In the realm of PLC outputs, relays are commonly utilized even without the need for signal conversion. However, when it comes to 24vdc input signals, the scenario is different. In my experience, there is typically no signal isolation for 24v discrete input signals into PLC cards. Recently, I was discussing a project with an application engineer where my customer was replacing a panel. The integrators sent a panel with 120vac relays to convert the input signals, assuming they were 120vac when they were actually 24vdc. The previous setup had the 24v input signals directly going into the PLC without a relay. When I decided to switch out the 120vac relays for the 24v input signals, I didn't see the necessity for using a relay for signal isolation. I believed that the input card didn't draw enough amps to warrant a relay, and saw relays as potential points of failure. However, the engineer expressed concerns about burnt-out inputs and disclaimed responsibility for such outcomes. I have not encountered many burnt-out inputs in the past, if any. What are your thoughts on using relays for input cards in situations like this?
If there isn't a shared network, utilize a relay for communication.
In a recent conversation with an application engineer, AutomationTechBrian discussed the use of relays for PLC outputs and 24vdc input signals. While relays are commonly used for PLC outputs, there is often no signal isolation for 24v discrete input signals into PLC cards. A customer was replacing a panel, and integrators mistakenly sent a panel with 120vac relays to convert the input signals for the 24vdc PLC input card. These input signals were actually 24v, not 120vac, as previously thought. The legacy panel being replaced had the 24v input signals going straight into the PLC without a relay. It is important to note that relays can be an additional point of failure, and it may be necessary to add interposing relays for dedicated IO to prevent issues. As long as the circuitry and devices are properly sized, this should not be a problem.
Swapping out a burnt-out relay is a quicker and more cost-effective solution than replacing an entire I/O card. Save time and money by opting for this simple fix.
ojz0r mentioned that it is quicker and more cost-effective to swap out a faulty relay rather than replacing an entire I/O card. This approach can save time and money, especially if you can pinpoint the component that has malfunctioned.
Incorporating relays into PLC inputs may compromise reliability if the input consumes minimal current (just a few milliamps) below the relay's minimum current rating.
While I understand the concerns you've raised, it might be worth considering the engineer's advice, even if your past experience doesn't align with it. Engineers often design with worst-case scenarios in mind, and while burnt-out inputs might not be common, they can cause a significant problem when they occur. Using relays as isolators can serve as a security measure to prevent unexpected failures. Remember, the principles of redundancy and isolation is sometimes not about the necessity, rather about safety and reliability, especially in crucial systems. So, in certain situations, this kind of precaution might be worthwhile.
In my opinion, it seems like the concerns of the application engineer are mainly based on potential worst-case scenarios. While you've rightfully noted that the input card doesn't draw much amperage, and hence, may not require a relay, his fears about burnt-out inputs could stem from experiences with high-surge environments or faulty installations. Also, the use of relays does provide an added assurance of safety to some extent. However, I agree with your viewpoint. Given that PLCs are designed to accept direct discrete inputs, and considering the added complexity and failure points that relays would introduce to the system, it might be an overkill to use them in cases where the input signals are within the PLC's input range. The need for relay depends upon the specifics of the application and the environment, and should be assessed individually.
It sounds like you're navigating a tricky situation! While I understand your reasoning for bypassing relays—especially considering the low current draw of the input cards—it's also true that unexpected voltage spikes or transients can occur, even with 24VDC systems. Relays can add that extra layer of protection, reducing the risk of damaging the PLC inputs. Ultimately, the decision might come down to balancing reliability with cost. If the application is critical, it might be worth considering relays for peace of mind, but if you’re confident in the stability of the current setup and have had no issues, it could be okay to proceed without them. It's definitely a good practice to discuss these concerns directly with the integrators to find a solution that everyone feels comfortable with!
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Answer: - Not necessarily. While relays are commonly used for PLC outputs, they may not be required for 24vdc input signals. It depends on the specific setup and requirements of the system.
Answer: - Factors to consider include signal conversion needs, signal isolation requirements, current draw of the input card, and the risk of burnt-out inputs. It's important to assess these factors before deciding whether to use relays.
Answer: - The use of relays for 24vdc input signals in PLC systems can vary based on specific applications and preferences. Some may opt for relays for added protection and isolation, while others may choose to directly connect the signals to the PLC without relays.
Answer: - Advantages of using relays include signal isolation and protection for the input card. However, relays can also introduce additional points of failure and complexity to the system. It's essential to weigh these factors when deciding whether to use relays.
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