Combining power and data wires, Part 2
In the first part of this discussion, I described a few design ideas for combining power and data wires, continuing here with some analog signalling thoughts.
It would probably not have occurred to me to include analog circuitry (including the 4-20mA current loop) into the discussion on the combination of power and data since the information conducted is a single continuous analog signal and it seems to me that the word “data” has digital implications, but in his article, Mr. Pickering is right that it does merit a mention.
The telephone (land-line) is an early example of the combination of power and analog signal on the same pair of wires. I also believe that distribution amplifiers for TV signals are also powered over the coaxial cable. These are solutions within their industries and I am not sure how well they extend into the industrial control area, which is where I practice. Let’s investigate some options that I have come across.
It is possible to configure some three-wire sensors for a current output and then use them as two wire devices. For instance the LM35 temperature sensor is a three-wire device — supply, ground, and signal. By connecting the output to the ground pin via a 200 O resistor, the device regulates the current flowing in proportion to its temperature. This allows the device to be remotely connected using two wires as you can see in Figures 16, 17, and 19 of the data sheet (linked above).
The 4-20 mA current loop technique uses an analog signal between 4 and 20 mA to indicate a measured parameter. The input may be from a sensor, current, voltage, or anything you may choose to encode. Aside from the facts that a current loop is inherently noise resistant and only two wires, and the offset allows for open circuit detection, the whole raison d’etre of the 4-20 mA is that the 4 mA can be used to power the module. Of course 4 mA is not a whole lot of current, but it is versatile enough to have endured in the industrial marketplace through more than five decades. For a more detailed analysis there are several app notes at Analog Devices and TI, especially the TI series on 2 wire 4-20mA transmitters. Might I also humbly suggest my article “The 4-to-20-mA Current Loop” in the August 2010 issue of Circuit Cellar. I have seen mention of 10-50 mA current loops in literature, but I have never seen one in reality.
The voltage to power a 4-20 mA receiver is derived across some device like a zener or resistor. From Kirchoff’s Law all the voltage drops in the loop (including the voltage drop across the transmitter itself and the drop across the resistance of the wires) must sum to equal the supply voltage. If the supply voltage is less than the total of the drops required then the current will tail off. The voltage (excluding the transmitter volt drop) is known as the “compliance” of the transmitter and should be specified on the data sheet of the transmitter. PLCs (Programmable Logic Controllers) usually use a series connected 250 O resistor to develop 1-5 V for an analog to digital converter. It is normal practice to connect several 4-20 mA receivers in series, for instance a panel meter and a valve. Most industrial applications use 24 VDC, and it is not uncommon for the compliance to be 15 V or more. The only limitation is that your circuit must work on 4 mA, however it splits up and then recombines as it passes through the receiver. Figure 3 shows how you might use a voltage reference to power the circuitry. Only a portion of the 4 mA passes through reference so you need a device that will hold its voltage with only microamps passing through it. Sometimes a reference is better than a zener diode, say, because it may require less current, but in addition you sometimes need an accurate reference voltage.
Figure 1. Power for the circuitry is derived across voltage reference U1. The actual signal is detected across R1 and fed to an instrumentation amplifier powered from the voltage on U1. What happens next is beyond the scope of this article.
Because of micros and other circuitry sometimes you need more than 4 mA. The way around this is to use some kind of switchmode power supply (power in=power out, in a perfect world) and you trade a higher input voltage at 4 mA to a lower output voltage at >4 mA. This switcher itself can be quite complex requiring a coil and having a high quiescent current, but Linear Tech has introduced the LTC3255 — a switched capacitor device that may answer your prayers.
Having opened the 4-20 mA can of worms, we need to consider how to power a two wire 4-20 mA transmitter. Figure 4 shows a classic implementation although it can be a fairly confusing undertaking because the common point of the internal supply is not the ground of the system.
Figure 2. 4-20 mA transmitter generating 5V from the loop. This configuration is often implemented within a monolithic IC. My Circuit Cellar article (see above) describes how to calculate the values of the components.
Most of the comments made for the 4-20 mA receiver are applicable for the transmitter as well. The compliance of the transmitter will be the supply voltage minus the volt drop from ILOOP+ to ILOOP- at 20 mA.
I have never seen the circuit that Mr Pickering presents in Figure 1 of his article (taken from the Linear Tech datasheet). Given that you could only get (0.3 V x 4 mA =) 1.2mW at the output, it would seem to me that the intention would be to derive a voltage without disturbing the voltage compliance of the current loop to any great extent.
And then there is the HART protocol which superimposes a Bell 202 Modem signal (FSK 1200 baud, 1200/2400 Hz) onto to current loop. Unlike adding data to the supply line (as with Foundation Fieldbus) HART can be introduced on any existing 4-20 mA current loop without needing a filter in the loop itself. The HART signal is AC coupled so as not to affect the mean value of the DC current in the loop. Not all 4-20 mA devices (e.g. a loop isolator) will pass the HART signal or will only do it in one direction (HART being bi-directional).
Have you any other techniques of combining power and data onto the same wires, or perhaps do you feel motivated to try and implement these techniques on a project of your own? Remember to save the gyrator design in your toolbox!