# coils ac/dc



## B4T (Feb 10, 2009)

ampman said:


> so i should know this but what is the difference between say a 24 volt ac and a 24vdc coil


AC won't work on DC circuits and DC won't work on AC circuits.

Normally I use a 24V ac transformer connected to a 24V DC power supply..


http://www.altronix.com/p_pdf/SMP5.pdf


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## Big John (May 23, 2010)

An AC coil is probably going to be physically smaller than a DC coil. 

In an AC coil, the wire resistance makes up a small portion of the total impedance. If you were to measure the resistance of the coil, then try to figure out the current draw at 24 volts using Ohms law, your result would show it would draw a heck of a lot of current.

In reality, the coil has a very high impedance because of the reactance the coil presents to the flow of alternating current. If you were to apply direct current to that coil, it would actually try to conduct the amount suggested by Ohm's law and it would burn up in no time flat.

A DC coil, on the other hand, presents pure resistance, which is why they are often larger because there are usually more turns of wire. If you measure the resistance and use Ohm's law to figure the current draw, it should be very close to what's printed on the relay/contactor.

-John


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## ampman (Apr 2, 2009)

Big John said:


> An AC coil is probably going to be physically smaller than a DC coil.
> 
> In an AC coil, the wire resistance makes up a small portion of the total impedance. If you were to measure the resistance of the coil, then try to figure out the current draw at 24 volts using Ohms law, your result would show it would draw a heck of a lot of current.
> 
> ...


 i did notice that the dc coil was larger and has more resistance ,your explanatoin is greatly appreiciated


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## Toronto Sparky (Apr 12, 2009)

An A/c Relay coil has a shading coil to stop it from chattering. 
I think some use diodes on A/c relay coils these days as well.

Opps.. Just noticed it was a coil question not a relay question :wallbash:


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## miller_elex (Jan 25, 2008)

Who uses a 24VDC coil?? Fire alarm?

24VAC is very common, if not the most common.

24VDC is used for 4-20ma devices, which suck, and

I think alot of fire alarm stuff is 24VDC, but am not sure.

The best thing to have is the RIBS which have 10-30V coils, ac/dc, and a seperate lead going to a 120V coil, all on the same relay!


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## B4T (Feb 10, 2009)

miller_elex said:


> Who uses a 24VDC coil?? Fire alarm?
> 
> 24VAC is very common, if not the most common.
> 
> ...


 
Every FA item I have used was DC.. even magnetic locks for doors.

Most FA panel have a set of terminals you can get DC power from, but they are limited...


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## Big John (May 23, 2010)

miller_elex said:


> Who uses a 24VDC coil?? ...24VDC is used for 4-20ma devices, which suck...


 That's the biggest one I can think of. A lot of PLCs are 24 volts, so all the output relays are 24VDC.

A lot of plants have hundreds/thousands of 'em.

-John


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## TheRick (Apr 13, 2008)

The last plant I worked at, we had some 1960s vintage GE Limitamp Medium Voltage Motor Starters, and they had 120 VDC coils for the time delay stepping relays. Those were big, heavy coils!

The control voltage in them was 240 VAC, which is why they were nick-named widow makers!


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## miller_elex (Jan 25, 2008)

Big John said:


> That's the biggest one I can think of. so all the output relays are 24VDC.


HVAC has got to be bigger than manufacturing, not to start another thread, but this is America after all.

Sure DC is great because two 12V batteries can be stacked in series in case the power sh1ts the bed, but these hvac controllers use thyristors and onboard 24vac on their outputs, its very convinient for turning stuff on and off. Most all the devices are powered by 24vac because it is still class 2 and the transformers are cheeep.


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## ampman (Apr 2, 2009)

miller_elex said:


> Who uses a 24VDC coil?? Fire alarm?
> 
> 24VAC is very common, if not the most common.
> 
> ...


 this was for a conveyor made in holland which is controled by a plc


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## MDShunk (Jan 7, 2007)

Duh-nuh-nuh-nuh-nuh-nuh-nuh-nuh-nah-AMPMAN! :laughing:


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## wildleg (Apr 12, 2009)

I think that would actually be:
POW !




BANG !





CRUNCH !







Could it Be ?










na-na-na-na-na-na-na-na-na-na-na-na-na-na-na-na-Ampman !


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## ampman (Apr 2, 2009)

MDShunk said:


> Duh-nuh-nuh-nuh-nuh-nuh-nuh-nuh-nah-AMPMAN! :laughing:


 sorry i don't get it


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## Introyble (Jul 10, 2010)

miller_elex said:


> Who uses a 24VDC coil?? Fire alarm?
> 
> 24VAC is very common, if not the most common.
> 
> ...



I see 24 VDC all the time. It's a common German/Italian Engineered control voltage in relay logic. Actually, I see more 24VDC than I do 24 VAC. 

