Terminals, Screw vs Spring

C

Thread Starter

Curt Wuollet

Hi all

As you know I am designing an open hardware project to provide a PLC form factor, PC compatible machine. I have a few questions regarding preference on terminals today.

I have used some of the Beckhoff stuff with spring terminals. It seems to work wery well in our environment as I haven't had to go tightening
things up in almost two years.

The last Open Hardware project used shielded screw terminals. Most stuff still uses these types of terminals. I have had to occasionally tighten screws on various stuff, my terminal boards haven't been in service very long, but I expect they will eventually loosen also.

What's the verdict on spring vs screw. The spring terminals are cheaper and seem to work better although it's difficult to visually verfy the connection. The screws are traditional. Is this a criteria for selecting IO?

How about spring with or without levers?

What about pluggable strips, are these a make or break feature?

Regards

cww
 
B

Bob Peterson

I don't think anyone that would use this device is going to care one way or the other. Since the whole thrust of the project is to make a cheap
substitute for a name brand PLC, why not save a few more bux and use the spring terminals. The pluggable strips are nice for changing I/O cards
without rewiring them, and my guess is that you should probably have them since it seems likely that to save a few bux those who use this thing will probably not be real religious about wire markers, drawing updates, and such.
I would only suggest one thing - use a pluggable strip that has a locking feature of some kind on it. I have seen some of these that do not stay on
real well and have had them work loose in the field. Best bet is some kind of screw to secure it in place, rather then a detent that just slips off.

Bob Peterson
 
G
Had a client a while ago who was very worried about the shock hazard from accidentally open circuit current transformers.

Particularly they were worried about screw terminals and plug in screw terminals (a la Klippon, Phoenix etc) because screws come loose and because the plug in types are easy to....not plug in!

They were quite happy with spring terminals though.

Personally I don't like spring terminals in tight spaces (where your hands block your view) as I don't think they give you as much tactile feedback as a screw terminal does (maybe that's just me though!?).

Obviously, if you go for screw terminals they HAVE TO BE rising clamp.

Another problem that can occur with screw terminals is stress on the solder joint (particularly on non-plated through PCBs where the copper only has a pretty tenuous grip on one side of the board), sometimes resulting in a broken joint, if the terminals are overtightened...

...and with either screw terminals or spring terminals you have to think carefully about the mechanical side of things to make sure the PCB is well supported around the screw terminal area to allow for the pressure during termination.

The one comforting part of making this decision is that the footprints of all the possibilities are pretty much the same (OK you need to choose 5mm, 5.08mm, 3.5mm pitch, one row, two row etc) so as long as you leave a little empty space around each connector you can probably swap between screw terminal, plug in screw terminal and spring terminal without re-laying the
board.

Are you going to have the IO connectors on the IO cards or are you going to have then on seperate termination modules?

If you're going for a seperate termination board arrangement it may be worth using a heavier board material (1.6mm FR4 is pretty flimsey).

Also if you have seperate termination modules you have the option of inserting signal conditioning modules on the inputs or interposing relays on
the outputs if you want to change the kind of IO you have without changing your basic IO cards.

...and you might have more chance of confining the charred remains of an accident to a cheap single sided PCB instead of your (relatively) expensive IO module!

Regards

Geoff Moore
====================================
Straight Forward Solutions Ltd
Maynooth Road, Prosperous,
Naas, Co.Kildare, Ireland
Phone : +353 (0)45 892739
Fax : +353 (0)45 893880
Mobile : +353 (0)86 8179683
email : [email protected]
====================================
 
L

Larry Lawver

Curt---

In my market, here's how my customers are lined up:

Volume OEMs go for the spring clamps as soon as they figure out the assembly time savings.

Integrators building one of a kind projects and the end users in general still prefer screw terminals. Adding one more wire to a random existing terminal is something they value highly.

End users that place a very high value on longterm stability are choosing spring clamps. The anecdotes are in, and systems built with spring clamp terminals are (at least anecdotally) performing more reliably than screw terminal systems, ten years later. Some of these end users previously required crimp terminations on wire ends; screw terminals appear to give them the reliability advantages without the initial expense.

Ideally, you should do both. If you had to choose only one, I'd recommend screw terminals to start, but have a plan to add spring clamps as soon as practicable. If the underlying value is there, the spring clamp fans will ride along until they get what they want.

Hope this helps!

Larry Lawver
Rexel / Central Florida
 
P
So far on this thread, there has been no mention Insulation Displacement Connectors.

