Comparison of output cards

I would like to know if a digital output card is better then a relay output card. I control outputs of 24 vdc and it's only contactors and solenoids. The relay cards are less expensive to put in my system, but is that the only advantage?

I would like some advice.

Relay Advantages - Isolation, can switch varying voltages. If individual commons are available for each output then can switch different voltages on one card. Often has higher current capability with appropriate snubbing protection.

Relay Disadvantages - Appropriate only for lower cycle rates. Lower overall switching lifetime.

DC Advantages - Higher cycle rates possible (with apropriate protection). Much longer overall lifetime. Higher density (more outputs per card available)

DC Disadvantages - Lower current capability. Usually limited to one voltage and switching mode (source or sink) per card.

Remember, relay outputs are mechanical. Therefore, they have the disadvantages that anything with moving parts has, such as a finite number of operations. An advantage is that they are "dry contacts", sometimes even isolated (no common), so they can utilize mixed voltages.

Solid state outputs have a certain amount of "off-state leakage current", so that must be dealt with (sometimes with bleeder resistors). Check your output card specs to be sure you don't need to make a "Radio Shack run" for 2K ohm 10 watt resistors because your solid state horn is beeping enough (when it's supposed to be off) to drive the operators to bury it in masking tape. Sonalerts can be notorious for this...

My simple rules of thumb are:

1) I use relay outputs if the devices being controlled are cycled rarely. (Example: drive enable)

2) I use solid state outputs for devices that are cycled often. (Example: solenoids - assuming that the output is rated to handle the current requirements of the coil and transient suppression is installed)

3) Pilot lights - I always use solid state outputs

Foty said:

I would like to know if a digital output card is better then a relay output card.

Click to expand...

Note that a relay output card is a type of digital (or discrete) output. The difference you are looking for is between a relay and a solid state discrete output.

Solid state outputs come in various types. Triacs are used for AC and are usually referred to as just "AC Outputs".

Transistors are used for DC, and are usually referred to as just "DC Outputs". Note that they come in sinking and sourcing versions, but I am not bold enough to go there!

Picky I know, but you may as well get in the habit of using correct terminology.

So if I have solenoids and contacters as outputs, I should use dc outputs and not relay outputs?

Bernie, Tom, and I were giving you more general advice.

Specifically, if you are just energizing starters and solenoids, both having DC coils, I personally think you are better served with solid state DC outputs and NOT relay outputs.

Be sure you follow the PLC manufacturer's recommendations for transient suppression across the load.

In addition, I'd consider individually fusing each output.

Ok, thank you for the advice, I ask the same question to my supplier and he told me that the dc outputs and relay outputs are somewhat the same.

The thing is, that I use relay outputs and so for I had no problems, I knew that dc outputs existed so wanted to know what is the difference between them.



Thank you guys

Foty,

As the others have mentioned the advantages of one over the other, much may also depend on the controller you are using and the type of I/O they provide.

Some I/O offers more protection or greater current driving capabilities, therefore much may depend on what PLC you are using.

It will also depend on your frequency of activation. If you are going to have a high frequency of activation between open and closed states, then you will want to consider a solid state solution. The inductive loads will reduce the life span of a mechanical relay, thus shortening the life for the controller / module.

With the majority of PLCs, you will have to add noise surpression.

Hope this helps.

God Bless,

By the way Foty...Happy Birthday.

God Bless,

Thank you,