PLC Digital Output Types | Solid State vs Relay

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⌚Timestamps:
00:00 - Intro
00:52 - Digital vs Analog
01:44 - PLC digital output module types
01:50 - Relay output module
03:09 - Solid-State output modules
03:24 - Transistor output modules
05:00 - Triac output module
05:36 - Output module comparisons

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In this video, we’re going to explore the differences between Solid State and Relay outputs on PLC Digital Output Modules and the pros and cons of each.

The purpose of a PLC Output module is to operate or control a physical device based on field device conditions connected to an input module and decisions made by the PLC program.

Digital or Discrete devices have only 2 possible conditions. These 2 possible conditions could be On or Off, Open or Closed, or Energized or De-energized.

Analog devices are a bit more complicated because they have an infinite number of possible conditions.

A fan is a digital device if it can be turned either on or off.
If the fan can be adjusted for any speed from Off to On, the fan is an analog device because it has an infinite number of possible speeds.

There are 2 types of PLC Digital Output Modules: 1) Relay and 2) Solid State.

1) When PLCs were first introduced, the only outputs were relay type with a separate relay dedicated to each output.

As with any typical relay, the PLC output module had physical coils and relay contacts contained inside.

The contacts were operated by applying a voltage to the relay coil. The contacts were connected to an external supply to turn on or off a discrete device. Not much has changed except today’s modules have miniaturized relays and contacts.

The Allen Bradley 1756-OW16I has 16 individual relay outputs.

We can use one output to operate an AC load and another output to operate a DC load as relay contacts are voltage-independent. That’s one advantage of relay type as solid-state type output modules are not voltage-independent.

Relay Output modules are available with Single Pole Double Throw, or SPDT outputs as well, as seen on the Allen Bradley 1756-OX8I module.

2) Solid-State output modules are often referred to as Switching modules. Output devices are turned on or off by using solid-state devices such as a Bipolar Junction Transistor (BJT) or a Triac.

A Triac output module is capable of controlling AC loads where transistor output modules can control DC loads only. When the PLC signal is applied, the Triac will turn on and provide an alternating current path for the load.

1) Relay
– Relays generally have a higher current rating than transistors, but, as with any mechanical device, it will wear out with time.
– A relay module can operate AC or DC loads, or both.
– Relays provide electrical isolation between the load and the PLC.
– Relays are slow-acting with typical response times of 10 milliseconds or greater.
Closed relay contacts are almost zero ohms thus ensuring no voltage loss.

2) Transistor
– Transistor output modules are for DC applications only.
– Transistors typically cannot switch a large load current as can a relay.
– A transistor is fast because there are no mechanical parts. Their response time is much faster than relays.
– In some transistor modules, there is a leakage current even when turned off.

3) Triac
– A Triac output is used to control AC loads only.
– As with the transistor, a Triac is much faster than a relay.
– Triac outputs are suited to low power AC loads such as lighting, motor starters, and contactors with current ratings of around 1 amp.

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#RealPars #PLC #Transistor

[renegadeflower]

With Realpars, you always learn something new.

[knkovja]

Last year i got my bachelor's in mechatronics, robotics and automation. Currently, I am on my master's studies and working in company which main job is industrial automation and machine vision using PLC/Vision System/HMI..etc.
Can't thank you enough for the amount of this quality content.

[natriumguy]

What an amazing video!
I am an experienced PLC Programmer but such kind of videos help me develop better understanding of things.

[munster]

Old school relays are still pretty dominant at the last plant I worked at.

[ardentdfender]

This was very helpful and a learned a bit. I remember on a huge project for a PLC Upgrade at a Power Plant in the South West about a year ago we used some new type Relays. I'd never seen them before and was like maybe a dozen installed. They were from Rockwell and looked like what is Transistors with LED Lights. At first, I had no idea what they were, but my senior said they were Relays. I said Ok and that was it. It now dawned on me they were like Solid State Transistor Relays, I just never knew. Good Stuff RealPars!

[ProfesionalRenacido]

Was s great video! This information will be very useful in my job!

[Dulalmahato]

I learnt a lot from your videos . I am just starting my working career . It's very useful.

[generaltechnologyknowledge]

Very clear explanation thanks you so much

[PhG]

Excellent video ! As usual, very well explained and entertaining. I'm always looking out for new content from you !

[MountainDewComacho]

Thanks for the video. I almost exclusively use relay outputs.

[DaDaDaddeo]

I only source SS output modules. They are much less expensive and seem to be easier to source. This decision is easy by using slim 6mm relays in conjunction with SS output module. The slim relays allow for different power feeds, custom interlocks, take up little real estate, and are inexpensive. The combined cost of both the SS output module and the slim relays is still less than a relay output module alone. Another benefit is the isolation of the controller from the field devices.

[shearzy]

From my experience, working at a machine integrator as a controls programmer, most of this is good information but largely obsolete for machine design. For any AC load, a contactor or VFD/Line reactor will be used regardless because of the limitations on switching such high currents and also because replacing the components if they become damaged is typically cheaper and less detrimental to machine downtime than replacing the PLC module. This is still good information to be aware of but it is not strictly required, sinking and sourcing are more important concepts in my opinion.

[aminokamad]

Damn I entirely watched ur all awesome uploads sir.. really friendly tutorial video easily to understand.. keep up the good work sir. More subscribers to come..

[JKiler]

I only use BJT modules to operate external triacs or relays. If an external relay fails, I can replace it cheaply and in seconds, as opposed to an internal relay that's part of a multi channel device.

[lexvanheel]

Really very instructive videos as always! Thanks for sharing

[redouanesayeh]

Thnx sir u really help me cuz am just beginner 🙏please can u tell me how i connect sinking module and sourcing module

[andreyace]

Thank you for the informative and succinct video!!!

[alakenno]

Thank you very much. I would like to make video about the relations between plc and HMI, and between inverter and HMI

[rahimbbc]

Possible practical training on DO 8*REL SM 322 230V

[قصيعليعبدالهادي]

This topic has been always confusing me. Not any more of course.
Thx a bunch