In 1997 we used the Krone IDC terminations extensively for field I/O wiring in marshalling cabinets. This was in a paper mill situation with a Kraft Pulp Mill next door, and in a geothermal region of New Zealand (high levels of hydrogen sulphide in the air). The cabinets were IP 65 rated, but had no internal air purification system. There were something like 3000 I/O connections, and I have not heard of any problems since installation..

We are all used to IDC terminations in communications circuits, and appreciate the advantages, but as far as I know, this technology has not been widely adopted in the E & C world.

The Krone product which we used is called "Terminator" and when introduced
it could accept stranded cables up to 1.5 sq mm. I believe it has now been extended to 2.5 sq mm.

I believe Entrelec now have a similar product called "ADO Insulation Displacement Terminal Blocks"

Regards

Peter Green
Semi-retired control systems engineer
 
C

Curt Wuollet

Hi Pete

I did see them in my search. Sounds like a good thing for the conditions you describe. Enough contact pressure to be gas tight. I don't know how good they'd be if disconnected and reconnected a few times. I'm kinda surprised at the acceptance spring terminals seem to have after only ten of fifteen years. Maybe next generation.

Regards

cww
 
G
At first glance they always do appear to be a reasonable solution but there are a number of issues that you need to consider, especially when you're using PCB mounted IDC connectors as opposed to frame mounted IDC strips:

1. When you're terminating you apply A LOT of pressure, and hence you need to ensure that the PCB is not going to flex and damage traces/joints.

2. When you're terminating patch cables onto frame mounted IDC strips the frames offer all the cable management you need to make things neat and tidy, but you don't get this if the IDC connectors are PCB mounted

3. If you need to repeatedly connect/dissconnect a particular wire it will get shorter and shorter (because you need to keep chopping off the crimped part of the wire) which can really mess up your tidy wiring

I think that IDC connections for patch panels (ie where you terminate your field wiring prior to connecting it into your control equipment) is a good idea. It's relatively quick, it's easy to find signals, it's neat and tidy if you're well planned, it's quite easy to work with (get yourself a collection of isolation combs, breakout connectors etc and life becomes very
easy).

However, in my experience this is only the case on frame mounted IDCs, the PCB mounted IDCs need a lot of special treatment (ie thicker PCB material, more mechanical support, careful consideration of cable management requirements etc).

We used Krone exclusively and never had a problem with the connectors themselves but I haven't seen the type you mentioned that will accept 1.5mm cable. Do these connectors have a lower limit on cable size? One of the advantages of screw or spring terminals is that they don't really care what kind of wire you stuff into them as long as it isn't too big.

Regards

Geoff Moore
====================================
Straight Forward Solutions Ltd
Maynooth Road, Prosperous,
Naas, Co.Kildare, Ireland
Phone : +353 (0)45 892739
Fax : +353 (0)45 893880
Mobile : +353 (0)86 8179683
email : [email protected]
====================================
 
C

Curt Wuollet

Thanks for the opinion Bob.
My electronics will actually cost more than theirs.
I prefer better value to cheap substitute :^).

Regards

cww
 
B
Here's the benefit (ahem;) of my opinions (aka 'ranting'). I've come to prefer screw terminal plug-in terminal blocks. The "wire loosening" issue is there, but it's never caused me a problem so long as the screws are retightened after a year or so of operation. After that initial retightening I've not seen them loosen again. Plug-in blocks make card replacement fast (a big winner from the maintenance perspective).

Depending on the manufacturer I've found spring clamps problematic, especially when they are the high density type. There's one card in particular (the Opto 22 G4MSERVO, which they used for motion control on their older Mistic controllers) that alway had me wanting to run away screaming. They used lever type spring clamp terminal blocks soldered onto the PCB - on, I think, 0.150" centers. My hands would be cramped up like crazy after swapping one of these out, and (as has happened at least twice) sometimes the lever assembly would get jammed in the open position, and I had to solder the wires directly to the terminal block. This one application has soured me on them, although other ones I've been exposed to weren't too bad.

I don't like using them on anything where substantial currents are present. I've seen some on power supplies where heating caused the spring to loose tension, (causing more heating), then avalanche to failure.

Although it's not good practice to shove a bunch of wires under one terminal block we've all seen installations (even new ones) where this has been done. Spring type terminal blocks are much less forgiving than screw terminals in this regard.
 
C
I do the most robust pth sizes I can for exactly that reason. Big pads on both sides and a hole diameter generous enough to allow full wetting helps distribute the load.

> ...and with either screw terminals or spring terminals you have to think
> carefully about the mechanical side of things to make sure the PCB is well
> supported around the screw terminal area to allow for the pressure during
> termination.

I agree, the last project used the robust OST terninals that tongue and groove together. The taller ones add a measure of stiffness. And I intend to have the card guides extend all the way to the end of the card.

