Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in

PLC Digital Output abilities 4

Status
Not open for further replies.
itsmoked

itsmoked

Electrical
Feb 18, 2005
19,114
0
0
US
I'm working up a control panel loaded with a customer's penchant for HOA (hand-or-manual) switches everywhere.

I'm using a Momentum (Modicon) PLC. Since the manual is pretty poor I haven't been able to figure out:

1) On a typical 120Vac output(not relay) what happens if I apply 120Vac to an OFF output? I can't think of a reason for damage but don't want to toast the thing "learning".

2) Also what is your recommendation about directly running something like a small hydraulics solenoid with these outputs directly? Bad idea or not?

I have 0.5A outputs. (170ADO54050) Page 9

Thanks for any insights.

Keith Cress
Flamin Systems, Inc.- <
 
Replies continue below

Recommended for you

Sort by date Sort by votes
itsmoked,

I have just completed my final year college project using a Modicon Momentum, I would be glad to answer your questions.

1. The internal circuitry of the discrete output base is arranged such that 120 Vac is supplied to row 1 on the base. This input is to be fused (according to Schneider Electric's advise in the databook) with a 10A fast blow fuse. This provides protection to all 16 output terminals in row 1.
The fused 120 Vac supply is connected to the MT1 leg of the triac which is internal to the controller, while the load to be controlled is connected to the MT2 leg of the triac (that is, the "load" leg), so that voltage (and current) is only supplied to the load when the triac is turned on by the processor.
So, to answer your question, applying 120 Vac to an "off" output is perfectly acceptable - in fact, that is how the device has been designed.

2. In my project I used 24 Vdc solenoids. Because of the reverse EMF genrated when current to a solenoid coil is interrupted (Faraday's Law of Electromagnetic Induction and Lenz's Law), I soldered a 1N4002 rectifier diode directly across the coil terminals. The diode was connected such that during normal operation (solenoid being turned on), the diode was in reverse bias. When the output was turned off, the relatively large reverse EMF (of opposite polarity to the original source i.e. the output terminal) would forward bias the diode and provide a low-impedance path for current as the magnetic field collapsed.

Since in your application the output solenoids are rated for ac, it would not be feasible to use diode. Rather, a suitably-rated MOV (Metal-Oxide Varistor), perhaps 120 Vac, 170 Vpk, wired across the solenoid coil terminals would provide the same low-impedance path as in my application.

I didn't know whether in the dc card such transient voltage protection was provided, so I aired on the side of caution and installed the diodes.

Also bear in mind that you cannot exceed the maximum current avaiable from the PLC output when driving any load. (Check your hydraulic solenoids for their VA rating to ensure that you can operate them safely). You may need to consider interposing relays if the coils draw too much current.

Hope this helps.

Richard.
 
Upvote 0 Downvote
Richard; Thanks much. That helps.
Thanks for clearing up the power structure aspect.

I actually will also be using 32-DC outputs and 32-DC inputs. I'll stick in the diodes though what a pain in the butt! I wish they'd put them in in the first place.

How'd you like the Momentum? Did you use proWorx Lite? Which form of programming did you end up using? LL or ? Did you use an Ethernet link or serial? Any, "I wish I'd done this differently" thoughts?

Wow, sounds like an ambitious project you did!

Thanks again.

Keith Cress
Flamin Systems, Inc.- <
 
Upvote 0 Downvote
I didn't expect to be using the Momentum at all at the start of my project. It turned out that my contacts at Schneider Electric only had that PLC to lend me, but it worked out to be great to work with.

While the programming software I used (ProWorx NxT) wasn't as powerful as Allen Bradley's RSLogix500 or Siemens' Simatic Step 7, I found it easy to learn.

I used Ethernet to connect a PLC and a Magelis touchscreen, via a 4-port D-Link router. The only problem I came across with the networking side of things was specifying the PLC's IP address (I had selected BOOTP as the method of assigning IP addresses, so that each time the PLC was turned off, it appeared to lose its address). A couple of calls to Schneider's tech-support guys and I was back on track. They sent me a DOS-executable file called BootP Lite, which assigns IP addresses to the MAC address of the controller you specify). Let me know if you need it.

One other thing regarding the IP address - I had to assign an area of the processor for use as "Configuration Extensions", so that networking data could be stored by the controller. Then, when the IP address had been specified, I wrote everything to the controller's flash memory so that every time the controller was turned off, it would load all it needed from the flash memory.

Programming was all ladder logic - all that ProWorx NxT can handle. The program I wrote was quite simple, involving inputs/outputs and a few math instructions for my touchscreen interface.

Again, I hope this helps,

Richard.
 
Upvote 0 Downvote
Hi Keith

Yep, them diodes are a pain. I've used Releco relays w/ great success in the past. They have a built in clamping curcuit, no need for diodes. Plus quite a few other nice features; Coil "On" LED, Armiture position indicator, Manual overide.

