# 24 VAC & 2-10 VDC sharing a common point



## Gravy (Feb 19, 2009)

Building automation question.

We use three wire humidity sensors (analog inputs to controller) and three wire e/p transducers (analog outputs from controller).

Both of these devices can be powered with either AC or DC (10-24V). Both devices either source or utilize 2-10 VDC to/from the controller. There is no common point between AC & DC.

How does it work :scratchhead?

I'm getting some erroneous readings from the sensors and tried tying the AC "neutral" (for lack of a better term, neither system is grounded) to the DC (-). The power supply did not like that.


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## John Valdes (May 17, 2007)

Courtesy of Wikipedia.

A _sensor_; is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument.

A *transducer* is a device that converts one type of energy to another. The conversion can be to/from electrical, electro-mechanical, electromagnetic, photonic, photovoltaic, or any other form of energy. While the term _transducer_ commonly implies use as a sensor/detector, any device which converts energy can be considered a transducer.


As you can see I am somewhat confused as to your question. I understand that sensors and transducers can accept different voltages, either AC or DC. But both produce a signal one way or the other.
With three wire sensors and transducers one wire is common, one is power and the other is the signal. If you are using standard shielded, 3 conductor cable, white will most likely be the signal.

Both are connected as shown on the controller wiring diagram. Common is common and as you found out ac and dc do not mix. 
I need to know more about your application and what type of equipment you are using. But to save time why not contact the manufacturers technical dept. Thats much faster and they know the equipment better than any of us do at this moment.
Give them a call and see. If not post all the specifics including brand and model number.
And please do not think calling someone makes you look less than remarkable. If you can correct your problem with a few minute phone call, that makes you the cats ass in my opinion.


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## PsiMan84 (Oct 29, 2010)

show us something that might look like this. this is a simple example


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## Gravy (Feb 19, 2009)

The manuf. is Honeywell. If you've never tried to get tech help from them, let me tell you, you'd have better luck getting milk from a bull.

Unfortunately, I do not have the ability to scan any of my termination sheets. 

I maintain 14 buildings so I went out to another and checked; I found, in this building, they tie one side of the 24 VAC xfmr to the DC (-). I think I'm going to go out tomorrow and have another look, maybe I missed something.


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## LARMGUY (Aug 22, 2010)

Gravy said:


> The manuf. is Honeywell. If you've never tried to get tech help from them, let me tell you, you'd have better luck getting milk from a bull.
> 
> Unfortunately, I do not have the ability to scan any of my termination sheets.
> 
> I maintain 14 buildings so I went out to another and checked; I found, in this building, they tie one side of the 24 VAC xfmr to the DC (-). I think I'm going to go out tomorrow and have another look, maybe I missed something.


 
I call Honeywell all the time. Not for building controls but I am sure once you get through the questions from barely English speaking Pakistani's they can usually help.


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## John Valdes (May 17, 2007)

Gravy said:


> The manuf. is Honeywell. If you've never tried to get tech help from them, let me tell you, you'd have better luck getting milk from a bull.
> 
> Unfortunately, I do not have the ability to scan any of my termination sheets.
> 
> I maintain 14 buildings so I went out to another and checked; I found, in this building, they tie one side of the 24 VAC xfmr to the DC (-). I think I'm going to go out tomorrow and have another look, maybe I missed something.


How old or new is this equipment? Do you know the name of the vendor that sold this equipment to your company or the company that owns the equipment?
There is a protocol to be followed in all cases of technical issues with any manufacturer.
First, call the vendor. They sold it and good ones rep it too. In most cases a call to the vendor will get you some relief. When I represented several companies, I looked forward to getting a call like this. It gave me an opportunity to come out with an appointment. A sales call in a way. Since I worked on everything I sold, this was how I made friends and new customers. Try some creativity. Make some calls and see. I bet the salesman for the vendor can tell you exactly what you are doing wrong, or if there is something wrong with the equipment.

Unlike in the past, most sales people in this industry are very knowledgeable. They have to be. Customers want more, they want help. Just like you do.


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## Gravy (Feb 19, 2009)

Thanks for your responses fellas.

I've been doing some research. The building in question uses a type of power supply that is not compatible for use on these types of devices (per manufacturer). 

Bottom line is (as far as I know anyway) if you power a three wire device with 24VAC and receive or utilize a DC signal from that device the AC and DC need to share a common point. What I have on my hands is a building automation system that was wired wrong, and it took 7-8 years for someone to notice. 

Here is a link to the type of electro/pneu transducers we typically use, it might illustrate my words:
http://products.ecc.emea.honeywell.com/europe/pdf/en0b0635-ge51r0908.pdf

Here is a humidity sensor similar to what we use:
http://specifyhoneywell.com/customer/Honeywell/UI/UserControls/ProductCatalog/Search/getliterature.axdXXLiteratureIDYY63-2579.pdf


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## miller_elex (Jan 25, 2008)

Three wire is correct for your AI's and sensors....

