Why DC voltage?

Because most PLC's (I don't know of any which work on AC internally) have DC power supplies, and therefore have DC inputs and outputs.

If AC control is needed externally, then it's easy to connect PLC outputs to DC input/AC output solid-state relays or something of that nature.

Does that clear things up? If not, please respond.

Good answer!

In addition:

• 24VDC is relatively safe in the hands of the inexperienced technician compared to higher AC voltages.
• The same power supply can be used for all the analog signals as well as the digital ones.

In addition to SparkY's reasons, DC circuits radiate less EMI and are generally less susceptible to interference.

AC's main advantage is the ease of converting one voltage level to another via a simple passive component (transformer), not so easy with DC.

I was told many year back to use DC Solenoids on any circuit that must maintain a constant time sequence. The reason given was and AC Solenoid needs the Sine Wave at or near the peak to get enough power to move the armature that shifts the valve. I was told DC Solenoids operated the same every cycle since the power was there no matter the input timing.

It sure sounded logical to a Non-Electrial person.

The other reason I have heard is due to Electrical Shock from the normal 110VAC or the occasional 220VAC is capable of that is less prevalent with the lower voltage DC ccurrent.

Like I say, it sounded logical to a non-electrical person.

Safe and simple.

James

...and cheaper and safer, easy to maintian!!!!!!!!!!

Also:- Many hydraulic components were originally devised for the mobile/automotive industry. It's mainly 12 and 24vdc there!

I would agree with all other contributers as to the ease of control on dc as, if we have a 240 ac input to a solenoid coil, we frequently need to convert to 207 dc with a rectifier plug so that we can 'adjust' the operating properties of the coil.

Hello there,

I am by no means an 'expert' but, bare in mind you are using power around water.......Using a DC power supply I would think is safer. If water gets into the DC power system the amount of damage will be isolated to the DC supply motor and relatively small area around it. If it is AC all kinds of precautions have to be taken in case water 'shorts out' any connection. It could go as far as starting a fire down the line away from the immediate power trip.

Of course I could be well off target with this answer but, I am sure a more expert person will answer your question.

Take care

jfmfit

Hello jfmfit

The electrical "leakage" problem are equal, whether AC or DC.

Kind Regards....

Hello Sparkstation,

I think...........I KNOW you are far more expert on this because I can barely wire up a three pin plug! So you have me there. If as I at first thought the electrics was around a possible source that could short them out, would not it be simple first, to have an all in one DC supply right where the power is needed, and, with a DC supply it could be say down to 12 or 24 volts and so everything is more manageable, including may be smaller motors if there is any and finite control, which is sometimes more necessary?

I am really interested in this as I can wire stuff 'parrot fashion', as long as there is an expert to check it before anything is plugged in.

I think people tend to take electricity for granted sometimes and do not give it the respect it deserves, with often dangerous consequences.think it should be like any gas installation where, you must have a qualified Electrician to do any final connections.

I know of people going from a 240v AC power supply to a home and, working on a 440 (I think it is in the UK) power supply in a factory. They are entirely different beasts!

I learned the hard way, though there was no damage to me, I got one, of I think six wires wrong in an industrial cast iron plug which weighed about a pound. I plugged it in and with the explosion the plug vaporised. I said my machine need re-wiring to my boss, but it was only a small work force of six and, he said have a go. needless to say I did not have a go after that. I copied what I was doing from another plug. Thought after 'double checking' I had it right. But I didn't. Don't take risks.

jfmfit

You should also consider the fact that almost all "old" car fires are caused by some electrical fault. DC currents are just as dangerous as AC currents at the same levels.

An aquaintance of mine had his car go up in flames while he was driving it around the welsh mountains. He said it was the most scary thing he ever had happen to him, sudden flames from the dash board.

Hello case491,

I did not mention I bought a car. It was a mini whose battery was behind the drivers seat. Problem was every time I brakes the battery would short circuit on the metal frame of the original mini seat! My seat was on fire twice before I realised what was the problem. The second time the battery totally discharged and welded my seat to the angle frame and battery terminal, which was part of a shelf going the full width of the cab.

jfmfit

Control supply has to be reliable.A supply from battery is the most reliable and hence most control supply is dc and voltage varies depending upon the application 24v DC, In high voltage substation 110V dc is used.Now we can also use ac by use of inverters with battery . But again reliability is compromised as if anything goes wrong with inverter control supply is off.The battery that is supplying will sufficient capacity and is on charge .Even for this reason railways prefer DC supply for signal and point machine operations .

Hey All,

Let me start by stating that I'm a licensed Journeyman Electrician.

The reasoning for D.C. controls is multi-faceted:

1. Safety( lesss chance of injury/fatality)
2. stability
3. readily available( in most cases)

PLC's (from the old TI 120's through A/B RS Logix500's have the capability of both D.C. and A.C. I/O's. As Sparky stated about "chatter" the most common solution is a capacitor across the line and neutral terminals. I have never used a "shadow ring" nor have I ever ran across one.

It all depends on the voltage. But AC is held to be more likely to stop your heart and DC is held to give a stronger "can't let go" factor. Under the wrong circumstances, either will ruin your whole day and can ruin the rest of your life.