> The one comforting part of making this decision is that the footprints of
> all the possibilities are pretty much the same (OK you need to choose 5mm,
> 5.08mm, 3.5mm pitch, one row, two row etc) so as long as you leave a little
> empty space around each connector you can probably swap between screw
> terminal, plug in screw terminal and spring terminal without re-laying the
> board.

Actually, that's the problem. The spring terminals seem to want two holes. And I can't find spring and screw plug in terminals that use a common header, at least in the DigiKey and Jameco catalogs I am using as standards of ready availability. Plugins add quite a bit of cost. I wantto make sure it's warrented. I may have to order some of each type and see if I can make a universal pattern that can use whatever the users preference is. That's the way it should be.

> Are you going to have the IO connectors on the IO cards or are you going to
> have then on seperate termination modules?
>
> If you're going for a seperate termination board arrangement it may be worth
> using a heavier board material (1.6mm FR4 is pretty flimsey).
>
> Also if you have seperate termination modules you have the option of
> inserting signal conditioning modules on the inputs or interposing relays on
> the outputs if you want to change the kind of IO you have without changing
> your basic IO cards.
>
> ...and you might have more chance of confining the charred remains of an
> accident to a cheap single sided PCB instead of your (relatively) expensive
> IO module!

Up to a certain density I will use board mounted terminals. There's simply no way to do 64 inputs or outputs without remote termination. It also raises a problem of where to put the blinkenlights, I really like leds to show status but no rugged, inexpensive way to get them and terminals facing the card edge. I meant to ask about indicators too.

Regards

cww
 
M

Michael Griffin

<clip>
> Actually, that's the problem. The spring terminals seem to want two holes.
> And I can't find spring and screw plug in terminals that use a common
> header, at least in the DigiKey and Jameco catalogs I am using as
> standards of ready availability. Plugins add quite a bit of cost. I want
> to make sure it's warrented. I may have to order some of each type and see
> if I can make a universal pattern that can use whatever the users
> preference is. That's the way it should be.
<clip>

I would strongly suggest plug-in terminals. A certain large PLC manufacturer
didn't have plug-in terminal blocks on a recent design of PLC, and it was a cause of major complaints from their customers. When they re-designed the PLC and I/O, they included plug-in terminals.
People do look for this feature on hardware. If you ever have to change an I/O card, you don't want to have to take all the wires out and put them all back in again in the right terminals. This can often be very difficult to do once the machine is installed.

> It also
> raises a problem of where to put the blinkenlights, I really like leds
> to show status but no rugged, inexpensive way to get them and terminals
> facing the card edge. I meant to ask about indicators too.
<clip>

I've seen cards that used plastic light pipes for this. It let them put the LEDs in a convenient place on the board, but still be able to bring indication out to the case of the module. I don't know if any light pipes are
available as an off the shelf item.
I think people will expect LED indication for both inputs and outputs. LEDs
are one of the basic troubleshooting tools which maintenance people use. However, you don't necessarily have to have the LEDs beside each wire. Some I/O cards have them as a small array at the top of the card.

--

************************
Michael Griffin
London, Ont. Canada
************************
 
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Hi Curt

> I agree, the last project used the robust OST terninals that tongue and
> groove together. The taller ones add a measure of stiffness. And I intend
> to have the card guides extend all the way to the end of the card.

Excellent point! We're using double and triple height terminals now in "clumps" of 8 or 10 wide and they really do stiffen the board quite effectively.

Watch out for the reduced pin to pin isolation voltage if you go this route (if you have two rows of terminals you probably need to run the trace from the "front" terminal between the pads of the "back" terminals, so the air gap between the terminal pins is almost certainly reduced).

This probably isn't an issue though.

It's worth shopping around for screw terminals, I'm using parts from ECE now (have also looked at parts from an Italian company called Sauro) that turned out to be a lot less expensive than what I could find from "the usual suspects" and the quality seems pretty good too.

I guess it helps to have a friendly (hungry) distributor locally...

> > The one comforting part of making this decision is that the
> footprints of
> > all the possibilities are pretty much the same (OK you need to
> choose 5mm,
> > 5.08mm, 3.5mm pitch, one row, two row etc) so as long as you
> leave a little
> > empty space around each connector you can probably swap between screw
> > terminal, plug in screw terminal and spring terminal without
> re-laying the
> > board.
>
> Actually, that's the problem. The spring terminals seem to want two holes.
> And I can't find spring and screw plug in terminals that use a common
> header, at least in the DigiKey and Jameco catalogs I am using as
> standards of ready availability. Plugins add quite a bit of cost. I want
> to make sure it's warrented. I may have to order some of each
> type and see if
> I can make a universal pattern that can use whatever the users preference
> is. That's the way it should be.

