Re: HVAC Actuator controls (4-20mA or 0-10V?)

[Steve in San Marcos <dr.dr.steve@xxxxxxxxx>]

On Jun 29, 10:54 am, rush <rushi...@xxxxxxxxx> wrote:
> I am trying to understand why certain flow actuators require a current
> signal as opposed to the standard 0-10V voltage signal.
>
> Is this something left over from historic applications or is there a
> real benefit for current controlled valves?
> (run a single wire through all valves to be controlled, no voltage
> drops to worry about etc?)
>
> Any thoughts?

Well there are a number of good reasons to use a 20 ma current
controlled device.

1. Yes , if there is enought voltage to drive the current , then you
can connect all of the actuators in series , they each provide a
voltage drop ( about 3 volts ) in the 20ma loop.

2. Noise immunity,  these are low impedence devices and as such they
are less succeptible to votage induction from a stray source. They are
usually immune to a 120VAC conductor a fraction of an inch away.

3. Accountablilty,  if you connect a 20 ma indicator light in series
with all the actuators , then you know that the power supply is good
and that you have continouity in your system.

4. Usually they are built with an LED that drive a photodetector ,
the LEDs usually want 4-20 ma to light up.  Its a legacy thing also.
back from the old telegraph days I suppose.

Steve