HVAC Actuator controls (4-20mA or 0-10V?)
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Posted by rush on June 29, 2007, 1:54 pm
Please log in for more thread options 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? | |||||||||||||||||||
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Posted by Steve in San Marcos on June 29, 2007, 2:44 pm
Please log in for more thread options 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 | |||||||||||||||||||
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Posted by Charles Sullivan on July 3, 2007, 3:07 pm
Please log in for more thread options On Fri, 29 Jun 2007 18:44:47 +0000, Steve in San Marcos 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 | |||||||||||||||||||
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Posted by MarcoS on July 16, 2007, 3:14 pm
Please log in for more thread options > On Fri, 29 Jun 2007 18:44:47 +0000, Steve in San Marcos 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 I'd say 20mA loop runs as far as you want....As long as the voltage source at the beginning can cope with supply enough voltage for 20mA (of course you would use a real current source anyhow). One of the source I know (back from early computer experience) is use of 20mA loops for driving TELETYPE printers! In industrial automation, 4-20mA is still used a lot (must be the FORTRAN of the field busses I gues :-)). And RS232 can (with good cabling ) be operated also much further than 100feet, not? And use of old-fashioned modems as repeaters used to work too. regards, Marco | |||||||||||||||||||
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> 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?