Ah yes! You're absolutely right! I'd forgotten this. The "perfect world" part of my brain was still harbouring this false belief despite the
"Murphy's Law" part of my brain having discovered the truth long ago! 8o)

> Up to a certain density I will use board mounted terminals. There's simply
> no way to do 64 inputs or outputs without remote termination. It also
> raises a problem of where to put the blinkenlights, I really like leds
> to show status but no rugged, inexpensive way to get them and terminals
> facing the card edge. I meant to ask about indicators too.

Hmmm..this can be a contentious issue. If your IO changes state in the middle of the forest does the LED really twinkle if nobody is there to see it ?!?

There are some applications where the added cost, board space and power required to add indicators to every IO point cannot be justified (or found!).

In an application where you only see the edge of your IO card then you're going to be hard pressed to find space for indicators, especially on high density IO modules.

An option is to use surface mount LEDs on the back of the PCB but then you need to allow a gap between the modules and maybe add clip on light guides to re-direct the light through the gap.

Another option might be to have a "multi-function display" of some sort on unit somewhere. This display could then be used to show the states of individual IO modules or other internal status info etc.

Depending on the budget you could use an array of LEDs or a small graphical LCD. Perhaps you design a generic "user interface" interface and offer a range of different display/keypad options starting with the array of LEDs, a 4 digit 7 segment display and one push button?

Or to keep the cost really low, don't include any indicators or controls and have a PC based plant simulator that just plugs into a serial port.

It's a pity nobody is producing terminal blocks with built in LEDs (you can get RJ45s with built in LEDs), or perhaps somebody is....

Geoff
====================================
Straight Forward Solutions Ltd
Maynooth Road, Prosperous,
Naas, Co.Kildare, Ireland
Phone : +353 (0)45 892739
Fax : +353 (0)45 893880
Mobile : +353 (0)86 8179683
email : [email protected]
====================================
 
R

Robert Dannenfelser

I have used and specified the Spring type (usually WAGO) for about 8 years now after we had outstanding performance by them. You correctly pointed out that they don't come loose. And on a large project, we literally saved hours of labor not loosening and retightening screws to insert the wires. They are more expensive per terminal, so some folks who always go for the lowest MATERIAL cost, choose the older screw terminals. But I believe, from experience, that the lowest INSTALLED cost is the spring loaded style.

Bob Dannenfelser ----- PowerWorks Project Manager
(V) 603-430-7084 (F) 603-431-9575 (C) 410-227-6718 (H)
410-282-3175

"What the Mind can conceive and believe, it can achieve, thus, we become what we think about!"
 
C
Hi Geoff

Yeah, despite remarks to the contrary, this Open Hardware gets the same care in engineering that paying projects get. I'm not sure why anyone would think it inferior simply because I'm doing it in my free time. It's a curious misperception. After all, I do get paid to design stuff. I am something of a generalist, but PLC hardware isn't particularly special as far as electronics go. The old aerospace stuff was far more difficult as you had to _make_ everything perform over temperature and environment. Now you can simply buy IC's guaranteed to work and it's far better stuff. And I can do a SPICE simulation on my Linux box in a few minutes. Lots of people like the emperor's garments I guess.

Regards

cww
 
I am curious about the final descision on the terminal questions.

As I never saw anywhere in the positive spring type descriptions that the Cage Clamps not only offers better connection quality, but also gives you a much lower resistance in the contact, resulting in less voltage drop over the terminal and thus resulting in a HQ electrical connection.
 
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Hi Gunther

My Open Hardware project has been severely impacted by my need to find gainful employment. I intend to continue it even though I may not be
employed in automation. The Evil Empire won out at the last place and R&D on Linux projects was banned. This lead to a parting of the ways. It may be some time before I am situated well enough to resume. What I decided to do is see if there are pluggable and non pluggable connectors that have the same or at least non-conflicting footprints (PC board layout). Then in a perfect world, the cost concious could use either spring or screw non-pluggable connectors and the pluggable variety could be either spring or screw. High density IO would have to use ribbon connectors and cable and mount the terminals on small boards that are DIN rail mountable or perhaps there are OTS ribbon terminal strips. I have considered finishing this up and trying to eke out a living selling kits for this and the many automation gadgets I've designed. But market research in automation s nearly impossible and
I'm not independently wealthy, and winter is surely coming to Minnesota. So, it's probably gonna be a while before there's an Open Hardware alternative and that right now, looks optomistic. With the age discrimination and the backwards technology market in MN, I may end up fixing cars.

Regards

cww
 
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