Shunts and 4-20 ma loops

[Snoring Beagle]

What purpose do shunts serve in 4-20 ma loops?

The web is full of references your responses will be greatly appreciated and useful. Doing my homework and studying on my own time.

Thanks, Arlen

[osiyo]

What purpose do shunts serve in 4-20 ma loops?

The web is full of references your responses will be greatly appreciated and useful. Doing my homework and studying on my own time.

Thanks, Arlen

In electronics, a shunt is simply a device that allows electrical current to bypass another device. Nothing more or less.

Now, as to the reasons why you might want to employ such a device ... they are varied and many.

i.e. In a typical string of Christmas lights many shunts are employed. Most people think they're fuses. They kinda look like fuses. Often they're labeled as "Christmas light fuses". But in fact, instead of breaking a circuit, by melting and producing an "open" in the circuit. Their job is to be "open" and then upon an excessive current flow, to melt and form a bypass around a burnt out bulb.

In the case of a 4-20 ma circuit, they're simply referring to the "dropping" resistor placed across the input and its common. It "shunts" the current around the two sensing points. And an electronic circuit designed to measure voltage drop across the resistor measures that voltage drop. Firmware translates that into a 4-20 ma value.

In modern electronics, their are several other ways to measure current, but the old "voltage drop" method is still alive and well ... and cheap.

[Snoring Beagle]

Like so?

[orion242]

I haven't heard of a shunt as far as 4-20ma loops too often. But in your diagram, I would assume that they are referring to the 250 ohm resistor. Which in my mind is not really a shunt, it is providing the proper voltage drop for the 1-5v indicator.

1v/250ohm = 0.004A or 4ma
5v/250ohm = 0.020A or 20ma

IMO typically a shunt is no more than a dead short or a near 0 ohm device, not a 250ohm resistor. The way they are using it and the technical definition of a shunt, it is the shunt in this circuit.

[orion242]

BTW the transmitter will be rated for the load it can drive. Adding another 250ohm load + the load of the of the other indicator may push a transmitter over its ability to drive the 4-20ma loop and causing it to fail.

[osiyo]

Like so?

Yep, in this case in the electronics world, the resistor would be called a shunt in this case. Meant to bypass the current around the voltage meter. Try to run the current through a standard voltage meter, and you'll likely burn it out.

[billcontrols]

Try to run the current through a standard voltage meter, and you'll likely burn it out.

If you were able to deliver the potential (Voltage) to maintain the current this might be the case, but in most cases the High resistance of the volt meter will prevent the current from flowing and you will read the maximum voltage that is driving the circuit. So without the "shunt" adding a voltmeter would effectively kill the circuit. The V would read high and the A would read low and the oprerator would be either scratching his head wondering what he had done, or tearing you a new one for what you had done. All for a resistor that costs chump change.

Ampmeters are prone to burning out if some means of limiting current through them is not provided. Typically there is a fuse in series with an ampmeter for protection in case of mis-application.

[osiyo]

If you were able to deliver the potential (Voltage) to maintain the current this might be the case, but in most cases the High resistance of the volt meter will prevent the current from flowing and you will read the maximum voltage that is driving the circuit. So without the "shunt" adding a voltmeter would effectively kill the circuit. The V would read high and the A would read low and the oprerator would be either scratching his head wondering what he had done, or tearing you a new one for what you had done. All for a resistor that costs chump change.

Ampmeters are prone to burning out if some means of limiting current through them is not provided. Typically there is a fuse in series with an ampmeter for protection in case of mis-application.

Chuckle, correct enough. I'm well aware of the high impedance nature of standard voltage measuring devices. In this case, essential since otherwise some of the 20ma flow would be flowing through it, instead of all of it flowing through the resistor. Which would result in inaccurate results for what it is intended to do in the particular circuit shown.

My only point was that the internal circuitry of voltage meters aren't built to handle any significant current flow. A sort at the connection points that'd allow even 20ma to flow through the internal circuitry of the meter wouldn't just result in in accurate readings, you'd need a new meter.

As far as amp meters are concerned, of course. They're rated as to how much current they were designed to handle.

[DeltaT]

I'll answer it this way and see if that is what you are asking.

Old control systems use to use voltage in controlling the end devices such as motors which operate dampers and valves for temperature control.

As control systems progressed using voltage alone became impossible because the supplied voltage would drop over long line distant runs making the contol system expensive due to needing larger control wiring to reduce the wire resistance and the fact that the lower voltages at the end runs would cause problems with the motors the voltage was attempting to control(drive).

So, by using a control signal other then voltage that eliminated the long distance voltage drop problems. Hence, with a shunt, the motor being driven is not control by a signal of amps and not voltage, if that makes any sense.

I have a bunch of training materials on this subject and I'll attach once I can find them. They will explain this type of control system much more better than me.

[DeltaT]

A 4-20mA Signal Goes The Distance
A 4-20 mA Signal Goes The Distance
by George Tsakir


A 4-20 mA control signal is one of the signals of choice in the Building
Automation industry as well as in the Industrial Process arena. Most of the
various transmitters and output devices available from Kele utilize a 4-20 mA
input or output signal. In large commercial buildings, expansive factories,
multi-building university campuses and other large projects, it is not unusual
to find an application where a transmitter is located a long distance from the
nearest building automation controller. We are frequently asked, "How far can
4-20 mA signal wires be run?". The answer is quite amazing.
A 4-20 mA current loop consists of a number of devices wired in a series loop.
These devices include a power supply, a sensor/transmitter and one or more loads
such as a building automation input, a chart recorder, an alarm module or a
digital display. Depending on the type of transmitter used, the power supply may
either power the transmitter external to the loop or the power supply may be one
of the devices in the series loop as shown in Figure 1.

