Voltage Output

A current output is independent of loop cable resistance and volt-drop.

Hi vscid.

In addition to PWSlack's response,

Advantages:

A voltage output generally takes an interpolation step out of the process, as equipment that receives current inputs will immediately convert it into a voltage that the rest of the circuit uses to measure with, anyway. This helps to reduce conversion errors that, while usually quite small, can be significant, depending on your application. Voltage outputs also are more quickly and easily read and interpreted by troubleshooting personnel with simply a voltmeter. More and more monitoring and control systems are moving toward voltage signals, especially PC based systems, as the A/D converters in these I/O cards are getting very sophisticated, making set up and tuning a much easier and quicker task.

Disadvantages:

NOISE. Voltage based control signals are much more prone to noise induced by nearby circuits. As such, extra precautions need to be taken to ensure accuracy and repeatability. Also, a lot of legacy and present equipment still use current based signals for these reasons, as well as those given by PWSlack.

Personally, I design exclusively with voltage-based systems, taking the extra time and expense to properly guard against noise, in order to take advantage of the newer PC-based systems, and their more direct voltage inputs and outputs.

I hope this helps.

CJ

CJ,

What do you exactly mean by:

A voltage output generally takes an interpolation step out of the process

"as equipment that receives current inputs will immediately convert it into a voltage that the rest of the circuit uses to measure with, anyway."

The original signal of any output card starts as a voltage, and if the card has a current output, it has to be converted to a variable current output. So, if you're set up to read voltages into, and send out of your I/O cards, your cards, (or other equipment) obviously don't have to convert the current signal, they can read and output the voltages directly, without the conversion error. They need only to be amplified, as necessary, which, in my experience, is easier to monitor and control.

This also affects the units receiving the voltage output commands, because if they're set up for current input, they will have to convert the variable current to a variable voltage in order to make it useful to the device being controlled.

So, in a current control scheme, the signal originates as a voltage, gets converted to a variable current output, is sent to the device being controlled, then gets converted back into a voltage by the device's onboard control electronics. That makes TWO conversions for every channel of output. I have seen error rates as high as 10% with this type of set up, and my projects typically require a maximum of 1%.

So, my reference to "an interpolation step" referred to the output and controlled devices not having to convert or "interpolate" the current signal.

If I have a sensor with a 0-20mA current output, can I convert it into a voltage output using a resistor across the output?

http://archives.sensorsmag.com/articles/0498/vol0498/index.htm

Vscid,

That is the most common method used. Check the wiring diagrams for many analog input devices which allow either a voltage 1-5VDC or 4-20mA input and you will see a precision resistor being used internally. The only real error in this is to make sure that your 250Ω resistor has a low-enough error, such as 0.1% tolerance. The conversion accuracy problems others have mentioned will be minimal. You will then be left with the design challenges of assuring that a current output has enough voltage driving its source to deliver its rated current to that input's resistor. Or, you will have to ensure that the signal is suitably protected or filtered from noise.

--JMM

Yes the current times the resistance will produce a voltage. So the accuracy of the voltage will be directly related to the accuracy of the current signal and the accuracy of the resistor being used for the conversion. This is very common practice but you need to remember that cheaper resistors have higher temperature coefficients and drift over lifetime so if you need accurate conversion for a long life and over wide temp rage you need to carefully select the resistor. Not so important if you just need a loose tolerance High and low output or you have a closed loop correcting the error. One more thing to keep in mind is that the current output will have a voltage limit that you will not be able to exceed no matter what the resistor value is, That may sound obvious to some but you would be surprised how many times it comes up.

Well said!

Output from what and into what?

The question is too open ended.

Most control circuits require a 4 to 20 milliamp input. Most control circuits normally supply a 4 top 20 milliamps.

This is a very high impedance circuit that has a very low voltage drop that can be used over a very long control wire system. Impedance being resistive, capacitance, and/or inductance.

Most control circuits are therefore a variable voltage to maintain specific amp output for a specific/input - specific/output requirements. This variable output will incooperate offset voltages and proportionment outputs to achive the desireable results. temperature, heat,speed, or other results.