# Difference between a voltage and a current signal?

1. Dec 30, 2015

### fog37

Hello Forum,

I have heard about voltage signals and current signals in the context of electronic circuits. What is the difference?

Is a voltage signal what is measured by a voltmeter between two points/wires while a current signal runs on a single conductor and is measurable by an ammeter?

Thanks,
fog37

2. Dec 30, 2015

### meBigGuy

Both require two wires, and source and a return, so to speak.

A voltage signal would be a switched voltage source, for example, 5V and 0V to represent ON and OFF. The detector would be a high impedance load, like a cmos logic gate.
A current signal would be a switched current source, for example 5ma and 0ma to represent ON and OFF. The detector would be a low impedance load, like an opto isolator or transistor
(do a google search for "current loop" to see some examples).

3. Dec 30, 2015

### CWatters

Fog37 - Your description is close enough, although for a current to flow you do need a completed circuit eg a return path to the source, so more than one wire is usually required.

4. Dec 30, 2015

### Averagesupernova

I recall long long ago working on a system that could output a 300 volt peak to peak signal. It was mostly direct coupled transistor stages balanced (differential mode) the whole way through and at one point one of the transistors was configured in common base mode. Its emitter was directly coupled to the collector of the previous stage. I would troubleshoot this thing and signal trace with a scope and all of a sudden the signal was gone. We often described this as: "You can't see it, now it's a current. Go to the output of the common base stage and you will get the voltage signal back."

5. Dec 31, 2015

### Hesch

The practical diffeference is of course, that the current signal is transmitted as a current level and vica versa.
So the current signal is induced by a current source, that has an infinit high impedance. Thus the current signal is very robust to disturbances along the transmission line, such as electric fields and magnetic fields in the environment, that will induce voltage, but not current due to the high impdance of the source.

Another practical thing is that current signals often are specified as e.g. 4mA . . 20mA, corresponding to a measured temperature 0C° . . 100C°.
That's because the device, measuring the temperature, may consume 4mA for its own internal purpose ( converting temperature to current ).
So within the specified temperature range, the device consumes an extra amount of current = 0mA . . 16mA.

In this way the device is power supplied through the signal conductors.

That's clever, and rubust to electric noisy environment. ( See attached ).

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6. Dec 31, 2015

### jim hardy

and there's added benefit of 4 to 20 ma (10 to 50 is also common)
in that an open circuit, ie detector has failed or somebody left it unconnected, announces itself as a reading well below zero.