Please don't delude yourself with wrong facts about AC or DC being safer. Safety of either depends on your attitude and procedures.

At 12 or 24V neither are very harmfull so no sweats about it but over 40Volts any can kill. Injury can happen at any Voltage simply by burning.

Sir, stick to the topic, we all know what it is. We started at 12-24 volts and all of a sudden you're at 40 thousand! How far do you have to go? Be objective for GOD sakes!

A little anecdote...If you've never been schocked, pinched, or cut; you've never been there!

I was working on a Lumonics Laser at a major Pharmaceutical; an Engineer had left the discharge electrode disconnected and sticking outside the operator door. I barely brushed my elbow across it when a spark jumped 6 inches and shocked me. I was in heaven for about 6 seconds propelled by 25Kvolts DC. And I'm still here! So tell me about DC voltage!

Read properly and than comment. Nobody mentions 40 thousand volts except you

Whatever he calls it; it does exist and is used widely. Look-it-up...(Google it)!

Thank you for your exceptionally clear write up, no more posts needed I think.

Excellent answer Marco could have written it myself ;-)

Sorry to disappoint other commentators but there is one more thing - power factor.

Most actuators, relays etc are highly inductive (coils of wire). This can produce large current flows in AC systems which can overload the transformer. Control systems have a habit of growing, so very common to hear the overloaded power trasnformer in the control panel humming away. There is no reactive power in a DC system.

By the by, car manufacturers are introducing 48volt car batteries to try and cut down on the cabling in modern cars.

That would be the size of cabling - not the footage?

You are right that most AC actuators have a very bad power factor but DC power supplies do generate a lot of harmonics. In most system the load from actuators and control is negligible on the main supply but could force you to upgrade the small control transformers. As far as the main transformer is concerned, one 10HP motor draws much more power than most complex control panel.

At the end you have to choose between to evils but if your control load is small with respect to the plant power feeder, it doesn't matter which one you choose. Take the control voltage that is more convenient. Usually it is the one you already use for other components in the plant.

AC current through a coil has a continuous waring EMF and capacitate coupling. There is also some chatter.

Most AC relays have a bridge rectifier and use a DC coil.

DC coils don't run as hot as an AC coil.

True there is an inrush current spike as well as a counter EMF when activating and deactivating a DC coil. This one time and not 60 or 50 times a second.

Also, AC neutral is usually grounded to protect against shock. DC does not need to be. This means that either the + or the - can short to ground (conduit or frame ground) without affecting the operation of the circuit.

Below 48V, you can usually have either AC or DC floating. You are right that floating supplies usually allow you to operate with one line grounded but this can be more of a curse than a blessing. By delaying the time when you figure out that you have a line shorted, it is more difficult to figure out what caused it. The shorted line can also generate unwanted ground loops giving you strange "noise" problems.

I prefer to ground the neutral of all my supplies even at 24VDC. Using a properly sized fuse you will know immediately when something goes wrong. Usually is it while you are modifying the system. You might as well fix it while you have your hands in it.

An even more compelling reason to avoid floating control supplies is that it could short to a 480V line. Now you have a safety issue. Assuming that the control circuits have enough insulation to take it (50% of the case), you will have a 24VDC supply floating at 277V with respect to ground (480V LL = 277V LN). Since you don't expect this, you might approach it carelessly and get zapped badly. If you had grounded the return, some fuses would have burned when the short occurred and you would have found and fixed the cause right away. I prefer having a machine stop because of a blown fuse than sending somebody to the hospital for electrical burns or simply falling off the ladder.

Grounded supplies are normally safer than floating supplies. The only exception are in explosive areas (oil refineries, various mills...) where the consequences of generating a spark are worst than the danger of getting shocked. Anyway, their people are trained accordingly and they have monitors that indicate if one line has been grounded.

Electrical Inspectors in the US and Canada typically spend more time reviewing voltages above 24vdc.

AC goes in two directions. DC goes in only one.

AC is net EMF. DC is total EMF.

Dear Sirs/ Madams.

My reply to:

Sparkstation #2

<< sometimes requires a "shading ring = short circuited turn in the form of a heavy copper loop" around each poleface to prevent that "chattering". >>

Dear Sir not " sometimes " but always & Not "poleface " but part of poleface; almost less than 1/2 the area to produce a 2nd leading phase.

It is in all AC shaded-pole motors; Relays; contactors.

It works as the 2nd winding + Series-Capacitor works in Capacitor-Start / Run AC motors.

Of cource it is to stop Chattering.

A well put reply.

Regards.

To: stevem #3

A good addition to Sparkstation #2

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Quote in Beej50's post:

<<"A common mistake people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools!" Douglas Adams 1952-2001. E&OE! >>

My Boss always suggested us to make products " FOOL-PROOF " but NOT "Bloody-Fool-Proof " You can't.

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Repl;y to: case491 #20 & #19

Really an exceptional write up.

< no more posts needed I think.>

but there is always a good addition to a good writing.

Let's say some-thing toi add, even if it is not as good as the original idea

Kind regards