4 to 20 What!?!

[itsmoked]

We have this English Oxygen sensor that puts out 4-20mA.

(see link below)

-BUT!-

It only puts it out into absurdly low impedance, like a DMM.

If you put it into 10ohms the 20mA sags to perhaps 7mA.

Put it into 500ohms and you get maybe 0.1mA.

It states 10 - 35VDC this is at 12VDC. If you crank up the supply to 30V it makes about a 10% improvement.

Is this thing goofed up or is this a type of 4-20 device I've not heard about?

And what's with that crazy output impedance?








Keith Cress
kcress -

http://www.flaminsystems.com

 

[Pud]

No idea what you are talking about, but if no joy via email to them, pm me and will call them with your questions. (Although SomptinGuy might be a better bet!)

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[danw2]

Since when do current outputs have 15 Meg ohm output impedances?

How much current does it really supply into a DMM? a full 20 mils?

Maybe it's really a voltage output, and 'mis-spec'd'? or the voltage output model mislabeled?

 

[waross]

I think that your device may have passed the end of its usefull life. BUT
12V at 4 ma should allow a maximum loop resistance of 600 Ohms minus the resistance of the transmitter.
I wonder if the 15 M Ohm is milli Ohms instead of 15 Meg-Ohms.
That would give a voltage drop across the transmitter of 0.06 millivolts at 4 ma. That seems low.
After a short break to check Omega.com, I wonder if you have just the sensor, and not the complete sensor-transmitter package.
Some of the Omega packages have a conventional 4-20 transmitter which is connected to a sensor, but the voltages/currents in the connection between the transmitter and sensor aren't specified.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Are you using metric volts or Imperial volts???

 

[IRstuff]

They're Californian volts, which are currently issuing IOUs for the missing current.

TTFN

FAQ731-376

 

[ijl]

The 15 MOhm output impedance might really be Mega-Ohms, not literally, but as a measure how constant the constant current is. A constant current source can be thought as a very high voltage connected to a very high resistance. In this case, a voltage of 150 kV would produce a current of 10 mA in a 15 MOhm resistor, for example. This current would be practically constant when the source is connected to a circuit with a moderate impedance.

 

[IRstuff]

But a 4-20 mA circuit is supposed to produce a constant current anyway, with a load like 600 ohm.

TTFN

FAQ731-376

 

[itsmoked]

Thanks folks. This is from a customer who wants this sensor monitored. We have him asking the manufacturer tomorrow. I just needed the clarification that this wasn't some sort of family of 4-20 devices that I'd somehow never heard of. Sounds busted or needing its 'adapter' board. It was just strange that it seemed to work correctly as long as the load resistor was infinitesimal.

Thanks for the communication offer Pud! Lets see what transpires tomorrow.

danw2; It really supplies the full 20mA to the DMM.

Bill, You have several good points. (as usual) Might be 15mOhms not Mohms. That would make much more sense.

Otherwise I agree the thing is messed up or needs an interface board. That is entirely it for the companies (*&^%}*^%( POS data sheet. Really really bad. Personally I would not even use a product that had so little useful info avaiable but what's a body to do.

dpc; IR is correct I O U volts.

ilj; interesting point, but...

Keith Cress
kcress -

http://www.flaminsystems.com

 

[benta]

Output impedance is 15 Megaohms.
Think back to your school days: an ideal current source has infinite impedance.
To me the spec makes perfect sense and is an indication of how good the current source is.
That being said, I'd say your device is kaputt.

Benta.

 

[itsmoked]

Just to get back...

The unit was believed to be a 4 wire unit. The leads brought out to us were from the factory test pins and have been used this way for over 10 years.!!!

In fact, the device is really a 2 wire unit and you put the sense resistor in series with the power leads as you would expect.

Cheers.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Thanks Keith.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[biff44]

I believe you are describing a HART sensor protocol. There are tons of descriptions on the web if you do a search.

http://www.MaguffinMicrowave.com

Microwave and wireless design consulting

 

[ScottyUK]

I believe you are very probably wrong! Read the data sheet in Keith's original post: it's a component-level device and there is no mention at all of HART capability.

Why did you reach this conclusion? HART just superimposes data onto a standard 2-wire 4-20mA loop.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[KiwiMace]

Doesn't it seem weird that the factory test pins will drive 4-20mA but only through a DMM? I would have thought you were either going to have an unamplified source signal, or you were going to have a full driving current. It's like they have two different amplifiers for the test pins and the transducer output.

 

[BJC]

What kind of indicator or signal conditioner do you have in your system? I have never had good luck using a "raw" power source ( such as 12 automotive source) in a system.
I would use an indicator with an internal loop power supply.

 

[roydm]

The test pins of most transmitters are connected across a diode in series with the output. That's so you can connect you DMM without having to break the circuit. The 4-20 load you can drive depends on the voltage you supply.
Roy

 

[itsmoked]

BJC; It has a linear open frame supply running it.

To reiterate. It was two miswirings strung up against years of using it the wrong way in a system that didn't care. Now as we were handed the device, (all nicely miswired), and added it to a "normal situation" the results were very confusing.

The system is working fine now.

roydm; I saw that as a jury rig hung in the chain for testing years ago but never thought to add it to a design! I like it a lot as it works well.

Keith Cress
kcress -

http://www.flaminsystems.com