# Telemetry(Two wire transmitter)

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#### yasser11

##### New Member
I have to design a two wire transmitter to convert a voltage signal in the range of 0-5 V into proportionate current of 4-20mA and that can drive two receivers having input resistance of 500 ohms each.Transmitter works on 24V and receiver takes power from curent loop.
what i need to know is how to design the V-I converter to achieve the above conversion.

#### mstechca

##### New Member
voltage to current :shock:

(he wants to emulate a power failure :lol: j/k)

Honestly, voltage AND current are required to make a circuit work. If you multiply them, you get POWER :twisted: !

but, a resistor can provide you with a desired current, BUT it doesn't take all of the voltage away.

You can use this equation:

Current (in amps) = Voltage (in volts) / Resistance (in ohms)

#### JimB

##### Super Moderator
MS

I think you should consider the saying:

"It is better to say nothing so that people think that you are a fool, than to open your mouth and prove it"

Yasser

I get the general idea of your project, but I have a few questions:

You are designing a converter which will take an input of 0 to 5 volts and give a proportional output of 4 to 20mA. Correct?
Where does the converter get its power, from a separate 24 volt supply, or from the 4-20mA loop?
In other words, is the converter powered from the loop or a separate supply?

JimB

#### heathtech

##### New Member
I am not entirely sure what you need, but, it sounds like you need a transmitter for a contol loop.....
You need to clarify this, because some people aren't familiar with control systems technology, and when you say "transmitter" they think you are talking about radio.
Especially when you say "reciever" in the same sentence. By reciever, Are you refering to a PID controller or an analog meter?

Fill me in on what your control loop is supposed to look like. What kind of transducer is creating the 0-5 v signal? This is irrelevant, I'm just interested.

Is this for a school project or an actual system? There are plenty of transmitters out there on the market!

BTW, another name for this is a signal conditioner.

#### heathtech

##### New Member
I don't have time today, but tommorrow I will post a drawing of a signal conditioner changing 0-5 v to 4-20 mA. Bear with me.

#### evandude

##### New Member
I agree with JimB... MS, that made no sense and had nothing to do with his question whatsoever :roll:

also try searching for "transconductance amplifier".

#### Roff

##### Well-Known Member
JimB said:
MS

I think you should consider the saying:

"It is better to say nothing so that people think that you are a fool, than to open your mouth and prove it"
JimB
Agreed. One of the most admirable qualities a person can have is to know where his expertise lies, and, more importantly, where it doesn't lie. In other words, be aware of what you know, and of what you don't know. We all have areas of expertise. I know that MS understands computers better than I.
I have a lot of electronics experience, but I keep my mouth (keyboard) shut on many subjects, because I don't have enough knowledge to comment intelligently. I still manage to stick my foot in my mouth occasionally.

#### heathtech

##### New Member
Design Formulas Simplify Classic V/I Converter
Ok, I found a good article on the web that shows a simple circuit and associated formulas for converting a voltage level signal to a 4-20 mA current loop. You have to scroll down to the second article on the PDF. Study it, and I think you can design a transmitter to do what you want.
I see that you will be having two instruments in the loop that add a total of 1kohm resistance to the loop. Please go into detail as to what these instruments are, because you said they get power from the current loop. (If they are analog style panel meters, or chart recorders or something else) I don't see a problem with that per/se , but if they add resistance to the loop, I think you have to take that into account and nulify it with a calibration circuit. I have professional training with transducer transmitters, proccess instrumentation, and proportional controllers, but since I work in a lab and not the field, I have forgotten a lot, and I never designed transmitters or controllers, just calibrated or repaired them. So, in other words, the best I can do is point you in this direction with this article, and maybe answer any question that might come up.

I still manage to stick my foot in my mouth occasionally
Don't worry, we all do! I like to feel like I know a lot about electronics in general, but a wise man knows what he doesn't know as well as what he does. MSTECHCA just obviously has never heard of control loops, and that stands to reason if he never worked in an industrial setting. That is OK, because I know very little about programming uControllers, but I won't be adding my 2 cents on that subject very often!

This might be a little oversimplified but I drew this:

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#### heathtech

##### New Member
I hope this guy comes back and reads his post, I hate to go through that research for it to be all forgotten!

#### heathtech

##### New Member
**broken link removed** is a very good reference site for signal conditioning and data aquisition. If all you need is hardware, and you don't have to build your own transmitter, you might browse the www.omega.com website for a suitable product.

#### Roff

##### Well-Known Member
heathtech said:
Design Formulas Simplify Classic V/I Converter
Ok, I found a good article on the web that shows a simple circuit and associated formulas for converting a voltage level signal to a 4-20 mA current loop. You have to scroll down to the second article on the PDF. Study it, and I think you can design a transmitter to do what you want.
I see that you will be having two instruments in the loop that add a total of 1kohm resistance to the loop. Please go into detail as to what these instruments are, because you said they get power from the current loop. (If they are analog style panel meters, or chart recorders or something else) I don't see a problem with that per/se , but if they add resistance to the loop, I think you have to take that into account and nulify it with a calibration circuit. I have professional training with transducer transmitters, proccess instrumentation, and proportional controllers, but since I work in a lab and not the field, I have forgotten a lot, and I never designed transmitters or controllers, just calibrated or repaired them. So, in other words, the best I can do is point you in this direction with this article, and maybe answer any question that might come up.

I still manage to stick my foot in my mouth occasionally
Don't worry, we all do! I like to feel like I know a lot about electronics in general, but a wise man knows what he doesn't know as well as what he does. MSTECHCA just obviously has never heard of control loops, and that stands to reason if he never worked in an industrial setting. That is OK, because I know very little about programming uControllers, but I won't be adding my 2 cents on that subject very often!

This might be a little oversimplified but I drew this:
As shown, this will work, but if you add the parasitic reactances of the loop, which could be thousands of feet long, it will be guaranteed to oscillate. Compensation might be possible, but there are alternatives.
Here is a primer on 4-20ma loops.
There are commercially available transmitters. Analog Devices has one. There are probably others.

#### heathtech

##### New Member
Very good primer Ron! You did your homework too!...LOL
Do you work in the industry too?

#### Roff

##### Well-Known Member
heathtech said:
Very good primer Ron! You did your homework too!...LOL
Do you work in the industry too?
Heathtech, I'm just an old analog design engineer. I have never used a current loop, but I've looked into them several times (sorta like, "I'm not a doctor , but I play one on TV" - anyone remember that?). I have designed lots of current sources, and lots of feedback circuits. Most of them worked. Some of them didn't.

#### heathtech

##### New Member
I thought the forum might benefit from a block diagram of a PID control loop using a 4-20mA signal.

I must remind everyone that this technique is used when devices are located significant distances from each other in an industrial setting. Thus, a transmitter is needed to relay the signal measured locally to a distant device, or several devices including indicators. Usually, in practical applications the controlling element is actually a control valve with a pneumatic actuator, but many types of devices can act as a controlling element including solenoids,pumps, motors, etc. In some of these cases an off/on digital type control might be more suitable than PID control. It depends on the requirements of the proccess.

#### Attachments

• controlloop.jpg
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