They still produce a lot of industrial process equipment that use relay logic rather than PLC's. 24VDC is in the majority of these circuits.

P.S. 4-20 ma has nothing to do with 24 VDC. Most control/logic circuits have phased out 4-20 but it's still out there. Matter of fact, seen a 2009 model extruder that used a modulated 4-20 from a speed pot to the drive as a speed reference.


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## wwilson174 (Apr 25, 2008)

ampman said:


> so i should know this but what is the difference between say a 24 volt ac and a 24vdc coil


Many years ago I had a major project which consisted of providing a turn-key job to a major U.S. company, which meant that we wired the building and the machinery and trained the people who operated the plant which was a manufacturing facility in the steel industry.It was a great job which lasted over 3 years and I enjoyed the experience.

All of the electrical equipment was designed and furnished By GE , Including switchgear and MCC's. 
We had a recurring problem with a starter on a compressor which would intermittently weld it's contacts. It was a size 5 across the line starter 200hp (big sucker) and it was fitted with a a DC contactor which was fed from an on-board rectifier bridge.

The plant was constantly crawling with GE engineers all of whom were stumped by the problem. We just kept replacing the contacts.

One day I saw an old timer GE engineer with whom I had worked before and I asked him to look at the problem, he took one look inside the cabinet and immediately saw the problem and the solution.

The contactor had been furnished with the wrong coils, instead of DC they were AC, problem solved!
I asked him how he so quickly recognized the problem and he told me that DC coils were always round in shape while AC coils were rectangular. I verified what he said and many years later was able to play hero just as he did. BillW.


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## Big John (May 23, 2010)

Introyble said:


> ...Most control/logic circuits have phased out 4-20 but it's still out there....


 That's news to me. What do new systems use for analog feedback? The only thing I've ever heard of besides 4-20mA is 0-5 or 0-10 volts, and I have yet to see those used.

-John


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## Introyble (Jul 10, 2010)

Big John said:


> That's news to me. What do new systems use for analog feedback? The only thing I've ever heard of besides 4-20mA is 0-5 or 0-10 volts, and I have yet to see those used.
> 
> -John


Heat controllers, various process controllers. Speed reference on an american maplan 2009 model extruder. I/O card rack logic. Power supplies.

If I think about it, I'll snap a picture of a schematic from an extruder less than a year old with 4-20 speed reference.

Funny thing, they then convert it to 0-10 V and send it to a red lion process controller which displays RPM's and alarms (opens the start latch) in error mode. Drive is water cooled, has integrated PLC, but uses 4-20 speed. *Go figure *


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## miller_elex (Jan 25, 2008)

4-20ma and 1-5V are about one and the same. Why fiddle-fart with those stupid resistors?

2-10V is the most common I've used with hvac. read that in bold capital letters hvac. Not industrial machines.


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## captkirk (Nov 21, 2007)

Nice thread.....bout time....


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## RIVETER (Sep 26, 2009)

miller_elex said:


> Who uses a 24VDC coil?? Fire alarm?
> 
> 24VAC is very common, if not the most common.
> 
> ...


We use thousands of DC devices operated from a PLC. It really become apparent what an improper voltage can do when a filter capacitor in the PS starts acting up. It is just about the last thing most people will look for is the AC ripple in the proper value DC voltage.


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## Nildogg (Jul 29, 2010)

Big John said:


> That's news to me. What do new systems use for analog feedback? The only thing I've ever heard of besides 4-20mA is 0-5 or 0-10 volts, and I have yet to see those used.
> 
> -John


 
I'm on a New construction sanitation plant and I'm also wondering what would be used for the analog input. We're using 120VAC for all control elements which are switching high or low inputs to the PLC, but for analog the devices are puting out 4-20 ma to give a reading of the level...not just normal or high or low. How would I get a footage off an ultrasonic level element without 4-20 ma?...or what would be more modern?


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## Nildogg (Jul 29, 2010)

Nildogg said:


> I'm on a New construction sanitation plant and I'm also wondering what would be used for the analog input. We're using 120VAC for all control elements which are switching high or low inputs to the PLC, but for analog the devices are puting out 4-20 ma to give a reading of the level...not just normal or high or low. How would I get a footage off an ultrasonic level element without 4-20 ma?...or what would be more modern?


 
Saturday night bump...come on yall.


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## Jlarson (Jun 28, 2009)

Nildogg said:


> I'm on a New construction sanitation plant and I'm also wondering what would be used for the analog input. We're using 120VAC for all control elements which are switching high or low inputs to the PLC, but for analog the devices are puting out 4-20 ma to give a reading of the level...not just normal or high or low. How would I get a footage off an ultrasonic level element without 4-20 ma?...or what would be more modern?


I would use 4-20. Why reinvent the wheel, 4-20 works, low cost, and pretty bulletproof.


