a minute, 24 hours a day, 365 days a year would require over 500,000 cycles, giving your PLC a life expectancy of approximately two years of continuous operation.

Keeping in mind that the above example is an extreme situation, however, the life rating of a relay is something that should be taken very seriously. Product reliability is essential to the continued success of an OEM's product. In an operation where the equipment is running continuously, down time is the
greatest cost when a piece of equipment fails, and probably the most dangerous. The potential for a relay failure is far greater than a triac for the reasons mentioned previously. A line down situation could potentially affect the entire company's ability to ship product.

The other cost, is service to a piece of equipment. The greatest costs in servicing a down piece of equipment are the labor and travel involved. In many situations, the OEM, who has supplied the equipment to their customer, are required to maintain the equipment through service contracts. Regardless of who is responsible for the costs, the end result is that someone will have to pay. By paying more up front, in the form of a more reliable output configuration, will constitute less expense down the road. When equipment goes down...it doesn't always happen when you want - a long weekend or holiday. The costs, then to service the equipment have no boundaries.

Now that you know the downfalls of using relay outputs, what are the advantages of using triac outputs?

Triac Outputs - Advantages

A triac output is an AC output. It has no moving parts to activate, therefore the potential for it to fail from life expectancy restrictions is nearly non existent. Triacs can fail, but the potential for them to fail is far less likely than a relay. As mentioned earlier, a relay requires approximately 8 - 10 milliseconds to turn on. For a triac to turn on, requires less than 1 millisecond. Not only are they more reliable, they are also much faster.

Triac Outputs - Disadvantages

The major opposition to using triac outputs, is really not the triac themselves, but the PLC manufactures and what they provide. As stated previously, the triac outputs that PLC manufacturers supply are the problem, around 500 mA / output, one quarter the current load capability of a PLC mechanical relay. Should an OEM want to use a reliable solid state PLC for the reasons stated above, in order to obtain the same performance as a mechanical relay, they need to add a solid state relay to handle the additional current load - an additional cost to the OEM.

Ask yourself these questions:

"If you could get a PLC that offers a triac output with a comparable rating to most PLCs with relays...would you use it?"

"Would you use a PLC that offers a rating comparable to a PLC with relays, while offering individual fuses and noise suppression (items that you normally have to add to the panel)?"

Believe it or not...there is such a PLC on the market today. An ePLC with a 2 amp continuous rating. Yes comparable to a mechanical relay. Its name? The Elite-2000 from Entertron Industries. Entertron has brought to the PLC marketplace a triac output design that has been proven through 20+ years of application success. The Elite-2000 has been on the market since 1994. Its predecessor, the SK1600 was first introduced in 1980 with the 2 amp triac design. The end result...Entertron receives back less than 1% of its SK1600 or Elite-2000 controllers with triac outputs for repair as a result of a failure of the triac. We are aware of product that has been in the field over 15 years, still in operation and NEVER returned to Entertron for repair.

To further drive home the point of just how reliable Entertron's solid state output is, we have extended our standard warranty from 5 years to 7 years.


When Entertron first designed their triac output, they took in to account the applications and incorporated the following:

All of the above are integrated into the controller design. The end result, less additional components to add to your panel.

The objections to using a solid state controller vs. a relay output controller have been eliminated, based on performance and reliability. The only issue that remains is cost. A 16 input / 16 relay output configuration costs between $400 and $500. The one piece OEM price for an Elite-2000 configured as 16 AC inputs / 16 AC triac outputs is less than $700. This product offers more memory for program storage (32K Flash Rom) than anything in the "small" PLC class (up to 128 I/O)

Conclusion:

The up front cost of using solid state over relay is more expensive. That cost comes in the form of greater reliability and less down time to your customer. Which would you rather pay? The issue should be choosing the right controller with superior solid state outputs.

If you want to supply a product to customers and get repeat business...it is best accomplished through supplying the most reliable product in your market, rather than getting business through service of your existing equipment in the field. A customer who does not have to worry about down time will be happier, resulting in the potential for repeat business. An added bonus is referral business. "Free" advertising.

Is $200 - $300 dollars cost up front too much to pay for long term success? The biggest problem today is people have trouble seeing the "BIG" picture. You decide.