You've got some jumpers wrong.

Is it the same 24vac that feeds the controller? yes/no

If the sensor is fed DC, then it needs a 2-wire, and is typically a 4-20ma, with a resistor on the controller AI to common. 500ohm. or 250ohm, I can't remember.

Edit: looked at the e/p cutsheet. That should be in the tub next to your controller, yes? That wiring diagram is correct.

What are you working with, delta?


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## DanS (Oct 30, 2010)

If both devices can operate with 10-24 Vdc, then why not connect both of them to the dc power supply instead of mixing ac and dc?


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## miller_elex (Jan 25, 2008)

2-10VDC and 24VAC are mixed all the time. The commons must be electrically the same. Most of the time it is tied in at the controller, provided it is not a full-wave device.

If you are doing 4-20mA, I can see having a DC power supply. Otherwise NOT.


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## DanS (Oct 30, 2010)

miller_elex said:


> 2-10VDC and 24VAC are mixed all the time. The commons must be electrically the same. Most of the time it is tied in at the controller, provided it is not a full-wave device.
> 
> If you are doing 4-20mA, I can see having a DC power supply. Otherwise NOT.


I wouldn't mix ac and dc signals. I wouldn't trust advice from a bozo installer


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## John Valdes (May 17, 2007)

DanS said:


> I wouldn't mix ac and dc signals. I wouldn't trust advice from a bozo installer


I have to agree. I am no expert, but have used similar equipment in plastic extrusion. Sensors and transducers. Not once in my career did I ever seen ac and dc mixed. It was one or the other. I mean if the controller can use a signal of one type and output another type, I get that.
But a three wire sensor requires a power source and outputs a signal for display and or operation.
Transducers convert an electrical signal to a mechanical signal so I really do not think the OP really understands what he has. Or, I have no idea about his equipment?

I am open to any new information that is available. But its simple process control. I wish I could see what he has.

Bozo installer? What do you mean. You are kinda new to be name calling.


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## DanS (Oct 30, 2010)

John Valdes said:


> Bozo installer? What do you mean. You are kinda new to be name calling.


I'm not calling anyone names, I was just quoting what someone wrote in one of my posts....hell I love you guys. By the way, a transducer converts mechanical energy to electrical energy.. not the other way around. You're right , you don't know what you are talking about


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## miller_elex (Jan 25, 2008)

You don't know WTF you're talking about.

Every one uses 24VAC for power and shares that common electrically with the 2-10VDC common on single-wave power supplies.

Why don't you get the 'Wiring Guidelines' for whatever line of controls you are installing, because they are very specific to grounding and grounded conductors.

BTW: Those honeywell E/P's are old as sh1t, so whose stuff from the nineties are you trying to take over, WTF? You're probably a fitter at best. Leave wiring to the big boys there junior. :laughing:


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## Gravy (Feb 19, 2009)

miller_elex said:


> WTF? You're probably a fitter at best. Leave wiring to the big boys there junior. :laughing:


Wow, rough weekend? Is this directed at me? I'd like to know before I try to get my panties un-bunched.

"mixing" low voltage AC and DC is common practice in automation wiring. What the installer did in this instance is forgot/neglected to make a common point in the two systems. The schematic for the system shows a common point (installed jumper) but the installer never put it in. When I try to add it the power supply trips its protection.

Maybe the installer did not keep his 24 AC "common" and “hot” the same through out the entire system?:confused1:


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## miller_elex (Jan 25, 2008)

Gravy said:


> Maybe the installer did not keep his 24 AC "common" and “hot” the same through out the entire system?:confused1:


Yes, you have an existing ground fault, OR the incorrect leg of 24VAC is grounded. It happens often and is not caught until changes are made to the system.


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## Hunter1151 (Nov 4, 2010)

PsiMan84 said:


> show us something that might look like this. this is a simple example


Looking at this the transducers are in a current loop, if you wanna check them put your meter in series with the device, I am unsure where. You are getting ac and dc, I do not see a ps on the board, the sensor can work on either an ac signal, or dc, depending on application, it's the current that matters


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## DanS (Oct 30, 2010)

miller_elex said:


> You're probably a fitter at best. Leave wiring to the big boys there junior. :laughing:


By "Big" I think you mean Big mouths. Keep your twinkie stained fingers outta the panel dipshyt: laughing


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## miller_elex (Jan 25, 2008)

DanS said:


> Keep your twinkie stained fingers outta the panel dipshyt


I'm in yur control panel.... and I'm changing the address to 00 :laughing:

That should keep you busy for a while. :blink:


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