Previous in Forum: Does Flowmeter Diameter Depend On Pipe Diameter?   Next in Forum: DP Type Level Measurement
Close
Close
Close
15 comments
Anonymous Poster #1

Current Monitoring Diodes

10/01/2013 6:21 PM

Hello All,

Can someone please explain, what is the need of a current monitoring diode in an analog (4-20mA) instrument loop?

Thanks

Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Apr 2009
Location: USA, Florida
Posts: 1561
Good Answers: 125
#1

Re: Current Monitoring Diodes

10/01/2013 6:40 PM

Can't tell for sure without knowing what PLC you are using.

Diodes and resistors are sometimes used as "end of line" (EOL) devices to indicate to a controller circuit that the wiring is intact, called "supervision".

__________________
An obstacle is something you see when you take your eyes off the goal.
Reply
Anonymous Poster #1
#2
In reply to #1

Re: Current Monitoring Diodes

10/01/2013 7:07 PM

Its a ROC 800 RTU. But does that make any difference though?

Reply
11
Power-User
Engineering Fields - Instrumentation Engineering - New Member Engineering Fields - Control Engineering - New Member Technical Fields - Technical Writing - New Member Technical Fields - Education - New Member Fans of Old Computers - Apple II -

Join Date: Sep 2011
Location: Bellingham, WA
Posts: 191
Good Answers: 46
#3

Re: Current Monitoring Diodes

10/01/2013 8:38 PM

A current-monitoring diode gives technicians a convenient way to connect a milliammeter in series with the loop without breaking the circuit. It's a clever exploitation of a silicon diode's forward conducting behavior and it works like this:

Diodes don't conduct current until you impress enough forward-biased voltage across their PN junctions. For standard silicon rectifying diodes, this minimum biasing voltage is approximately 0.6 volts, depending on temperature. For all practical purposes, the diode will not conduct any current while the voltage drop across it is less than this threshold value.

When a diode is connected in series with a 4-20 mA current loop, it typically drops this amount of voltage, which has a negligible effect on the performance of the circuit. If you connect a milliammeter in parallel with the diode (i.e. across the diode), however, the ammeter's very low impedance provides an easier path for current, dropping much less voltage than the 0.6 volts or so required to make the diode conduct. As a result, the diode stops conducting entirely and all the 4-20 mA current passes through the milliammeter.

I've also seen these "monitoring" diodes connected across the terminals of field indicating meters. Their purpose here is to maintain loop integrity if ever the indicator suffers an "open" internal fault.

__________________
They call me "lightning" when wielding a hammer, because I never strike twice in the same place
Reply Good Answer (Score 11)
Anonymous Poster #1
#4
In reply to #3

Re: Current Monitoring Diodes

10/01/2013 9:03 PM

Thanks for the explanation. Further to what you have mentioned, can you also please explain the following in a bit more detail?

"I've also seen these "monitoring" diodes connected across the terminals of field indicating meters. Their purpose here is to maintain loop integrity if ever the indicator suffers an "open" internal fault."

What do you mean by 'open internal fault'? and How does the integrity of the loop maintained by using a diode.

Also is there a specific diode (I mean part number) to be used for the purposes you've explained?

Thanks for taking the efforts in explaining.

Reply Score 1 for Good Answer
Guru

Join Date: Mar 2007
Location: by the beach in Florida
Posts: 31843
Good Answers: 1750
#5
In reply to #4

Re: Current Monitoring Diodes

10/01/2013 10:12 PM
__________________
All living things seek to control their own destiny....this is the purpose of life
Reply
6
Power-User
Engineering Fields - Instrumentation Engineering - New Member Engineering Fields - Control Engineering - New Member Technical Fields - Technical Writing - New Member Technical Fields - Education - New Member Fans of Old Computers - Apple II -

Join Date: Sep 2011
Location: Bellingham, WA
Posts: 191
Good Answers: 46
#7
In reply to #4

Re: Current Monitoring Diodes

10/01/2013 11:09 PM

If a field indicating meter fails open (i.e. fails in such a way that it no longer presents a continuous path for electric current through it), the diode connected in parallel with that meter provides an alternate route for the current so that the 4-20 mA signal doesn't become interrupted (i.e. forced to a value of 0 mA). This was especially a concern back in the days of D'Arsonval movement analog field indicators, where the indicating meter relied on a very fine coil of wire through which the 4-20 mA current flowed to generate the torque to move the meter's needle. If this fine wire ever broke and there was no diode in parallel with the meter, the 4-20 mA loop would become an "open" circuit and fail.

General-purpose rectifying diodes work great for this purpose. The common 1N400X series of diodes is probably your best bet.

__________________
They call me "lightning" when wielding a hammer, because I never strike twice in the same place
Reply Good Answer (Score 6)
2
Guru
Engineering Fields - Electrical Engineering - Been there, done that. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 15283
Good Answers: 940
#8
In reply to #7

Re: Current Monitoring Diodes

10/02/2013 12:26 AM

BRAVO! An excellent collection of explanations of why and how 4-20 mA circuitry should be intended to work. The addition of the strategically placed diodes the OP asks about allows for a robust system design so that a common failure mode at one location will allow another location to still accurately sense the measured signal by the transducer producing the signal.

__________________
"Don't disturb my circles." translation of Archimedes last words
Reply Good Answer (Score 2)
Anonymous Poster #1
#9
In reply to #7

Re: Current Monitoring Diodes

10/02/2013 6:47 PM

Another question arises from this is, why then just have a terminal in the loop and then check current across the terminals, do you think this will affect the mA reading on the multimeter in any way? I know you have mentioned that this method was used in old circuits but, I came across such set of diodes in 4-20mA loops and was a recent installation.

Sorry if I am sounding too naive here...

