Transistor or relay outputs?

What are the advantages of transistor and relay PLC outputs?
Obviously a relay output will give more current for heavier loads, but if the output is driving, say, a small 24 volt relay, which is best to use?

Student, hea...

Ok alot depends on alot. I try to use relay outputs only because you can use a wide variaty of voltages ac or dc. However with applications with a lot of switching (frequency) at low power, it's usually best to go with solid state, because they dont wear.

Be Careful with Solid State

I have seen where using solid state outputs would cause certain solenoids and other devices to stay energized even though the output was off. If you are going to use solid state outputs, you need to check and see what the off state current is. In a solid state output, current still flows even though it is off. Using devices connected to them that have very low hold in currents can be a problem getting them to turn off. I found this out during check-out once. It was not pretty and is one of my favorite war stories.

transistor output

Potter
As you said relay can handle more current then transistor output but the transistor output is faster then relay because of that there is no mechanical time constant like the rekay mechanical contact,also the durability of the transistor output is higher from the number of ON/OFF point of view.
I alawys use transistor output cards and put an interface relays to protect them against any damage.
Br

A relay output might give you a second chance if you $crew up. A transistor will not.

I agree with Hesham, I use a transistor output to triger a relay,and connect the load to the relay...


one thing to consider is that, most PLCs with transistor outputs, is cheaper than plc with relay outputs...and of course it has a longer life...it is faster(you can drive PTO/PWM,etc)...so I think it offers more options that a simple relay output.


take note..when dealing with DC inputs and outputs, there are PNP and NPN, those two terms determine what signal the PLC will be outputing...if PNP, connect the negative to load and PLC output will supply a positiv signal for your load to be acctivated...whereas if the it is NPN then, connect the positive to load and the PLC will output the negative...its that simple.

good luck with your efforts.

hope this help.


best regards.khalil.

:site:

I also use DC outputs and routinely connect to the load via a relay.

Reasons:
.Standardisation (I only have 32-point DC input cards, 32-point DC output cards and 8-point analog input cards).
.The "real" relay has a higher rating (both voltage and current) than the relay in the card.
.You can exchange a defective relay with ease. If it was a card, then it is more expensive, and require you to shut down the proces.
.In some cases you can save yourself of having to expand to several racks because DC outputs are typically 32-point, wheras relay outputs are typically 16-point.
.You have NO/NC contacts whereas PLC relays are typically NO only.
.Some relay output cards are "strange". Example: The 16-point relay output card in S7-400 have the outputs grouped together in pairs.

edit:
If you have machines where short cycle times is high priority, then it is a good idea to use AC outputs !
The combination of AC output + AC contactor is much faster than DC output + DC contactor or DC output + DC relay + AC contactor.

mechanical time constant

Although as mentioned in an earlier post concerning a time mechanical time constant there is however access times which is the time from recieving a valid input to a valid output. often referred to as the switch times. However this is always faster thqn mechanical relays being in the nanosecond ranges. or an average of 3 ns per gate on TTL devices.

JesperMP said:

I also use DC outputs and routinely connect to the load via a relay.

Reasons:
.Standardisation (I only have 32-point DC input cards, 32-point DC output cards and 8-point analog input cards).
.The "real" relay has a higher rating (both voltage and current) than the relay in the card.
.You can exchange a defective relay with ease. If it was a card, then it is more expensive, and require you to shut down the proces.
.In some cases you can save yourself of having to expand to several racks because DC outputs are typically 32-point, wheras relay outputs are typically 16-point.
.You have NO/NC contacts whereas PLC relays are typically NO only.
.Some relay output cards are "strange". Example: The 16-point relay output card in S7-400 have the outputs grouped together in pairs.

edit:
If you have machines where short cycle times is high priority, then it is a good idea to use AC outputs !
The combination of AC output + AC contactor is much faster than DC output + DC contactor or DC output + DC relay + AC contactor.

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That makes a real good assingment report. beerchug

I have seen where using solid state outputs would cause certain solenoids and other devices to stay energized even though the output was off

Click to expand...

Transistors are generally OK. SSRs can be a problem and can stay on if load is light. You normally need to add some load with an RC setup.

Use transistors almost exclusively. Use them to drive LED lights directly on 24VDC and relays if a mixed setup. BEWARE, make sure the relays are fitted with flywheel diodes or you can cause yourself a lot of grief with blown up transistots from voltage spikes.

