Hey guys,
I come across 4-20 mA control systems and sometimes have to spec parts and modifications. So...
Some devices generate a carrier DC voltage, about 10 to 30 volts, and some don't. I figure I need one device generating the voltage and the others not, in a loop.
My problem is that some suppliers don't clearly state whether the carrier voltage is supplied by them or not.
What is the vocabulary or 'phraseology' (ask Paulin), that I should use when clarifying these devices and the specs needed?
Thanks for the lesson in controls-tech-talk.
And while you're at it, throw in your 2 cents worth on dry and wet contacts. I've been curious to know.
Dave
I've no idea what you mean by using the term "carrier" here.
4-20 ma signal generating devices have been around a long time. As have 0-20 ma devices.
In the industrial/process control and instrumentation worlds 4-20 ma signaling devices are seen to have a couple advantages over the 0-20 ma devices.
First, if the instrument, sensor or whatever had a low enough power requirement (the power needed to operate the sensor's internal electronic circuitry), it needed no additional power supply plus associated wiring. i.e. Typically you just needed two wires to connect to it. One providing 24 volts DC. The other being the output signal. The instrument drew its own internal power needs from that 24 volt supply, with a maximum draw of less than 4 ma. The "signal" is the modulated current flowing through the loop. With 4 ma being the "low signal" and 20 ma being the "high signal". The sensor modulated the current flow at some point between those two values to represent a proportional value of the MV (measured variable). Such as pressure, temperature, or whatever.
It was quickly realized that this had an additional advantage. A second reason to prefer such method. And that is if you monitored the signal (current), and its value went significantly below 4 ma or above 20 ma then you probably had a fault condition. A failure of some kind. Wires were cut, sensor was playing dead cockroach, or whatever.
In fact the line of controllers we use most often recognize this and will flag a 4-20 milliamp input as "unreliable" if the input goes below 3.5 ma or above 22 ma.
If the sensor/instrument can get by with needing less than 4 ma for its own internal circuitry needs, its most often "loop powered". Meaning that it draws its power from the current loop. The loop formed by the current flowing from a 24 volt DC source (external from the sensor), to the sensor, out of the sensor, back to the input of the controller, back to the common of the power source.
For instance, all two wire ma sensors are "loop powered". They are consuming some of that current below the 4 ma level.
Some 4-20 ma signaling instruments/sensors require more than 4 ma to power their internal circuitry. For instance, CO2 sensors of every make I've seen so far used in the BAS industry.
In such cases, those will be 3 or 4 wire devices. The additional wires are needed to provide adequate power levels for the sensor circuitry. Typically a 12 or 24 volt AC or DC power supply on two wires. Then internal circuitry within the sensor powers and modulates an isolated 4-20 ma signal on the remaining wire(s).
Typically a device manufacturer provides cutsheets that indicate whether the device is "loop powered", needing only two wires, or whether it needs a separate power supply (2 wires) plus signal carrying wire(s).
For instance, a particular CO2 sensor we commonly use requires (consumes) about 70 ma for its own internal circuitry needs. Obviously, it can not be "loop powered".
As to wet and dry contacts.
Typically "dry contacts" refers to the opening and closing of contacts that have no voltage or current applied to them by the equipment having said "dry contacts". They're "dry". You sense the opening or closing by whatever method your equipment requires, can handle. Typically by placing a potential (voltage) on one side and checking to see if there is a voltage/current appearing on the other.
"Wet contacts" usually mean that the equipment providing such is presenting a voltage on the two wires to indicate closure, or an absence of voltage to indicate open contacts. You'd sense contacts "closed" by reading/detecting a voltage on an input, or by having said voltage pull in a relay (of proper coil voltage) and then reading the condition of that relay's dry contacts. Or whatever.
For instance, recently I needed to pick up an input from a machine that would indicate a fault had occurred and its controls were locked out. All that was available inside the device's internal circuitry was a "wet contact". Meant to power an alarm bell or light. No voltage, everything was fine. 120 volts AC present on the two wires mean a fault condition. I just put in a little 120 VAC relay, coil connected to the two wires. And connected my wires for my controller input to dry contacts of said relay.