Reply
2
Power-User
Engineering Fields - Instrumentation Engineering - New Member Engineering Fields - Control Engineering - New Member Technical Fields - Technical Writing - New Member Technical Fields - Education - New Member Fans of Old Computers - Apple II -

Join Date: Sep 2011
Location: Bellingham, WA
Posts: 191
Good Answers: 46
#12
In reply to #9

Re: Current Monitoring Diodes

10/02/2013 9:56 PM

Installing a pair of current-monitoring terminals in a 4-20 mA loop will work so long as their is a disconnect switch between those two terminals to break the circuit on demand. The tech would then have to follow these steps precisely:

(1) Connect ammeter between the two terminals

(2) Open the switch (thus breaking the original current path and forcing current through the meter)

(3) Take the current measurement from the meter's indication

(4) Close the switch

(5) Disconnect the ammeter

Failure to follow these steps in the correct order would result in interrupting the current's path, which of course would cause the signal value to drop to 0 mA (-25% of range). This would be a Bad Thing if the current signal were part of a safety shutdown system or part of a working control loop. This is what makes the diode concept so appealing: you just connect the ammeter across the diode and take the current measurement. The "switching" of loop current is entirely automatic, and cannot be done incorrectly so as to interrupt the integrity of the loop.

As far as age is concerned, current monitoring diodes are entirely appropriate even for brand-new 4-20 mA loop circuits. All I meant to say in my original post is that back in the days of moving-coil, permanent-magnet field indicators (which were prone to failure due to broken coil wires), a diode connected across such an indicator served a different purpose than technician convenience. Instead of merely providing a convenient point for current measurement, it helped ensure loop integrity in the event of an indicator failure.

__________________
They call me "lightning" when wielding a hammer, because I never strike twice in the same place
Reply Good Answer (Score 2)
Guru

Join Date: Nov 2007
Location: Eastern Arizona mountains on Route 666 about a mile from God's country
Posts: 1676
Good Answers: 122
#15
In reply to #9

Re: Current Monitoring Diodes

10/04/2013 9:58 AM

In addition to all of the excellent responses in this post I would add that use of the diode(s) allow effective and efficient critical process loop tuning without signal interruption.

It is extremely difficult and time consuming to perform tuning of a process loop if the technician is required to open circuit the loop and place a meter in series with the signal at each node within the loop.

__________________
They said; "Brain size?" I heard; "Train size?" so I said: "I'll take a small one, thank you."
Reply
3
Power-User

Join Date: Nov 2009
Posts: 333
Good Answers: 63
#6

Re: Current Monitoring Diodes

10/01/2013 10:50 PM

Sometimes the diode is built into the field instrument. Most industrial pressure transmitters have test terminals, behind which sits a diode.

mA meter probes put across the test terminals allow the meter to read the current in the loop without having to break the loop circuit to insert the meter in series.

Reply Good Answer (Score 3)
2
Guru

Join Date: Jul 2010
Posts: 669
Good Answers: 176
#10

Re: Current Monitoring Diodes

10/02/2013 8:07 PM

So everyone can see what a diode in a 4-20mA current loop looks like, the illustration below shows the diode and a meter across the diode.

It's an excellent way to check the current in a current loop circuit without having to open the circuit. Although pressure transmitters have test diodes, many other field instruments do not.

The only down side I know of is that the diode can rectify 'random' noise in the current loop. Integrating amplifier/A/D's on the front end of an analog input will only 'see' the positive noise, not the negative noise, and the integration results in an offset error. It is not a common occurrence.

Reply Good Answer (Score 2)
Power-User
Engineering Fields - Instrumentation Engineering - New Member Engineering Fields - Control Engineering - New Member Technical Fields - Technical Writing - New Member Technical Fields - Education - New Member Fans of Old Computers - Apple II -

Join Date: Sep 2011
Location: Bellingham, WA
Posts: 191
Good Answers: 46
#11
In reply to #10

Re: Current Monitoring Diodes

10/02/2013 9:48 PM

Thankfully we don't have to worry about noise being rectified when we place a diode in a 4-20 mA current loop, because the "live zero" of the DC signal maintains the diode in a constant state of forward bias. The diode would not rectify unless the noise were so extreme in amplitude that it actually caused the signal current to pass through 0 mA. If the noise were that bad, you'd have huge problems with or without a diode in the circuit.

You're absolutely right, however, about noise rectification in circuits where the noise can realistically push the diode between its conducting and non-conducting states. Diodes intended to clamp a signal to within a few tenths of a volt from ground (or from a power supply rail) can also cause this to happen. I've even heard of cases where such a diode functioned as an AM demodulator (like the old "crystal detector" circuits), allowing high-frequency "carrier" noise to impose a lower-frequency noise signature on the circuit.

__________________
They call me "lightning" when wielding a hammer, because I never strike twice in the same place
Reply
Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17996
Good Answers: 200
#13

Re: Current Monitoring Diodes

10/03/2013 1:18 AM

Nice blog, thanks to all.

A simple and effective solution.

__________________
"What others say about you reveals more about them, than it does you." Anon.
Reply
Anonymous Poster #1
#14

Re: Current Monitoring Diodes

10/03/2013 1:40 AM

Vote tonykuphaldt for the President of the USA!!!!! Great explanations. Thanks for sharing the information in your valuable time..

Reply Off Topic (Score 4)
Reply to Forum Thread 15 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Andy Germany (1); Anonymous Poster (4); Carl_E (1); Iris (1); redfred (1); SHOCKHISCAN (1); SolarEagle (1); tonykuphaldt (4); WJMFIRE (1)

Previous in Forum: Does Flowmeter Diameter Depend On Pipe Diameter?   Next in Forum: DP Type Level Measurement

Advertisement