4-20ma loops?

Little confused! in a 4-20ma current loop where is the current that is consumed by the electronics in the sensor? I assume the process adjusts the current via a constant current source type arrangment, but where does the current come from to drive the electronics in the sensor? I have a Vega level sensor driving a display unit and then a 4-20ma loop to the PLC this being the only power to the unit?, also does this mean I can size the cable for 20ma max?

If the Vega is a 2 wire loop powered transmitter, then the power to drive the transmitter comes from the loop current up to about 3.5mA.

Most 2 wire transmitters have a selectable, self-check diagnostic 'failed state', in which the transmitter will either go high (21.5mA) or low (3.5mA) in order to signal that it is dysfunctional.

The mA current below that failed state level is consumed by the transmitter electronics to make the measurement and produce a signal.

If your display is a separate device in series (with the PLC) in the loop then you have to be aware that the combined loop resistance has limitations. Many times an intrinsically safe display designed for loop power and which drops only 1 volt or so has to be use in order to get sufficient voltage to the transmitter for its operation.

Somewhere in the documentation is a graph similar to one below, (not a Vega device) that defines how much loop resistance a given power supply will drive. I found out the other day that AD PLCs are only 125 ohm inputs, others I've run across are typically 250 ohms.

See if these help:
http://www.datel.com/data/meters/dms-an20.pdf

http://www.sensorland.com/HowPage028.html

RayK said:

Little confused! in a 4-20ma current loop where is the current that is consumed by the electronics in the sensor? I assume the process adjusts the current via a constant current source type arrangment, but where does the current come from to drive the electronics in the sensor? I have a Vega level sensor driving a display unit and then a 4-20ma loop to the PLC this being the only power to the unit?, also does this mean I can size the cable for 20ma max?

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This woke me in the middle of the night, electricity is not "consumed" per se. In other words the current that flows is based on the resistance in the ckt, so the electronics in that sensor is basically a varying resitor that changes due to its interaction.

I am not sure I am saying this properly but current is electron flow, the electrons just move from hole to hole, they do not get used.

So if there is current flowing a potential difference will develop across the device, the device will change resistance values because of its interaction with whatever it is "sensing".

Where consumption comes in is in the form of "watts"; which is the result of voltage and current. The primary form of wattage is heat.

Maybe Keithkyll or someone can express this better.

rsdoran said:

This woke me in the middle of the night, electricity is not "consumed" per se. In other words the current that flows is based on the resistance in the ckt, so the electronics in that sensor is basically a varying resitor that changes due to its interaction.

I am not sure I am saying this properly but current is electron flow, the electrons just move from hole to hole, they do not get used.

So if there is current flowing a potential difference will develop across the device, the device will change resistance values because of its interaction with whatever it is "sensing".

Where consumption comes in is in the form of "watts"; which is the result of voltage and current. The primary form of wattage is heat.

Maybe Keithkyll or someone can express this better.

Click to expand...

I think you expressed it quite well, as a matter of fact I've been trying to explain it to Florida Power and Light for the past hour and a half!!!!

THOSE idiots think I "used" $300.00 worth of electricity last month. (THAT wakes ME up in the middle of the night)

Of course I'm just kidding, But once again it is the "terminology" that causes the confusion. While electrons are not "used up", ENERGY is. (actually lost, or converted, but gone none the less) As rsdoran stated, energy is measured in "watts" ( or in the case of the sensor's LED for example, MICRO-watts.) It is the function of the sensor to "limit" the potential current flow in the loop to the correct MA to express what is being sensed.

The total resistance that results in the total MA in the loop INCLUDES the resistance caused by the voltage drop required by the components that need "energy" to function. Thus "energy" is consumed (converted to light, heat, etc).

These "loop powered" sensors can QUALIFY as loop powered because they use so little energy, that is: they are so efficient. Very little power is required to "operate" them. Very little energy is "lost" to heat, or converted to light.

As far as the "wire size" part of your question goes: If you notice the FACTORY wire on a 2 wire loop powered sensor is usually VERY small, 22gauge or so. As long as there is only ONE loop being fed by the wire, and it is properly FUSED, and LENGTH is not such that it becomes a factor, you can run the entire loop with this same size wire. I go larger sometimes just for durability.

Stationmaster

Stationmaster said:

As rsdoran stated, energy is measured in "watts" ( or in the case of the sensor's LED for example, MICRO-watts.)

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Not to be too nit-picky, but Power is measured in Watts.
Energy is measured in Joules, or kwh (kilowatt-hours).

Power and Energy are not interchangable terms.

Where consumption comes in is in the form of "watts"; which is the result of voltage and current. The primary form of wattage is heat.

Click to expand...

Now that I think of it I did not say energy or power, I said the above albeit the last part could have been expressed better the rest should be appropriate.

Thanks for that guys, so the sensor sets the current according to the process, then its acts like a constant current source, the voltage varies according to the resistance in the loop, so more devices in the loop the higher the resistance the higher the voltage but the current remains the same according to the process, so volts x milliamps = watts consumed, is this a ressonable summary?

jimbo3123 said:

Not to be too nit-picky, but Power is measured in Watts.
Energy is measured in Joules, or kwh (kilowatt-hours).

Power and Energy are not interchangable terms.

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Thank you.

I hope I didn't do any irreparable harm to RayK's comprehension of "consumption".

My use of the word "energy" was more general than definitive. I was trying to relate "heat", "light", and electron "movement", all of which are (generally speaking) "energy".

Besides, even though the 4-20MA sensor may use "energy", a kilowatt-hour meter may not be the best way to measure it.

Stationmaster

RayK said:

Thanks for that guys, so the sensor sets the current according to the process, then its acts like a constant current source, the voltage varies according to the resistance in the loop, so more devices in the loop the higher the resistance the higher the voltage but the current remains the same according to the process, so volts x milliamps = watts consumed, is this a ressonable summary?

Click to expand...

My understanding is that the voltage source stays constant, the resistance of the sensing device is varied causing "voltage drop" to vary. The "resistance" includes the power consumed.

RayK said:

so the sensor sets the current according to the process,

Click to expand...

correct

RayK said:

then it acts like a constant current source,

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let's avoid this for the moment.

RayK said:

the voltage varies according to the resistance in the loop,

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The power supply voltage is constant in the loop.
The transmitter's output stage varies the output current by varying its resistance which in turn, varies the current

RayK said:

so more devices in the loop the higher the resistance

Click to expand...

The more receiver devices in the loop, the higher the loop resistance.

RayK said:

the higher the voltage

Click to expand...

No, the loop's power supply voltage is fixed and constant. The transmitter has to vary its output stage resistance to get the desired current (say 11.2mA) to flow through two receiver devices, which would have more total resistance than just one receiver device.

RayK said:

but the current remains the same according to the process,

Click to expand...

Correct, the loop current representing the sensed value remains the same, as long as the power supply has sufficient voltage to drive the loop resistance of the receiver devices (the reason for the voltage vs loop resistnace graph above)

RayK said:

so volts x milliamps = watts consumed,?

Click to expand...

watch the units:
volts x amps = watts, example 24V x 0.0112A = 0.2688 watts
volts x milliamps = milliwatts, ex: 24V x 11.2 mA = 268.8 mW

Dan

Dan, VERY well put.