Transistor Vs Relay Outputs

I am new to PLC in the sense that I started my current job just over a year ago withouth any knowledge of PLC systems. I have picked up all that I know so far. Which is not a lot by watching others and also by doing an apprenticeship where I am in college 1 day per week. Unfortunatly PLC is not covered in my course. Basicly I have no idea as to which output is better. So if someone could enlighten me towards which is suited and generaly not used for different applications I would be greatfull.

I have been wondering this as well. As far as I have been able to gather, a relay output is an actual switch that has a current rating associated with it. The transtor is a digital on/off signal.

if i understand correctly, if you are going to be controling something like a pump that draws say 10 Amps, you will not want to use your plc relay to control the on/off switch to this pump as plc relays are rated at .5 Amps. in this situation, it would be better to have a transistor give an on/off signal to a relay (purchased separately) that is rated for 10 Amps.

I have been wondering if I have this correct. Could someone back me up and/or pwn me like a noob?

The biggest advantage to transistor outputs is that they have no moving parts, so they are rated for a larger number of cycles than contact outputs. A transistor output module is generally less expensive than a contact output module with the same number of outputs, although the price difference is probably not a significant percentage of the total cost of the controls.

The biggest advantage to contact outputs is flexibility. You can use a contact output to turn on or off devices with a wide range of voltage ratings. Often, different outputs on the same module can be used to turn on or off field devices with different voltages. Whether or not this is a good idea depends on the design of the system. It is also easier to find contact output modules with higher current ratings than transistor outputs.

Also a contact output, when off, is an open circuit. There will always be some leakage current through a transistor output. Generally not enough to cause a problem, but something that must be taken into consideration.

You are correct in your assumption that a relay output is like a switch; it is actually an electro-mechanical relay contact. As for current ratings, it varies; typically, a relay output can handle h;igher current than a solid-state output. Thsi is dependent upon the "comman scheme" of the relay output module. Individually-isolated relay outputs (one common per output) typically handle higher current than relay modules that share commons (1 common per every 4 or 8 outputs). Other considerations besides current load to be considered when choosing between solid-state vs. relay outputs. The number of times during a given period that the output will be switched off and on: if you are controlling a device that wil only turn on/shut off 3-4 times and hour or less, a relay is o.k. If you will be cycling the output frequently, you will wear out the relay contacts and solid-state would be a better choice.

Cwiz said:

if i understand correctly, if you are going to be controling something like a pump that draws say 10 Amps, you will not want to use your plc relay to control the on/off switch to this pump as plc relays are rated at .5 Amps. in this situation, it would be better to have a transistor give an on/off signal to a relay (purchased separately) that is rated for 10 Amps.

I have been wondering if I have this correct. Could someone back me up and/or pwn me like a noob?

Click to expand...

I'm assuming you meant "own me like a noob"? Funny to see that here!

No noobiness implied in my response.

If you have an output to control that requires 10 amps, I recommend a contactor controlled by the solid state output.

I use solid state outputs in most applications. The exceptions being things like controlling a VSD. Usually these little drives will have their own power supply (24v) and it is "switched" (using a relay contact) to the run, fwd and reverse inputs in "2 wire" and "3 wire" control configurations. When using these, it is just sensible to use the power supply on the VSD, so I use a relay output to connect these terminals to their own local power supply to be enabled. Does this make sense?

That makes things a whole lot clearer.