using resistors to change signal type

[luke mcbride]

I was wondering if anyone would mind taking the time to explain using resistors to change signal types. Im a little confused as to which way they can be used, i.e they can change ma signal to vdc signal, but not the other way around(or vice versa) which way is it and why? I guess from what ive seen of them being used they are placed in parallel with the device that you are sending the signal too? I know this is probably an elementary question to most of you here, but it just seems like when I ask questions at work, I get different and conflicting answers, and I cant really find anything online. I just want a good undrestanding of when I can use them, how to use them, and what they are actually doing.

thanks alot for any help

[timebuilder]

I was wondering if anyone would mind taking the time to explain using resistors to change signal types. Im a little confused as to which way they can be used, i.e they can change ma signal to vdc signal, but not the other way around(or vice versa) which way is it and why? I guess from what ive seen of them being used they are placed in parallel with the device that you are sending the signal too? I know this is probably an elementary question to most of you here, but it just seems like when I ask questions at work, I get different and conflicting answers, and I cant really find anything online. I just want a good undrestanding of when I can use them, how to use them, and what they are actually doing.

thanks alot for any help

First, this is more of a question for the Pro Technical forum. Apply for pro membership.

Second, an understanding of circuit design on this level is not typical for an HVAC tech. This is more of a question that a design engineer or electronics hobbyist would ask. I would need some circuit designs to look over to give an opinion on this.

Can you share the basis of this curiosity with more info?

[nyrb]

I think your refering to converting a 4-20ma signal to Volts DC. It's basic ohms law, using a 500 ohm resistor a 4-20ma signal will give you 2-10VDC across the resistor.

Ohms Law E=IxR
.004 amps x 500ohms = 2 Volts
.02 amps x 500 ohms = 10 Volts

Yes it only works converting milliamps to volts, it does not work the other way around.

Hope that's what you were asking about.

[Snoring Beagle]

Look here might shed some light...

http://hvac-talk.com/vbb/showthread.php?t=721471

[timebuilder]

Yes.

A design engineer would use this technique to convert a mA signal to a voltage input.

Again, not something a typical HVAC tech would need to know.

A controls guy, yes.

[flange]

woa, this is commonly done by certain controls manufacturers to integrate an industrial type instrument to a commercial controller. for example, using an industrial flow meter on a chilled water plant such as a rosemount coupled to a honeywell system. to convert the signal, they simply use a five hundy ohm resistor across their terminal input, and this is probably the type of thing he is asking about.

[Dallas Duster]

Yes.

A design engineer would use this technique to convert a mA signal to a voltage input.

Again, not something a typical HVAC tech would need to know.

A controls guy, yes.

Don't sell hvac tech's short on needing to know this info . Don't forger the need for a 250 ohm resistor either.

[timebuilder]

Don't sell hvac tech's short on needing to know this info . Don't forger the need for a 250 ohm resistor either.

Of course, this depends on who the tech is, and what he does.

My point is that most HVAC techs neither know nor need to know this type of info.

As Flange points out, there is a need for some techs to know this...those who work with controls.

Most techs are not "that guy."

[luke mcbride]

thanks alot , havent had a chance to to go to the links yet, but im sure that will clear up some questions. Its amazing to me how many people say that it works both ways, how can so many people be misled??? why is it that it only works converting ma to volts?? when I visualize it in my head, it seems to make more sense that it would work the other way, if you have a voltage why can you not insert a resitor to generate a current? and if I have a milliamp signal, dosent it have to have a voltage pushing it already?? obviously im missing something!! just gotta figure out what! thanks again for all your help, really appreciate your input. I am a controls tech by the way.

[timebuilder]

I'm not a controls guy, but I have gone through a couple pounds of rosin core solder in my life.

Yes, you can connect a resistor to a voltage and there will be a current through it. However, it strikes me that this is the extent of the loop you just created. In order for this to be useful, you would have to create a loop that sends that current somewhere. Since the point of a dc current loop is to reliably convey info using a range of current, I'm not sure why you would want to take a voltage and convert it to a current. What are you hearing about the reasoning for this?

