You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

Reply
 
Thread Tools Display Modes
Old August 17th, 2022, 01:18 PM   #1
RisingEdge
Member
Belarus

RisingEdge is offline
 
Join Date: Aug 2022
Location: Belarus
Posts: 11
Short circuit protection of PLC's digital outputs

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)

Iíve been working on options with current limiting for a week now and it doesnít lead to anything good, Ohmís law still canít 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 itís 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!
  Reply With Quote
Old August 17th, 2022, 02:01 PM   #2
parky
Member
United Kingdom

parky is offline
 
parky's Avatar
 
Join Date: Oct 2004
Location: Midlands
Posts: 4,295
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.
  Reply With Quote
Old August 17th, 2022, 02:16 PM   #3
RisingEdge
Member
Belarus

RisingEdge is offline
 
Join Date: Aug 2022
Location: Belarus
Posts: 11
Are you talking about external fuses on each discrete output?
  Reply With Quote
Old August 17th, 2022, 02:18 PM   #4
parky
Member
United Kingdom

parky is offline
 
parky's Avatar
 
Join Date: Oct 2004
Location: Midlands
Posts: 4,295
Yes & in some cases only one fuse per card feeding the supply These were mainly fused terminals for external connections.
  Reply With Quote
Old August 17th, 2022, 02:27 PM   #5
RisingEdge
Member
Belarus

RisingEdge is offline
 
Join Date: Aug 2022
Location: Belarus
Posts: 11
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...
  Reply With Quote
Old August 17th, 2022, 08:38 PM   #6
James Mcquade
Member
United States

James Mcquade is offline
 
Join Date: Oct 2007
Location: Nashville, Tennessee area
Posts: 3,387
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
.
  Reply With Quote
Old August 18th, 2022, 02:28 AM   #7
Saffa
Member
New Zealand

Saffa is offline
 
Join Date: Feb 2012
Location: Bay of Plenty
Posts: 1,384
If I understand the OP correctly, he's designing the circuitry for some form of PLC card. So this is all internal to a card?
  Reply With Quote
Old August 18th, 2022, 04:11 AM   #8
parky
Member
United Kingdom

parky is offline
 
parky's Avatar
 
Join Date: Oct 2004
Location: Midlands
Posts: 4,295
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.
  Reply With Quote
Old August 18th, 2022, 04:48 AM   #9
BryanG
Member
United Kingdom

BryanG is offline
 
Join Date: Feb 2005
Location: Manchester
Posts: 1,776
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.
__________________
Knowledge is power, share the knowledge.
  Reply With Quote
Old August 18th, 2022, 05:03 AM   #10
JesperMP
Lifetime Supporting Member + Moderator
Denmark

JesperMP is offline
 
JesperMP's Avatar
 
Join Date: Feb 2003
Location: ᚴᚬᛒᛅᚾᚼᚬᚠᚾ
Posts: 15,570
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.
__________________
Jesper
NOTICE:
JesperMP can be impolite, has passive-aggressive tendencies and may use sarcasm !
Also: ᛁᚠ ᚢᚬᚢ ᚴᚬᚾ ᚱᛅᚬᛏ ᚦᛁᛋ ᚦᛅᚾ ᚢᚬᚢ ᚼᚬᚠᛅ ᚴᚬᛁᚾᛅᛏ ᛘᚢ ᚱᛅᛋᛒᛅᚴᛏ
  Reply With Quote
Old August 18th, 2022, 06:07 AM   #11
saultgeorge
Lifetime Supporting Member
United States

saultgeorge is offline
 
saultgeorge's Avatar
 
Join Date: Jul 2015
Location: Detroit
Posts: 525
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!
__________________
"Instant success teaches one nothing. Failure provides the tools and instructions one needs to succeed."
  Reply With Quote
Old August 18th, 2022, 07:49 AM   #12
RisingEdge
Member
Belarus

RisingEdge is offline
 
Join Date: Aug 2022
Location: Belarus
Posts: 11
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!
  Reply With Quote
Old August 18th, 2022, 08:00 AM   #13
RisingEdge
Member
Belarus

RisingEdge is offline
 
Join Date: Aug 2022
Location: Belarus
Posts: 11
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.
  Reply With Quote
Old August 18th, 2022, 08:08 AM   #14
RisingEdge
Member
Belarus

RisingEdge is offline
 
Join Date: Aug 2022
Location: Belarus
Posts: 11
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...
  Reply With Quote
Old August 18th, 2022, 08:10 AM   #15
RisingEdge
Member
Belarus

RisingEdge is offline
 
Join Date: Aug 2022
Location: Belarus
Posts: 11
Saffa, yes, short circuit protection will be located inside the terminal
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
Short circuit protection of ET200S HF outputs oregonsam LIVE PLC Questions And Answers 0 January 11th, 2007 12:08 PM
BCD input to 13 assigned outputs on a FX Skeggy LIVE PLC Questions And Answers 7 March 12th, 2006 03:58 PM
Stepper control direct from PLC Digital Outputs? girevik LIVE PLC Questions And Answers 4 January 3rd, 2006 02:30 PM
PLC'S DC OUPUT and its application for ambient temperature digital display jebrietszkie LIVE PLC Questions And Answers 2 August 29th, 2003 06:59 PM
PLC'S DC OUPUT and its application for ambient temperature digital display AND DIGITA jebrietszkie LIVE PLC Questions And Answers 0 August 29th, 2003 01:36 PM


All times are GMT -4. The time now is 03:33 PM.


.