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

[Charles Sullivan <cwsulliv@xxxxxxxxxxxx>]

On Fri, 29 Jun 2007 18:44:47 +0000, Steve in San Marcos wrote:

> 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

One of the major advantages of the old 20 mA current loop as opposed
to RS232 is the distance.  The 20 mA loop is supposed to be good
for up to about 1/4 mile versus perhaps 100 feet or so for RS232
if you're lucky