Short circuit protection of PLC's digital outputs

RisingEdge

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Join Date
Aug 2022
Location
Belarus
Posts
11
Hi group!

Repeatedly found answers to some questions in the PLC's programming world, now I decided to ask the question directly) Now I am designing I/O terminal, discrete signals, analog, PROFIBUS, MB, and so on... The plans are cool) But the conversation will not be about this, but about discrete outputs, if not strange... I have already been tormented with protecting the output transistor from a short circuit, the main question is what the device should do when one of the outputs is short-circuited. In my subjective experience, we have 2 options: limiting the current or disconnecting the transistor from the switched bus, 24 or 0 volts. Self-restoring fuses or just using fuses is somehow not nice ... Although using an internal fuse is still in the plans)

I’ve been working on options with current limiting for a week now and it doesn’t lead to anything good, Ohm’s law still can’t be deceived ... To limit the current, you need a couple of parts: an amplifier to measure the shunt current and a comporator that will control the gate of the output transistor. But in the event of a short circuit, all voltages will drop across the transistor, and if we limit the current to 500 mA, then 12 watts will be released on the transistor. The digital is brutal) I immediately refused this option. There is an option with switching to the current source, in case the load current exceeds the setpoint. But the best option that I found is integrated voltage / current stabilizers. In this case, the power released will not be a high, up to 2 watts, but the internal stabilizer circuit will take 2-3 volts. And from beautiful 24 volts it will be 20-22...

I like the option of disconnecting the transistor from the switched bus more. We measure the current with an isolated amplifier and push it into the uC. In case of excess current, disconnect the transistor from the bus. It seems to be beautiful ... But there are some minuses, and the main problem is the processing of this signal in uC. Although, I have enough experience to competently and accurately respond to overcurrent events.

However, the very concept of disabling the output is in question, because i have never seen such functionality.

The reverse of some controllers also did not please, somewhere there is no protection at all, and where I did not figure it out)

Disconnecting the transistor from the bus is good, but it’s not possible to do such a function for every channel) In addition, I want to get away from the usual source/sink output and use a push pull. My Wishlist) And why not ...

In general, I have the following questions for you: what should be the reaction of the device in the event of a short circuit (limitation or shutdown)? Is the concept of such an exit good (push pull)? Is using resettable fuses doomed to failure?

I will be glad to give reasoned answers and wish you good luck in mastering the world of PLC's programming!
 
To be honest, I have been in this game for over 45 years, only on one system have I had a short destroy outputs, that was 110v Triac card, this was because no protection i.e. fuses & 110v direct acting solenoids for steam control so heat was also a major factor, replaced these with a 24v card, 24v actuated air solenoids, so never really had a major problem only others were occasional relay failure but these were relay output cards where they were driving coils at a rate of 45-50 per minute & as a quick calculation had done over 4 million operations, All systems I have come across generally if a short occured would recover or blow a common fuse to the card without destroying the transistors.
I have done projects where the specification required every output to be protected with either a fuse (fast acting) or semiconductor fuse.
 
Yes & in some cases only one fuse per card feeding the supply These were mainly fused terminals for external connections.
 
I understood you, I also saw such solutions, but the E-cabinets in which I saw it were very old) I would still like to have internal protection / short circuit resistance...
 
i'm not sure if this will help, but this is one of the systems we use.

we use AB plc's and Balluff i/o link blocks connected to the ethernet network.
we have aoi's that detect a short in the output and gives us an alert. Balluff has aoi instructions but the boss does it a little different.
doesn't blow fuses, just shuts off the output when a short is detected.
you never mentioned the type of plc you were using.
i hope this helps
james
.
 
That's where I'm a little confused, Reading it again seems to imply that it is a design of output cards of some description, why ? have a long way to go to get it on the market I would guess.
 
Every system design has to balance cost against function, you can design any system to have multiple protections that NASA would be proud of, but at what cost. Is it cheap and easy to replace a PLC output module, or expensive, and a real pain. Does the failure of a machine cause huge financial losses, or can they make things up on the next shift. My personal choice is to only use PLC outputs to drive relays, mechanical or semiconductor, relays are easy and cheap to replace. I always choose relays that have LED and mechanical indication, it makes finding problems a lot easier, though again, it adds to the cost. So far, and hoping not to anger the Electrical Gods :), I haven't had a PLC output fail, and I am very very old, and have done this for many many years.
 
On many PLCs (most PLCs I believe), the outputs have current limiting built in.
So the outputs should not take any damage from a short circuit. The output will not function, but when the short circuit is fixed the output will return to functioning.

Some PLCs supports test functions that can detect short circuits even if the output is not activated. A small testing voltage is applied instead of the full voltage. Usually safety (F) outputs supports this, but I have seen it on some non-F outputs as well.
 
Hi, a few places I've worked the company had interposing relays on all outputs. All the output itself did was fire the relay. As an extreme example, one could have 120VAC going through the relay contacts and the output would be isolated from any short circuits or other electrical overload issues. Most places now don't spec out panel space for all those relays. Hope this helps and good luck!
 
James Mcquade, I assumed that such functionality exists, but before your words, I had never heard or seen about it anywhere. Thanks for the info!
 
JesperMP, everything is right. Today, almost all PLCs have short circuit protection, but at what cost. As BryanG said, in many cases they just change the whole module, it's easier and faster. At least that's what I do too. Of course, then everything is restored) But still, protection is needed and Ohm's law still cannot be beaten. With a current limit of 500mA, we will have 12 (24 * 0.5) watts of power on the key element, in which case each of the discrete outputs will have to be equipped with a boxed processor heatsink) But I didn’t know about the low test voltage for diagnostics, the idea is not bad, thanks for the information.
 
parky, I'm developing a bus I/O terminal. To work with many PLCs. Everything should be relatively inexpensive and affordable, I'm talking about components. Many issues have already been resolved, even issues that I planned to devote to several weeks were resolved in a few days, but the protection of discrete outputs resists) Who could think...
 

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