They are distributed by TURCK. You might want to give them a check.

Ed
 
Upvote 0 Downvote
For those in our reading audience that may not be familiar with BOOTP...
When BOOTP (short for Bootstrap Protocol) is enabled, the device expects to be assigned an IP address each time it is powered up.
If your system is like most PLC based systems, the device needs to have a static IP address, so that other devices which communicate with it can always find it.
There are a couple of ways to do this. One way is to simply disable BOOTP, and assign an IP address to the device.
The other way is to use a piece of software known as a BOOTP server. This is a program which listens for devices requesting IP addresses, and then assigns an IP address to the device. By knowing the MAC address of the device, you can configure the BOOTP server to always assign the same IP address to a particular MAC address. This works fine if your system always includes a PC for the BOOTP server to reside on...
Allen-Bradley users may be familiar with the BOOTP server which typically comes as part of RSLinx.
 
Upvote 0 Downvote
Hiya-

Just an aside from the computer side of things. The protocol
you are refering to may also be referred to as DHCP
Dynamic Host Configuration Protocol.

The BOOTP protocol is also available on most distributions
(distros) of linux. Windows provides the same service on
their server distributions.

If you are on the corporate lan with your PLC, might I humbly
suggest that you coordinate with LAN powers that be. Either
They can:

1. Provide the BOOTP protocol either tied to a specific IP
address (where indeed, the PLC looks more like a server).

2. Can assign a static IP address for your particular PLC
or other computing device.

3. Can assign a static ip addres to your "system always
includes a PC for the BOOTP server" to reside on, which I
would suggest doing some NAT work, let it be the server
for any data base and data collection, and mail server for
the subject PLC and it's brothers/sisters on a private lan
of the PLC world. The PC acts as the gateway into the PLC's
private LAN space. This PC too can act as the data
reduction engine and web server showing performance/history
information to all or a select few.

With this, then data collection from the PLC may be done at
the desk of the engineer, not wandering down to the factory floor just to
"take a look at the critter".

So, even if you do establish your own LAN for the PLC(s), it would be
worthwhile to at least have an insulatled connection to it
via the corporate lan.

Hope this helps.

Cheers,
Rich S.
 
Upvote 0 Downvote
Keith,

I'm curious: how did you find the pricing on the ProWorx and Concept software? I was fairly shocked - shocked enough to consider dumping the whole Schneider Quantum PLC and replacing it with an SLC-5 or an S7. I figured the time spent moving the application over onto the A-B or Siemens platform would offset well against time spent learning how to use the new software. I was only discouraged by the likely loss of warranty on the whole system which the PLC formed part of.

Check out some obsolescent software called MODSOFT - it's a clunky DOS programming tool but is copmpatible with some if not all of Schneider's PLCs. You might get a cheap copy somewhere out there on the 'net.

----------------------------------
image.php
I don't suffer from insanity. I enjoy it...
 
Upvote 0 Downvote
ScottyUK; I think the price is ridiculous.. They will be slowly run out of the market by such nonsense.

The job required "a Modicon, Square D" PLC or I wouldn't have gone there at all. I'd probably have whacked out an embedded controller and provided a spare in the box. But that's a different story.

Presumably the customer is familiar with this "brand" and has blown the bucks on the ProWorx32 full Monty. I just got the "Lite" version for just over $1k instead of $5k.

I'll make a note of MODSOFT but hopefully the Lite will work.(since I now own it)

fangas; I dove on that relay-with-diode thing... Cripes. The idiot relay makers charge about $20 to add that $0.03 diode into the relay. I could pay someone to pry them open and add them for less than that!!!
A $4.82 relay goes to $25.32 for a diode adder.. Hence no one uses them so then no one stocks them. It was a nice dream. [cry]

Keith Cress
Flamin Systems, Inc.-
 
Upvote 0 Downvote
We got the same nonsense on pricing, except the $'s were £'s. And I thought A-B were expensive. Hell, A-B are expensive!

Telemechanique are a Schneider company - they have some other PLCs under the brand name 'Twido' in Europe which is pitched below the Modicon range. I'm not sure if the pricing is any better - when A-B and Siemens make good reliable products and their software is sensibly priced, I haven't had the urge to find out more about a something named after a character from Buck Rogers.

----------------------------------
image.php
I don't suffer from insanity. I enjoy it...
 
Upvote 0 Downvote
Hahaha Twido(like PLCs) are the thing to come. It is so inexpensive they hate selling it. I was told they make like 32 bucks on each one. You have to pay for any tech support(makes sense) but they come with Ethernet for programming and a lot of power. Someone in here said the programming SW for it is abysmal. I have no direct experience. I was eager to try it on this job until tech support pointed out that it was most definitely NOT a Modicon PLC. Shucks.... The Momentum is even over kill.

