Relay IO vs Digital IO

Sorry for the kind of newbie question, but I've always wondered why someone would want to use a Relay In / Out over a Digital In / Out.

Essentially they accomplish the same thing, so are there some situations that relay output cards are more suited for, over a digital out (and the same for inputs).

For example, on a project I just worked on, all the outputs to a VFD were relay outs (1794-OE8). Are there certain power requirements that only a relay out can meet?

Thanks.

KDcui,

There are several reasons for using a relay instead of a digital (usually a transistor) output. Sometimes the voltage of two systems do not match, so a relay can be purchased that has the coil voltage of 1st system and the contact voltage of the second system. In other cases, the digitial output may not have the current-carying capacity needed to energize an output. That is common for large motor and heating contactors. There are cases where a digital ouput will have enough bleed-through voltage (current through a load even when the output is supposed to be Off) to create problems, so a relay may be needed to prevent this problem.

For VFD's, often the VFD has an internal power supply, and digital outputs tied to that power supply. Now if one or more of those ouputs need to be used in a PLC program as an Input, then often a relay is needed to interface from the VFD voltage to the PLC system voltage.

My understanding is for 2 reasons.Relays switch slower,are not for hi frequency switching,but can handle larger currents.Digital/Analog I/Os typically are used to control inputs of another device that dosent sink/source to much current,but can switch faster than the relays can.
Check me if Im wrong.

kdcui said:

Sorry for the kind of newbie question, but I've always wondered why someone would want to use a Relay In / Out over a Digital In / Out.

Click to expand...

First lets sort out some terminology. A digital out is any kind of output that can be in one of two discrete states: on or off. Digital outputs come in a variety of hardware styles such as relay, triac, transistor.

Essentially they accomplish the same thing, so are there some situations that relay output cards are more suited for, over a digital out (and the same for inputs).

For example, on a project I just worked on, all the outputs to a VFD were relay outs (1794-OE8). Are there certain power requirements that only a relay out can meet?

Thanks.

Click to expand...

Relay outputs handle a wider variety of voltages and voltage types, low voltage DC to 120 or even 240VAC. A triac output cannot be used on a DC circuit. A transistor output cannot be used on an AC circuit. Solid state triac and transistor outputs also have a very small leakage current which in certain rare circumstances is undesirable. Relays are mechanical devices and have a limited switching life and a limited switching speed. Triacs are zero-crossing switch off devices, so although they turn on fast, they have to wait for the AC sinewave to cross zero to turn back off, but triacs can often handle large currents.

Triacs nearly always fail in the ON, or shorted, condition. Relays can fail in either the ON condition (welded contacts) or OFF condition (faulty coil or mechanical failure).

Last, certain devices provide their own power source for their inputs and require a "dry contact" for their inputs. Relays are best suited for these kinds of devices.

You can get relay output cards were each output is isolated. I typically use these for controlling motor starters, since each mcc bucket has it's own control transformer.

Yes its trut that many a times relayed Outputs are Preferred due to foll reasons:
1) generally the Digital O\p give 24 V DC signal but many a times the device may have control voltage other then 24V DC.
2) in Case of VFD generally the common which we use of the VFD is of 5V DC so relayed OP's are the solution
3) the output Current rating of each output is generally around 200mA (not having exact Value) which is not sufficient to glow Lamps of higher rating so relayed OP's are used to glow bigger Lamps


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Pawan Rathi

I second that....very undesirable.. Specially when its raining out and you are changing a valve...LOL

Alaric said:

Solid state triac and transistor outputs also have a very small leakage current which in certain rare circumstances is undesirable.

Click to expand...

the only reason people are aware of triac leakage current is when they try to measure voltage on energized output with no load. troubleshooting AC I/O has more pitfalls such as false readings due to capacitance of the conducts etc.

i've never seen an issue with transistor output leakage current. unlike with triac, leakage current on transistor output is very small. in fact how many forum member have ever tried actually measuring leakage current of the transistor output? it's so simple thing to do...

relay output may have higher survival rate in hands of inexperienced (at least short term), but they are not suitable for all application (otherwise there would be just one type of output - relay).

also relays cannot produce clean signal - result of contact chatter or bounce.