Difference between Relay and Transistor

domint

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D
Can someone explain to me the difference between a relay output and a transistor output? I'm working with Keyence KV 24AR or KV24AT (Relay or transistor), and I need to know what's different between them.

Thanks,

John Garst
 
Well, I've never worked with Keyence before, but I can speak generally.

A relay output sends current to a relay inside the output card, which pulls in a coil, which allows current coming in from the field to flow through a contact. So you need to provide an incoming current wire as well as an outgoing current wire which leads to your field device (light, solenoid valve, etc.) for each output.

With a transistor output, when the PLC turns the output on, it sends a voltage to switch the transistor "on", which allows current to flow through the transistor. Usually, (at least with the A-B stuff I've used) there is one incoming current line to the whole card, which goes to each transistor. Then, when a transistor is turned on, the current flows to the outgoing terminal and to the field device. So with the transistor output card, you have one wire per output, instead of two per output on the relay card.

One advantage of the relay card is in most cases you can have AC and DC outputs on the same card.

I wish I had time to put some pictures together to make this clearer, but I've got to run to a meeting now. Hope this helps!
 
Relay output provide a complete isloation but the transistor don't. In other word, the relay may be used for other safe voltages i.e. 24, 120, 230, or 277 volts ac or dc. Whereas the transistor output can be used for its rated voltage only.
 
Transistors week points:
1. lamp load, ordinary pilot-light (not LED) gets 10 to 20 times nominal-current in switching-on moment, becourse resistance is very low in cold wolfram-wire.
Control from transistor base could be so slow that whole current try to go thruth from small semiconductor area and it could destroy it. (The control have not yet opened whole semiconductor area). Same phycsical property is present with empty capacitors.

2. Inductive load as solenoid. Solenoid includes magnetic-energy and Current likes continue in switching off. Voltage is R*I, and in switching off situations R is growing up and it means that voltage likes gworing up too. Without commutation diodes or corresponding devices, transistor could be damages for high voltage.

Relay contacts are not so 'finicky' for over-voltage and -current.
 
Relays will deteriorate over time when constantly switching, whereas transistor outputs will last a lot longer. I have had numerous failures on my AB OW8 output cards but the transistor ones have not failed me yet.
They are more sensitive to voltage spikes though but providing your system is protected well, you should be OK
 
If I understand your question, you want to known about the Keyeance output.

If you want to use the output from it to a PLC then use the Transistor Type Device.

If yout want to use the output from it to something else, maybe a relay coil, where more current will be needed then use the Relay Type output.
 
There are several types of solid state outputs available with PLC’s. Three popular types are transistor, triac and TTL. All three of these output units will generally have a common terminal although triac output units are available in an isolated configuration.

Transistor output units are usually open collector with the common terminal connected to the emitters of all outputs.

A transistor output unit providing three open collector outputs is shown in Figure below. In most, if not all transistor output units, the transistors optically isolated from the PLC.

The transistors shown in Figure below are optically isolated devices. This unit has all the emitters of the output transistors connected to one common terminal labeled OUTCOM. The transistors contained in this unit are NPN although PNP units are available.

There are two different types of transistor units available and they are described as sourcing and sinking. The NPN units are referred to as sinking and the PNP units as sourcing.


SABARINATHAN ARUMUGAM
 

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