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## Big John (May 23, 2010)

Nildogg said:


> I'm on a New construction sanitation plant and I'm also wondering what would be used for the analog input. We're using 120VAC for all control elements which are switching high or low inputs to the PLC, but for analog the devices are puting out 4-20 ma to give a reading of the level...not just normal or high or low. How would I get a footage off an ultrasonic level element without 4-20 ma?...or what would be more modern?


 That was sort of my question. How _else_ would you do it except by using 4-20mA? :confused1:

I think if you were using the 0-5VDC/0-10VDC system, you'd have to adjust it for wire resistance. You don't have to do that with 4-20mA.

I've done a number of of those liquid level switches and they all used 4-20 outputs. I agree with Jlarson: It's simple, it's reliable, it's easy to find parts that use 4-20. I'd use it without a second thought.

-John


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## sparky970 (Mar 19, 2008)

Introyble said:


> I see 24 VDC all the time. It's a common German/Italian Engineered control voltage in relay logic. Actually, I see more 24VDC than I do 24 VAC.
> 
> They still produce a lot of industrial process equipment that use relay logic rather than PLC's. 24VDC is in the majority of these circuits.
> 
> P.S. 4-20 ma has nothing to do with 24 VDC. Most control/logic circuits have phased out 4-20 but it's still out there. Matter of fact, seen a 2009 model extruder that used a modulated 4-20 from a speed pot to the drive as a speed reference.


Most control circuits do use 4-20, unless you're at a plant that still uses 3-15 psi. I consider a logic circuit to be either on or off, so it wouldn't use 4-20


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## Big John (May 23, 2010)

sparky970 said:


> Most control circuits do use 4-20, unless you're at a plant that still uses 3-15 psi...


When I did pharmaceutical, a lot of the plants had these old systems in place where there would be hundreds of 1/4" copper tubes carrying air pressure between all these pieces of equipment. I never understood what it was for, but maybe that's how they did analog transmission before widespread use of electronic transducers!

Where's the smiley-icon that has a lightbulb going off over it's head? 

-John


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## Introyble (Jul 10, 2010)

Big John said:


> When I did pharmaceutical, a lot of the plants had these old systems in place where there would be hundreds of 1/4" copper tubes carrying air pressure between all these pieces of equipment. I never understood what it was for, but maybe that's how they did analog transmission before widespread use of electronic transducers!
> 
> Where's the smiley-icon that has a lightbulb going off over it's head?
> 
> -John


Ah yes, good ole low pressure , solenoid logic


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## miller_elex (Jan 25, 2008)

Big John said:


> these old systems in place where there would be hundreds of 1/4" copper tubes carrying air pressure between all these pieces of equipment.


What's great is that instead of these anaconda tube bundles, where every signal no matter how insignificant is a dedicated hit, now most all of that travels over one single pair comm cable. 

Where before you'd have a sensor or two, a receiver-controller, and an actuator working in a control loop, now that loop is inside a small computer. The way the loop works is programmable, and not subject to the mechanical limitations of springs, diaphrams, orofices, and such.

Its all about the loop baby. Not good to run loops over the network, but it is cheaper. I've noticed that drives are running solely on the network more and more, without pulling cable for the commands, and when the network has problems you're stuck running all the drives in hand.


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## AussieApprentice (Aug 16, 2008)

Toronto Sparky said:


> An A/c Relay coil has a shading coil to stop it from chattering.
> I think some use diodes on A/c relay coils these days as well.
> 
> Opps.. Just noticed it was a coil question not a relay question :wallbash:


The DC relay coils have a diode attached to shunt the voltage spike which occurs when the coil is de-energised.


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## miller_elex (Jan 25, 2008)

You call it the snubber?


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## Toronto Sparky (Apr 12, 2009)

AussieApprentice said:


> The DC relay coils have a diode attached to shunt the voltage spike which occurs when the coil is de-energised.


Last time I worked with dc relays I had to add the diode shunt myself.
Also used to add them to Mag locks.


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## John Valdes (May 17, 2007)

sparky970 said:


> Most control circuits do use 4-20, unless you're at a plant that still uses 3-15 psi. I consider a logic circuit to be either on or off, so it wouldn't use 4-20


0-5 VDC, 0-10VDC and 4-20ma are reference signals. They are never on or of If you want to start something or stop something you would most likely use a digital contact closure. If you have a process you must have a reference signal and it is never on or off, it's always sending a signal to the device you are using.


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## miller_elex (Jan 25, 2008)

You can use an analog input to monitor a binary input with EOL resistors.

Its whats called a multi-state variable. It can tell you, ON, OFF, OPEN, GROUND FAULT. With most all controller inputs being universal inputs, it makes sense on critical equipment.

I learned that from ASHRAE - HVAC Controls. Never programmed one, but I know it is possible.


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