The relays I normally use are Omron G2R series with LED indication and flywheel diodes. AB have the same units but are generally a little more expensive. The LED indicating lights I normally use are Izumi HW series but also use Sprechur & Schuh (if I have to) and Schneider ZB5 series. All of these have diodes built in to enable use on 24VAC or 24VDC and are bi-directional. Polarity does not matter. The Izumi units use BA9S multi chip LED lamps. The Sprechur & Schuh and Schneider units use a very small single point LED but a surprisingly bright and easy to see.

For the coil & load common side of the relays, I use Finder bridging bars. Saves making up heaps of loops. Just do not like the fiddly little tags on the Finder relays. Ham fisted electricians always bend them when inserting the relay into the base. Although if they try hard enough they can do it with the others also. How often do you go to a job and a relay is not working? First place to check pull out the relay and see if the tags have been bent over. Also only use relays with LED indication. Nightmare commissioning a job with a couple of hundred relays and no LEDs. Spend the day on your knees trying to see what is on and what is off. Do not use relays with manual push buttons, or even worse, built in latches. Can really cause some problems if someone latches the relay, the PLC program has it off and closes something else. BANG.

In a mixed setup I use the relays to then drive contactors on 24VDC/AC or 240VAC. If there are enough contactors to almost fill a relay card I then use a relay card to drive the contactors and use transistors for all lights. Use a mixture of 64 bit cards, 32 bit cards and 16 bit cards. The 64 & 32 bit cards I use are plug type connection. Make my own cables from the supplied plug. Much cheaper, do not have to find room for terminals hanging off proprietary cables (save money too) and I can make the cables the length I want. This allows me to make longer leads to the lights on the door and shorter leads to relays. Also allows for very long leads to run into the cubicle next door. Prefer 32 & 64 bit cards as they save a lot of space and money.

On PLCs of Schneider TSX Premium, AB SLC500 and Omron CS1 size I prefer 96 bit cards. Omron were the only ones with these but perhaps the others have extended their range to include this I/O density. Saves lots of racks and room on a large job.

Finder and Phoenix make some really nice 6mm wide relays that stack together really nicely as output relays. About half the size of the Omron G2R's and much easier to common up.

Yes the narrow Phoenix relays are really nice too.
But the OMRON G2R1 has a higher rating and is sligthly cheaper.

On PLCs of Schneider TSX Premium, AB SLC500 and Omron CS1 size I prefer 96 bit cards.

Click to expand...

You are right BobB i am also prefer to use this premiumu 96 or 64 outputs .because it saves a lot of space.
Some times some clients and consultants refuse it saying that if on channel is defected we have to pay for 96 o/p card even it only one channel is defective.so they ask to replace it with 32 or 16 o/p cards

Potter,

There are advantages and disadvantages to both. Which is best...it depends on your requirements.

If we are talking specifically for PLCs...

Relay Outputs typically can handle a larger load and either AC or DC signals. Most PLCs are typically 2 amp relays. A typical transistor output is .5 amps. Unless your running lights or other low current devices, you will require an interposing device, be it solid state relay or mechanical relay.

The greatest advantages of a transistor output are speed and life cycle. They can turn on in 1 ms or less and with no armiture or moving parts, will last much longer than a relay.

Keep in mind that whether relay or transistor, life cycle will depend on the amount of current required to turn on the output device. The higher the current, the shorter the life cycle and vise versa.

I have never really understood people that purchase a transistor output PLC and interpose mechanical relays for a higher load. The only advantage is triggering the interposing relay 10ms faster than with a PLC relay output.

Here is an article I ran across when researching this post - http://www.kpsec.freeuk.com/trancirc.htm#npn It should give you some additional insight.

When Bob mentioned transistors staying on (sometimes), he is refering to the leakage current they generate. This is typically in the microamps.

The 32 transistor output cards only advantage is the number of outputs you can interface on one card. Typical ratings for this configuration are 100 mA...good for driving lights, relays and very small devices...not much else. Not the best option for industrial devices, because you are adding additional components and cost to the product / application.

It is amazing how many PLC manufacturers offer products with less than adequate output ratings, given the many uses for PLCs. Even with a 2 amp relay output, many applications still require interposing relays. You would think that they would take this into account when designing their products.

Hope this information helps you with what you are looking to do.

God Bless,

It is amazing how many PLC manufacturers offer products with less than adequate output ratings, given the many uses for PLCs. Even with a 2 amp relay output, many applications still require interposing relays. You would think that they would take this into account when designing their products.

the problem with that is the power regulations would probably exceed the ratings of many of the IC components. Usually board manufactures place their line drivers and buffers at the beginning of a given board. which in turn gets distributed throughout the rest of the board. So the problem exists that if you want to drive 15 amps as an example to your output relays. Then you would need several times that as your supply current. Or a seperate power supply using optical isolation with the isolators acting as a switch only.