[luke mcbride]

well, I guess im thinking of a situation where you have a vdc output on a controller and a end device that wants to see a ma signal, and you need to fix it asap. and when you say the extent of your loop, I guess im visualizing the 3 components, an output on a controller(0-10vdc) a resistor in series and the device receiving the signal all in the loop. like I said, I know im off base somewhere, I think some drawings are what I need.

the reason I asked this question originally, was I found a controller, with 0-10 vdc output controlling a chilled water valve that operated on a 4-20 milliamp signal, it had a resistor across the terminals of the chilled water valve, I was told that was converting the signal, and the engineers said the valve has always worked!! somebody has to be wrong here .....right?

[nyrb]

I’ll take a crack at trying to explain this.

When a control system generates a 4-20ma signal to control a device it does not know what the input impedance (resistance to make it easier) is, it just drives the DC voltage up until the desired current output is attained. Generally speaking current driven devices have relatilely low input impedances and Voltage driven devices have relatively high input impedances.

For example if I am driving a valve with an input impedance of 100 ohms it will take 0.4 VDC to create 4ma of current and 2 VDC to create 20ma of current.

Now if you change out that valve to a different manufacturer and the new valve has an input impedance of 250 ohms now you will require 1 VDC for 4ma and 5 VDC for 20ma and the controller will automatically adapt to provide the required signal.

Additionally in the case of multiple damper actuators being driven by a single analog output, all the damper motors are in series and the current is the same throughout the circuit regardless of input impedance. This is only limited by the maximum DC voltage that the control system can deliver. 4-20ma control signals also deal with line loss by design.

In the case of DC voltage control there is no way that I know of to set up a 0 – 10 volt circuit so that it will constantly drive a 4-20 ma actuator. Just think about it 0 VDC output means 0 ma of current flow. Potentially a 2-10 VDC signal could be configured with a resistor in series with the driven device so that the effective input impedance was 500 ohms and therefore create 4-20 ma. A resistor in parallel with a 4-20ma driven device would not affect the current flow through the device at all it would simply create an alternate path for current to flow also known as a current divider circuit.

There are devices available that will accept milliamp or DC voltage input and convert it to whatever you want and these are what I usually see used when converting a 0-10 VDC signal to 4-20ma. See Attachment.

That's about the best I can do without pictures, circuit diagrams and a thourough lesson in ohms law and basic DC theory.

[luke mcbride]

Thanks nyrb, that was exactly what I needed!! I appreciate you taking the time to really explain it, and thank you to the other posters as well. cant wait to share with everybody at work, I have a feeling some of them aren't going to take this lightly, they are positive it works both ways!!

[timebuilder]

Thanks nyrb, that was exactly what I needed!! I appreciate you taking the time to really explain it, and thank you to the other posters as well. cant wait to share with everybody at work, I have a feeling some of them aren't going to take this lightly, they are positive it works both ways!!

You can feel free to direct them to this thread.

There is a converter in electronics for almost any purpose. It just depends on how badly you want to convert something.

[Randy S.]

Have done it many times, when I am given a 2-10VDC actuator to run on an existing 4-20mA loop.

You simply end up with the 500 ohm resistor in series with the 4-20 mA and read the voltage drop across the resistor as your 2-10VDC. Ohm's Law 101.

[nyrb]

Have done it many times, when I am given a 2-10VDC actuator to run on an existing 4-20mA loop.

You simply end up with the 500 ohm resistor in series with the 4-20 mA and read the voltage drop across the resistor as your 2-10VDC. Ohm's Law 101.

That's the method we have discussed as working, the query was whether or not you could incorporate a resistor in a 2-10VDC output to control a 4-20ma actuator.

[The Doctor]

Had to do it maybe once. It was a Pure humidifier with failed internal resistor and the options were these:
repalace the main control board, or add 500 ohm resistor and change the control type in the configuration menu.

That was fun and the tech support was helpful in listening to the symptoms in order to suggest the option. Besides, the humidifier right next to this one was a great reference point for "things that work right for 100, Alex".

Since it doesn't happen much I barely remember which came first--the 2-10 or the 4-20.