Voltage vs Current control loops

[AVGTech]

I would like to get some feedback on what method of control most of you prefer 4-20ma or a voltage loop like -10+10, 0-5v etc. I understand the noise considerations with voltage. I use a mix in my plants but determining a standard seems to be a Coke vs Pepsi argument. (all control loops are relatively short, under 30ft.)

 

[danw2]

Some devices like some valve positioners and two wire loop powered transmitters, can only use a current signal because part of the current signal is used to drive the electronics of the device. If you use those devices, current is inescapable in the loop, although the inputs are expected to have a precision dropping resistor which then becomes a voltage to measure and troubleshoot from.

I prefer the live zero that 4-20mA or 1-5V provides versus the zero based low end. An open circuit is really obvious with the expectation of a live zero.

A voltage output has the benefit of paralleling to more than one input, which seems to be more intuitive to the uninitiated than a series loop for current outputs.

Can I have a root beer?

 

[djs]

Current loops are low impedance, voltage loops are high impedance (at least for the recievers). For short loops no biggie, but you are luckly that your loops are soo short. Usual around here is several hundred feet.

 

[FrancisL]

You say

"A voltage output has the benefit of paralleling to more than one input, which seems to be more intuitive to the uninitiated than a series loop for current outputs."
But
You can take as many 1-5 inputs as youu like from a 250ohm resistor that carries the 4-20mA. In fact most times you combine 1-5V inputs with 4-20mA loops and 250ohms.

4-20mA provides power to the transmitters so you only have 2 wires.


Francis

http://www.controldraw.co.uk

http://www.s88control.blogspot.com

 

[mhyworld]

I prefer 4-20 mA for following reasons :
- it is less vulnerable against noise and voltage drop along the wire, especially for long distance.
- the current signal can be converted easily to 1-5V by simply fitting a 250Ohm resistor.
- you can use smart devices with HART technology which have a lot of advantages, including diagnostic and recalibration.

 

[dpc]

The 4-20 mA current loop is much more immune to electrical noise in an industrial environment. We would typically use 4-20 mA for any analog signal running outside of a control panel (field wiring). Two-wire transmitters can also be powered directly from the current loop.

Inside of control panel (assuming no 480 V or high current), the noise is much less of an issue and voltage signals are much easier to deal with. So we generally use voltage signals inside the panel and convert to 4-20 mA when it leaves the panel.

Beware wary of trying to run a 0-10V voltage signal into a VFD enclosure - a very noisy environment.



David Castor

http://www.cvoes.com