The transmitter is a 4-20 mA current source that requires a DC voltage supply to
operate. The transmitter in Figure 1 is a model PTX1 pressure transmitter.
Reviewing the PTX1's specifications in the Kele catalog shows that it can
operate with a power supply voltage from 10 to 30 VDC. However, the actual power
supply voltage used will directly affect the output capability of the
transmitter. To provide its full output, the transmitter must be capable of
supporting the total resistance of all loads that are wired in the loop. Again
looking to the specifications of the PTX1, there is a graph (Figure 2) that
indicates the output load capabilities of the PTX1 for supply voltages from 10
to 30 VDC.

A quick study of the graph shows that when using a 10 VDC power supply, the PTX1
can support a maximum load of zero ohms! In other words, the PTX1 requires 10
VDC just to power its internal electronics (power supply overhead). For the
transmitter to drive its 4-20 mA signal through any load at all, the power
supply must be increased above the 10 VDC overhead required by the PTX1. As
indicated by the graph, a 30 VDC supply will allow the transmitter to handle
over 900 ohms of load in the loop. At 24 VDC the transmitter is rated for 650
ohms.
Looking back to Figure 1 shows that the building automation input is 250 ohms
and the chart recorder adds 250 ohms. The total load of 500 ohms is within the
650 ohm capability of the PTX1 with a 24 VDC supply.
So what does all of this have to do with how far 4-20 mA signal wires can be
run? There is an additional load on the loop that has not yet been considered.
This additional load is the resistance of the wiring connecting the devices in
the loop. For shorter signal wiring runs the resistance added by the wire is
negligible, but what effect will long wiring distances have on the loop? By
considering the wiring resistance as an additional load, the allowable length of
wiring in the loop can be determined.

In Figure 3 the loop is powered by 24 VDC which means the PTX1 can drive its
4-20 mA output into a maximum of 650 ohms. The distance from the control panel
to the PTX1 transmitter is 1000 feet and is to be wired with 18 gauge wire. The
resistance of 18 gauge wire is 0.00651 ohms per foot. The total resistance of
the wire in the loop, Rw x 2, is 13.02 ohms (1000 ft x 0.00651 ohms/ft x 2 =
13.02 ohms). The total resistance of the loop then is 513.02 ohms, which
includes the resistance of the wiring added to the building automation input and
chart recorder. In the real world, additional resistance may be added to the
actual total loop resistance by wiring connection points such as terminal
blocks, splices, etc. The total loop resistance of 513.02 ohms is less than the
maximum output rating of 650 ohms so the PTX1 will work in this application.
The amazing part is to do the math to determine the maximum allowable wiring
distance for the example in Figure 3.

Even with two loads (500 ohms), a building automation input and a chart
recorder, the signal wiring from the control panel to the PTX1 transmitter could
be as long as 11,520 feet or more than 2 miles! The characteristics of the PTX1
are not unique. Kele has 4-20 mA transmitters available to handle loads from 300
to over 1000 ohms.
Of course, there are additional factors to consider when running signal wiring
over a long or even short distance. These factors include protecting against the
effects of radio frequency interference (RFI) and electromagnetic interference
(EMI) by never running signal wires in the same conduit as AC power lines,
considering the use of shielded twisted wires, employing surge protection when
running between buildings, and always following national and local electrical
codes.
Remember, when you face a long signal wiring run, a 4-20 mA transmitter from
Kele will go the distance.

[crab master]

To add pics and keep under the 'copyright' rules...http://hvac-talk.com/vbb/showthread.php?t=167548
Here's the link
4-20mA Signal Goes the Distance

[osiyo]

A 4-20mA Signal Goes The Distance
A 4-20 mA Signal Goes The Distance
by George Tsakir
.... snipped

A good and decent article. Even tho its a barely disguised marketing ploy by Kele to sell a little more of their product.

Not a slam against Kele. I use a lot of their products.

But there is NOTHING new about 4-20 ma signaling. It became a standard, reliable, accurate, signaling method that was the choice of industrial, process control, and aviation systems way back in the 1930's and 1940's.

For darn good reasons.

It's accurate, even over long distances and under conditions of high levels of induced noise and voltage spikes and drops. (Think, NOISE)

A well designed 4-20ma transmitter, will drive the voltage to whatever it needs to (keeping in mind what it has available to work with) to establish the correct ma current to represent the measured value. Voltage, as measured, may jump all over the place as conditions change, but current (in milli-amps) will remain relatively constant. Granted a well designed and made circuit.

i.e. Just recently, last week, I was at a job site where customer was complaining. Main complaint? DDC controlled VFD's were jumping all over the place. A lot of hunting. When I looked at the signaling, set up for 0-10 VDC ... a whole lot of noise and wandering. Pumps, controlled by the DDC controller were on the 17th floor of building, sensor that was sensing DP up on the 30th floor.

Solution? All the heck I had to do was switch DP transmitter from 0-10VDC signaling to 4-20 ma signaling. And, of course tell controller to look for 4-20 ma signal. (Okay, a little rewiring, reset of config, etc ... but didn't have to pull more wires nor anything like that)

Once I switched everything over, piece of cake. Signal nice and steady with normal "wander" instead of big jumps. Tuning PID loop was simplicity itself.

The point is ... I took out the "noise" factor. Old way, with some regularity as something or other turned on, between 17th and 30th floor, old 0-10 VDC signal started jumping all over the place. Think this way, you're in the middle of calculating some complex numbers, several factors to be taken into account. And right in the middle of the calcs someone changes one of the variables. Darn, yah gotta start all over again to re-figure the solution. As a result, your solution is always a day late and a dollar short.

That's what happens in control loops if the input keeps jumping all over the place due to "noise" from any of many possible causes.

My thoughts. Nothing else. Worth ... zero.