Keith Cress
Flamin Systems, Inc.- <
 
Upvote 0 Downvote
Keith,
18 years ago I worked for the Automation Products Division of Square-D (prior to the Schneider buyout). At that time, we (field people) BEGGED them to reconsider their scalping policy on software. Their attitude was that if Allen Bradley can get away with it, so can we. At that time A-B had a 42% market share on PLCs, so they felt that customers must be OK with forking over their hard earned cash for something that only works with one product and is necessary to make it work anyway.

As much as I disagreed then, and still do, I have to admit that they were apparently correct. A-B is still the US market leader in spite of their gouging people for the software necessary to make their stuff work. I suppose that if you are an also-ran but want to be considered a serious player, you have to stick your knife in and twist it along with them! (Do I sound bitter?)

By the way, as to applying 120VAC to the back side of a triac output; I know this sounds basic, but you would be suprised how often I have seen this problem. Just make sure the 120V source is the same (phase) as the one feeding the card! If it comes from an outside source and that source was derived from a different phase, the potential between them is now 200- 240V and the traces to the triac may flash over. Many of them have only 150V isolation spacing, not the typical 250V.
 
Upvote 0 Downvote
Oh yeah jraef thanks for the heads up. I considered this but infact am locally deriving the the 120Vac from the in-enclousre 480/3ph. So there will only be the one species.

It also turns out that the triac outputs on the cards I already chose are rated at 120V,0.5A and I have been informed that the chosen(buy others)directional hydraulic valves draw 0.49A steady state.[cry] So I will be forced to interpose relays.

Software;
That is a bona fide business method. Charge the victim enough that if they actually spend the money they will have a mental problem spending the "same" money again elsewhere.. Locking(mentally) themselves into your business.


Keith Cress
Flamin Systems, Inc.-
 
Upvote 0 Downvote
As cumbersome as it sounds, I am a fan of interposing relays in non speed critical applications. Relays are sooooo much cheaper and easier to replace. I did a few prison wide door lock systems, and we used interposing relays on all of the lock solenoid circuits, thousands per prison. It was worth it in the long run though. Solenoids can be a nasty load for a triac, especially as they get old and/or dirty and start to chatter.

Eng-Tips: Help for your job, not for your homework Read faq731-376 [pirate]
 
Upvote 0 Downvote
jraef,

Even the steep pricing by A-B at approx £500 for RSLogix 500 (a few years ago) pales into insignificance against the £5000+ I was quoted by the thieves at Schneider for their equivalent software. The salesthief was quite offended when I asked if he realised that it made economic sense to throw his PLC in the skip and replace it with an A-B or Siemens product, and that I would still have money in my hand after the application was migrated.

If we had a huge installed base of Modicon equipment I wouldn't mind the software price so much, but this is a one-off. We'll just subcontract any work on that PLC out to a third party, and make sure no more of the damned things get on site.

----------------------------------
image.php
I don't suffer from insanity. I enjoy it...
 
Upvote 0 Downvote
The Twido is a re-badged Idec (Izumi) with a different set of firmware. The software is terrible and I have thrown it away. It is even worse than Schneider's normal software. By the way, try pricing the up market stuff, it is a dear as ABs up market stuff.

For relays with diodes and LEDs I normally use Omron G2R series. They are much more reasonably priced and Omron are the world's largest manufacturers of relays.

I do not consider AB and Siemens software to be reasonably priced. In fact it is quite expensive in Australia. I do find Omron software reasonabley priced. When you also consider that Omron CX-One programs all the PLC's, except the smart relay, touch screens, Device Net Configurator, Simulator, all NC type cards, Thermo etc etc it is quite a bargain.
 
Upvote 0 Downvote
Idecs huh? Thanks for the info. I think a door to door sales man hit me up with the original. It looked like a tiny stereotypical PLC that had been run through a hot wash cycle. The price was incredible. It was less than a hundred bucks for a sizable I/O inventory. I had no use for any PLC at the time so I never sampled the fare.

I went with Omron MY relays in sockets.

I would've used used something like the Omron mini one-box PLCs if been allowed for this job.

Keith Cress
Flamin Systems, Inc.-
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

HydroBuff
  • Question
Remote PLC Updates and Diagnostics Via Ethernet
Replies
0
Views
14
HydroBuff
HydroBuff
506818
  • Locked
  • Question
Multiple PID Loops (PLC)
Replies
3
Views
18
xnuke
xnuke
cames
  • Locked
  • Question
PLC Control for small lab setup automation help
Replies
5
Views
12
3DDave
3DDave
SA07
  • Locked
  • Question
Can we operate Emerson Scada system/Server with Siemens PLC 400 ?
Replies
1
Views
18
thermionic1
thermionic1
Gor_10
  • Locked
  • Question
GE Fanuc series 90-30 plc
Replies
2
Views
20
cliff reed
cliff reed
Back
Top