View Full Version : Relay vs. DC out
May 21st, 2002, 10:31 AM
This is probably a very simple question. But I am a PLC-rookie.
All of my output devices will be DC. I can use either the DC output off of the PLC or I can use the relay output off of a different PLC. My question is, what is the difference between DC out and relay out? Which is better? Is Relay out for higher current devices? Or is it just in case you want to use AC devices too?
May 21st, 2002, 10:56 AM
My opinion is that DC outputs are best if you know you are going to need only DC or if you have some very fast cycles on the outputs. I tend to use relays when I have a mix of voltages or voltage sources required for my outputs. If I have a high current load I generally use an external relay anyway, with whatever coil voltage is most convenient.
May 21st, 2002, 12:23 PM
Given the choice I would go for relays. You eliminate all the problems associated with current leakage, grounding faults etc.
The drawbacks are that they tend not to come in high density versions and maybe more expensive, but that depends very much on the PLC being used.
If you are sure of all of the outputs you need, and if you require lots of them, then DC outputs are probably the way to go.
May 21st, 2002, 01:58 PM
Another consideration is lifetime. Relay outputs tend to have shorter lifetimes than DC outputs.
May 21st, 2002, 05:44 PM
Thanks for all the info. Very useful.
Any other info is much appreciated.
May 22nd, 2002, 05:05 PM
Whether you are to use Relay outputs or Transistor (DC) outputs can depend on several factors.
1) Is leakage current going to present a problem of creating a false output? DC outputs tend to emit a small amount of current when off. If your devices are sensitive to this, then you should not use transistor outputs.
2) Current requirements - Most PLCs offer anywhere from 100 mA to 1 amp current driving for their transistor outputs (Entertron offers a controller with a 2 amp transistor output). Most relays for traditional PLCs are capable of approximately 2 amps, some offer higher ratings with fewer outputs (Entertron offers a minimum 7 amp / output ratings). If your current requirement to turn on your output device is higher than the PLC is capable of, you will require an additional relay to interface (solid state or mechanical) to interpose between the PLC and your device. This would also be required if your relay output rating was not sufficient to drive your device either.
3) Reliability - A transistor output will provide a longer life than a relay output due to no mechanical or moving parts. It is solid state. Keep in mind that the higher the inductive load, the shorter the life span, whether relay or transistor. Most PLCs rate their relay outputs for 1 million mechanical cycles (no load). The higher the inductive current, the lower the lifecycle rating.
When using transistor outputs, you will need to check and determine if the module you are using provides diode protection or if you will have to add it.
Remember that not all transistor or relay outputs are created equal. You should let your application requirements determine the controller and the capabilities you will need. Solid state reliability will cost more in the beginning, but over time will save in down time and aggravation.