Analog transmission techniques-problem

Hey there,

I have a small problem and hopefully someone here can enlighten me.
Regarding analog transmission, i know that there are several methods, such as 0-20mA, 4-20mA, 0-10V, etc, but i am having difficulty determining the characteristics and uses of each, as i currently have little practical experience with PLCs.
Can someone help? All help is greatly appreciated.

Hello CFisher, and Welcome.

The basics are current transmission (ma) and voltage transmission (V).

With current transmission, you connect your devices in a series loop, and usually only have one 'Sending' device, one 'Receiving' device, and one source of loop power. It is possible to have more than one 'Receiving' device, but for the most part, 'Current Loops' are one to one. They are very good in noisy enviornments, as the impedance of the overall loop is fairly low (typically less than 1000 ohms total, and more commonly around 250 ohms.

When using a 1-5ma, 2-10ma, or 4-20ma (the common levels for current loops), you have what is known as a 'Live Zero'. That means, that you can tell easily if there is a wire break, as the current will drop below the minimum value. The 0-xx ma loops don't offer this, and aren't used as much for analog transmission (they are often used for digital transmission, tty-style).

Voltage transmission can have one 'Sender' and multiple 'Receivers'. Voltage transmission systems are probably easier to work with, but have more problems with externally impressed noise on the actual signal. This is because they run at a much higher impedance (generally). Even when using a low source impedance signal source (as in an active, op-amp type output), the receivers (PLC Inputs, Drive's, Meters, etc) usually have an input resistance of well over 10,000 ohms. In some cases, you need to pay attention to the source impedance (such as when using a potentiometer) so that the load impedances don't cause the signal to actually be loaded down.

Voltage devices can be configured with a Live Zero, but it isn't a standard, and might still not recognize a wire-break due to impressed noise.

I hope that helps a bit, the smarter folk here will contribute better explanations I'm sure

Good luck!

thanks rdrast, that has cleared most of my problem up. Though one more thing-which sort of transmission would be most suitable for use over a small distance (approx. 5 metres) in a low-noise environment? I presume voltage transmission would be, though would any particular signal range, i.e. 1-5V, 0-10V, be preferred?

Today only +/- 10V, 0-10V, and 4-20mA signals are used in new installations.
If some of the older types, +/- 20mA, 0-20mA, 0-50mA, 0-5V etc. is used it is because of old habits.

It is easier (=cheaper) to generate a voltage signal than a current signal, apart from that a current signal wins in any other respect (*).

If you have the option to chose, then chose the 4-20mA as your standard. Even for signals that run a short distance.

In addition to the above-mentioned analog signals, there is the possibility of "raw" analog signals from load cells, thermocouples or RTDs.

*: The only other advantage of a voltage signal over a current signal is that the current transmitter will work as a low-pass filter. So if you need to monitor a signal with VERY fast changes, then voltage may be better.

One other HUGE advantage of using a voltage signal over a current signal (especially from a troubleshooting point of view), is that you can directly measure a voltage signal without interrupting the circuit.

I make it a standard practice of putting a 250 ohm resistor in series of the terminal strip in the device loop. Then if I ever need to read the milli-amps, I just do a voltage check across the resister without breaking the loop. 1V/250ohm=4mA 5V/250ohm=20mA You can figure out you mA's by reading your 1-5V across the resistor.

For the 24V supply terminals to 4-20mA loops I allways use terminals with an isolating switch.
Useful for tracking down shortcircuits.
And also for measuring the signal without interrupting it:
Attach meter, open switch, take reading, close switch, remove meter.

Jesper,

If you open a switch, which opens the loop, don't you just read the power supply voltage on the meter?

Dan

danw said:

Jesper,

If you open a switch, which opens the loop, don't you just read the power supply voltage on the meter?

Dan

Click to expand...

Not if you put your meter probes across the switch first! I use disconnect terminal blocks all the time this way.

Dan,
the meter has to be in current-measuring mode.