Allen Bradley Current Loop

[Goodknight436]

Hey Guys

I have a Micrologix 1400 that has Analog output card 1762-OF4. I need the analog output card to send a signal to a receiver (I am using ahttp://www.sensaphone.com/pdf/LIT-0171_v1.5_WEB.pdf). However no matter how I hook it up the the Analog output blows up on me. I have gone through three of these and is starting to get real expensive fast. So can someone tell me how to properly wire this thing up?

*I have tried this so far in descending order.
*Note Transmitter has non isolated common
*Note Receiver has non isolated common

(#P#) = DC Power Supply
(#T#) = Transmitter
(#R#) = Receiver

Combination 1: (#T+)(+R-)(-T#)
*After it blew I found transmitter was configure for a Thermocouple so I thought the resistance blew it so I switch it to 4-20ma for the next try.

Combination 2: (#T+)(+R-)(-T#)
*From my understanding the transmitter sends out a 24VDC signal and has a max current of 132ma so thought I be able to power all 3 analog signals without a DC power supply in series.

Combination 3: (#P+)(-T+)(+R-)(-P#)
*I set it up that way as it was the recommendation from the Receivers Technical support. I had to reverse the Transmitter Polarity because both transmitter and receiver are non isolated.

Any help would appreciated Thanks Goodknight436

 

[GaryS]

first the OF4 is not a sensor it is an output so the external power supply is not necessary.
The OF4 supplies the power to drive the outputs from the back plane (internal to OF4)

The connection should be as follows
OF4 common (Com) connects to the Sensaphone G term common to common
the output of the OF4 connects to the analog input of the Sensaphone A1~A6
make sure the jumper in the Sensaphone are set per channel for ma input
and of course make sure the setting on both the OF4 and the Sensaphone are correct for 4 - 20 ma
Good Luck
Gary

 

[Goodknight436]

That's why I set it up as combination 2 and I switched the ma jumper before hand. The only thing I found that could go wrong was the Cell682 was not configured correctly in the settings. That's what confuses me is that it still blew even though the jumper was set correctly. Shouldn't it not pull to much current if the jumper is switched? Far as my knowledge goes that switches the resistor that measures the signal so the input resistance wont be to high. Yes you are correct I shouldn't have wired it up as a sensor, I couldn't see any other options
at the time.

Now that settings are changed I will try again with combination 2, as its wired like you described. I am still afraid it wont work though.

 

[pal]

Before you hook up the two devices , you should check the voltage between the common on the output card and the senserphone receiver . If they are running from different power supplies , then there could be a large potential difference which is causing the burn-out .

Paul

 

[widelto]

Be careful when you tie ground from OF4 module to sensaphone unit ground as shown on fig 10 of sensaphone manual. In this case external power supply ground is floating, so there's no problem. This is not the case for OF4 module.
My recommendation is to use a galvanic isolator between them.

 

[Goodknight436]

You will have to explain that more widelto. I know the common on the OF4 is shared but what do you mean by the external power supply is floating? Which Power Supply? Why does the Cell682 not have this problem?

 

[OkiePC]

+1 for the signal isolator.

Also check whether the devices are sinking or sourcing. I came close to connecting two incompatible devices directly but was cautious enough to notice the issue before I landed the wires.

I did not know about sinking versus sourcing with 4-20mA current loops until that day, but the polarity symbols on the little schematic that came with my new device puzzled me enough to take extra care and not fry anything.

In my case, there were 3 PLC inputs already in use powered from the same supply and all three inputs to the PLC shared a signal common tied to the low side of the power supply. The new device wanted me to apply the +24 to the signal common which I knew would let the smoke out.

Anyway, a loop isolator took care of the issue for me.

 

[Goodknight436]

I would agree with okie but I have three separate analog signals wired like so.

Channel 1 : (x T +1)(+1 R c1)(g1 T x)
Channel 2 : (x T +2)(+2 R c1)(g2 T x)
Channel 3 : (x T +3)(+3 R c1)(g3 T x)

Thus I don't see the advantage of a loop isolator so you might need to dumb it down for me.

 

[Goodknight436]

Bump

 

[OkiePC]

Well, one advantage is cost. If your signal isolator does not solve the problem, it will at least be cheaper to replace. In my case described above I used this device because that is one they were already using in that panel for other things:

http://www.automationdirect.com/adc...Voltage,_Current_or_Potentiometer_Input/FC-11

I have seen smaller and less expensive versions without the adjustment pots.

 

[widelto]

You will have to explain that more widelto. I know the common on the OF4 is shared but what do you mean by the external power supply is floating? Which Power Supply? Why does the Cell682 not have this problem?

Goodknight436:

What I mean is exactly the same as PAL explain:

Before you hook up the two devices , you should check the voltage between the common on the output card and the senserphone receiver . If they are running from different power supplies , then there could be a large potential difference which is causing the burn-out .

Have you measured voltage between grounds, if there any voltage you must use an isolator. An example of floating ground is a battery that has no reference to any ground so is floating. Please take a look at fig 10 of Cell682 manual, that external power supply is the one I'm talking about.

 

[cbuysse]

This looks extremely straight-forward though I'm confused somewhat by your terminology in the original post. Are you using the term "transmitter" to talk about the output card or is there another device in the mix? I'll be interested to see how this turns out. If I were to wire this up myself, I would go with a 2-conductor cable from the output card to the Sensaphone. No other power supply should be needed.

Red wire --- Output Card (Iout#) to Sensaphone (A#)
Black wire --- Output Card (Com) to Sensaphone (G)

Assuming the PLC is mounted to a well-grounded panel, the Com terminal on the output card is internally connected to ground. It may be beneficial to confirm that both the PLC and Sensaphone are grounded, but otherwise I've rarely had problems with connections like this.

 

[Goodknight436]

To clarify guys I was using the wrong terminology its not a transmitter/sensor that I am getting the signal from, its an analog output from a plc addon card for a Allen Bradley Micrologix Controller.

I haven't measured the voltage difference. I do know they have two different power supplies though. The Receiver has its own AC adapter while the controller has its own built power supply. So Since the PLC has it own power supply and I am not sending the signal from a sensor/transmitter there should be no power supply in the current loop. Which is why I am disappointed in myself for trying option 3 BTW. So what Cbuysse described is the correct down to the T, they have the same earth ground.

With all that I called the receivers company support and found out that even though all the common(G) in figure 10 are labeled the same they are completely different (I had specifically ask if all the grounds were the same even though they are labeled the same). I should have figured this out knowing that you cant mix contact commons with analog commons. I only really noticed and figured to ask that question after reading the paragraph abov figure 10 thoroughly that you have to wire it the """adjacent""" "ground" not any "ground". So when I wired it the analog output, it found some type of ground in the solid state circuitry and pulled to much current from it frying both the analog output and receiver. So while going through all this I now discovered I fried the all receiver contacts (which have all the same ground) along with the output from the analog output.

All in all I will try wiring it to the proper ground next and check the voltage difference for giggles. I will post the results today or tomorrow.

I wish my boss would let me use fuses on everything. Probably would have saved me even though I know its mostly overkill. Ill just put in a temporary one for testing.

 

[Mickey]

Its is always best to use isolators, especially if different power sources are used.

Just do it and save yourself a lot of grieve.

 

[Goodknight436]

Honestly I would, the company I work for though has had the same type of equipment for so long work without it they feel its unnecessary. I couldn't sell them on buying stuff like that standard for every project that uses analog signals. Plus if my reasoning correct from earlier is correct it fry that too anyway. My last job would only use them to clean up signals or to convert from voltage to current and vice versa.