PLC

good question...

I was also looking for the same ....

Hope u guys can help him solve the problem.....

"Transistor" based is a triac.

From "Wikpedia"

A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two silicon-controlled rectifiers (SCRs/thyristors) joined in inverse parallel (paralleled but with the polarity reversed) and with their gates connected together. Formal name for a TRIAC is bidirectional triode thyristor. This results in a bidirectional electronic switch which can conduct current in either direction when it is triggered (turned on). It can be triggered by either a positive or a negative voltage being applied to its gate electrode (with respect to A1, otherwise known as MT1). Once triggered, the device continues to conduct until the current through it drops below a certain threshold value, such as at the end of a half-cycle of alternating current (AC) mains power. This makes the TRIAC a very convenient switch for AC circuits, allowing the control of very large power flows with milliampere-scale control currents. In addition, applying a trigger pulse at a controllable point in an AC cycle allows one to control the percentage of current that flows through the TRIAC to the load (so-called phase control).

Low power TRIACs are used in many applications such as light dimmers, speed controls for electric fans and other electric motors, and in the modern computerized control circuits of many household small and major appliances. However, when used with inductive loads such as electric fans, care must be taken to assure that the TRIAC will turn off correctly at the end of each half-cycle of the ac power.

Triac semiconductor construction

A snubber circuit is often used to assist this turn off. Snubber circuits are also used to prevent premature triggering. For higher-powered, more-demanding loads, two SCRs in inverse parallel may be used instead of one TRIAC. Because each SCR will have an entire half-cycle of reverse polarity voltage applied to it, turn-off of the SCRs is assured, no matter what the character of the load.

Mechanical relay is a "dry contact" meaning a physical openning/closing of metal contacts.

From "Wikpedia"

Since relays are switches, the terminology applied to switches is also applied to relays. A relay will switch one or more poles, each of whose contacts can be thrown by energizing the coil in one of three ways:

• Normally-open (NO) contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called a Form A contact or "make" contact.
• Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called a Form B contact or "break" contact.
• Change-over (CO), or double-throw (DT), contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal. It is also called a Form C contact or "transfer" contact ("break before make"). If this type of contact utilizes a "make before break" functionality, then it is called a Form D contact.

The following designations are commonly encountered:

• SPST - Single Pole Single Throw. These have two terminals which can be connected or disconnected. Including two for the coil, such a relay has four terminals in total. It is ambiguous whether the pole is normally open or normally closed. The terminology "SPNO" and "SPNC" is sometimes used to resolve the ambiguity.
• SPDT - Single Pole Double Throw. A common terminal connects to either of two others. Including two for the coil, such a relay has five terminals in total.
• DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST switches or relays actuated by a single coil. Including two for the coil, such a relay has six terminals in total. The poles may be Form A or Form B (or one of each).
• DPDT - Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the coil.

The "S" or "D" may be replaced with a number, indicating multiple switches connected to a single actuator. For example 4PDT indicates a four pole double throw relay (with 14 terminals).

• transistor output modules are usually used with DC voltages while relay outputs can be AC or DC.
• transistor output modules are used for up to 48 volts DC while relay outputs depend on the rating of the relays.
• generally speaking, transistor outputs can be "packed" into a module up to 32 outputs. Relay outputs usually come in 8 output points.
• When you buy a transistor output module, you have to specify if it's pnp or npn (sinking or sourcing). With some relay output modules, both the NO and NC contacts are available for use.
• Transistor output modules can usually provide several milliamps of current. Relay modules can handle higher currents.

Transistor output is much more faster than relay's, so if your application is crucial for time response it's better to use one.

Relay's ops number is restricted from some 100.000 up to 1 million times.

But I'm preferring to use relay as more foolproof decision.

There are controllers equipped a Mosfet output as well.

Transistors are DC, but triacs can also be AC. You need to look at you application before deciding on one type of output over another.

Triacs/transistors drive output devices by sending power out directly out the terminal. There is a common power feed to one terminal of the card and that power is sent out each output. The advantages are higher output densities. Typically twice as many outputs per card than relays. This is good if you are driving small cube relays in the same panel. The drawback is that you have a lower power budget for each card. You are sharing one power supply for all outputs.

Relays need two terminals per output. These are typically a normally open single pole single throw type. You need to provide your own power supply to one side of each contact and the load connects to the other. Advantages are more power available to each oint. It eliminates the need for an external cube relay in cases where you only need one pole. you can drive MCC motor starters directly (if they are not too big). Each relay can get power from a different source. The disadvantages are a smaller density of outputs. And if you burn up a relay contact you cannot easily replace it without replacing the entire card.