Interposing relays

[Lancie1]

I think that there is a general trend of lowering all control circuit voltages, not just those devices used with PLCs. One reason is safety: it certainly is less likely that someone will get killed working on 24 volt devices than 120 volt or 240 or 480 volt devices. At one time, I worked on 480 volt control circuits, with stop and start buttons having 480 volts across the terminals. I had much rather work on control circuits with 24 volts, whether 24 VAC or 24 VDC.

 

[Geospark]

...24 VDC has been a standard for a long time, the main reason it is used is because there are many devices to choose from and because they are made in large quantities the pricing is kept in line.

Chicken?...0?....Chicken?...0?

The industry steered towards 24VDC for a few possible reasons.
PLC's grew out of the automotive industry.
12/24VDC car batteries
It's high enough to prevent false positives that may occur at 12VDC, but low enough to be relatively safe.
DC is 0Hz, no inductance or capacitance.
AC over long runs creates capacitance in the cable, storing energy, that can potentially hold a coil closed with an output off.
DC supplies tend to be more stable, withstanding shorts.

There are cons to using DC as well, but whatever the reason we have moved that way, in the main, for technical advantages, reliability, durability, safety.
It's supply and demand, but demand comes first. Manufacturers supply, but they didn't create the demand, the PLC industry did, the innovators, designers, the end users, you did!

The PLC industry sets the market, DC is best suited, but AC is also available.
The manufacturers of PLC auxiliary equipment just follow suit.
Nowadays, though, more manufacturers are doing it all. So they have some control over the shape of the market.

I always try to look at the bigger picture. Things are evolving. What are we trying to achieve at a basic signal level?

To start a simple process, Joe presses a "message sender" to a "message receiver" which tells a "decider" to move a "persuader", if a "preventer" allows it.

We've always used the tried and tested electric signal to achieve this as it made sense to switch the voltage the intended "persuader" requires to be started and use similar voltage to monitor the status. But we no longer should need to do this? Now that the "decider", i.e. PLC/PAC, can receive a message from a "message sender" and send a message to a "persuader" in different ways, only the "persuaders" voltage to drive it is required.

So DC beware! AC beware! Control circuits beware!
With the rapid development of Ethernet/IP, it is quickly replacing the requirement for any kind of conventional control circuit in industry. Everything will be packets, not signals, message and receive, not energise and de-energise, only the motive power required to drive "persuaders" will be left at the control voltage level. Less and less push buttons, selectors, lamps, pots, thumb wheels, readouts. All will be done on a touchscreen. Discrete and process I/O devices will all be Ethernet.

In 10-15 years this conversation will look very old to the younglings seeking advise, if it's still even relevant!

 

[Lancie1]

In 10-15 years this conversation will look very old to the younglings seeking advise, if it's still even relevant!

Yes, well there is still that little nagging problem that Thomas Edison had with his early DC power stations: low-voltage DC does not travel well. In large plants spread out over big areas, 120 VAC control circuits are still very common.

 

[Geospark]

Yes, well there is still that little nagging problem that Thomas Edison had with his early DC power stations: low-voltage DC does not travel well. In large plants spread out over big areas, 120 VAC control circuits are still very common.

Yes true Lancie1, one of the cons of using DC I didn't bother mentioning. As I said earlier, it's horses for courses. Everyone, has different regulations and preferences. You can use DC for relatively long distances for signalling, but it's prohibitive for anything else. I've seen the effects of both at long distance. But on the whole I feel those reasons were why they predominantly leaned towards DC. Just my humble opinion. I'm very interested in others.

Either way, my overriding statement was that this topic may be nostalgia sooner than we realise. Why use voltage based I/O at all anymore when there are safer, easier methods available?

Those long runs are now starting to carry protocols instead of potential and POE(Power over Ethernet), although the jury's out on that one. I can see panels becoming bus-based with little to no wiring, other than patch cords and motor leads, supplying the only inductive circuit to be seen.

I'm generalising alot, I know. Ther are many specialised control systems that require specifics. I'm referring mainly to the fundamental controls we all use every day, particularly the discrete I/O we are discussing the best practices of switching here.

I'm not ringing the death knell of electricity! Just questioning it's necessity in today's control world, beyond supplying and driving devices.

 

[RussB]

Chicken?...0?....Chicken?...0?

"A little nonsense now and then is cherished by the wisest men".

Credits:

 

[iant]

the early shoebox PLC's (1980's) came out with there own 24Vdc power supply.
the bigest issue in the early late 70's to early 80's was being able to purchase non contact sensors othe than reed swicthes.
these (prox's etc.) were available in 110/240 Vac and 24dc
there were very few 24VAC or 32VAC units available.
We usually fitted in a V?Prim/24V sec transformer with a rectifier and Cap for DC.
As the progress moved forward more 24v DC gear came on the market.
I might add that Temperature controllers are still heavily AC powered.
Now days there is a huge push to lower all control voltages to less than 50V Extra Low Voltage (ELV).
the principle for this is to make it safer
AND ALSO TO
remove the restrictions on Panel access by non qualified professionals.
In Australia only an 'A' grade electrician can access panels with voltages between 1000V ~ 50V Low Voltage (LV) - voltages above this requires additional qualifications because they bite with out touching.

 

[BobB]

I use 24VDC interposing relays for several reasons
1) I use 24VDC powered PLCs and have 24VDC readily available.
2) The relays are driven by transistor outputs that do not wear out and, generally, do not fail because the relays I use all have flywheel diodes in them.
3) They have a very low current draw.
4) They are predictable with respect to their switching times, both on and off.
5) They are low voltage.

On the other hand, I normally use 24VAC for switching contactors via the 24VDC interposing relays - 24VDC contactors are huge, expensive, and the PLC transistor outputs will not switch them due to the load they would place on the transistors.

 

[iant]

Hey BobB there are some realy nice Schneider low current DC contactors now. And they have competitive pricing.
Also ABB has a range:beerchug:

 

[BobB]

Hi Ian - still not competitive for a board builder though - will have to talk to my rep.