4-20mA current loop

[PFakhir]

I want to make a Voltage to Current converter that has an output of 4-20mA and these are my general specs for it.
Output signal current:0 - 20 mA
Load impedance:< 500 Ohm
Linearity:± 2 LSB or better
Resolution:12 Bit minimum
Conversion time (typ):2 ms maximum
Measuring error (25°):< ± 0.1 % of the full scale value
Temperature coefficient: < ± 0.01 % / K of the full scale value
Isolation system/supply: 500 V (optional for this project).

however i notice that alot of current source converter have very high output impedance and i wanted to check how i can achieve this low impedance current source? or did i go completely in the wrong direction...

 

[Ian Rogers]

Mine is about 250ohm..... I use a 12 bit DAC from Microchip... and a 3 transistor TTL convertor with a VERY low temp spec resistor.... I get reasonable results...

I think the reason that most 4..20mA devices are high impedance is that some devices have reverse protection....

 

[PFakhir]

huh... ok. is there any available to purchase. or should i go about making 1 myself and if so do you have a circuit diagram i can follow. im very new at this so ive been doing research on this for a few days now and it seems like there is a different method for each circuit. it gets confusing not knowing which to follow as a guideline.

 

[PFakhir]

Also if i have a 16 bit register from my micro controller should i be using a DAC with a 16 bit resolution?
currently i have these 2. that have 12 bit which is my minimum requirement
https://ca.mouser.com/Search/ProductDetail.aspx?qs=1THa7WoU59GA7WbHsL7LkA==
https://ca.mouser.com/Search/ProductDetail.aspx?qs=wTZ/Fzl837aRRkKyTB8rGQ==

 

[Hayato]

Hi Fakhir,

Theoretically, if you have a current source, your main concern is not about the load impedance, but how much voltage your source is able to provide.

For a 500 ohm load at 20mA, your source must be able to supply at least 10V to the load.

For the current load, if the project is not too critical, you could use a simple bipolat junction transistor, such as BC548 or 2N2222, with a emitter resistor, which you vary the base voltage to set the desired current.

A more elegant solution is to use a current mirror, wich provides some isolation from the load to the control circuit.

 

[KeepItSimpleStupid]

Any reason why you can't try to buy one? You forgot to mention Bandwidth.

An IDEAL current source has in infinite input Z. Output Z is somewhat hard to determine, but it's basically delta V over delta I of the output.

Compliance voltage and the amount of current it can output is important.

I did a design once of a voltage controlled (-5 to 5 V) current source that could do +-50 mA with a +-10 V compliance, but it wasn't for an industrial process.

 

[dougy83]

What's the need for a DAC? If it really is a voltage to current converter that you want, then there's a simple circuit attached. A low offset opamp is required.

 

[PFakhir]

Hi Fakhir,
Theoretically, if you have a current source, your main concern is not about the load impedance, but how much voltage your source is able to provide.
For a 500 ohm load at 20mA, your source must be able to supply at least 10V to the load.
For the current load, if the project is not too critical, you could use a simple bipolat junction transistor, such as BC548 or 2N2222, with a emitter resistor, which you vary the base voltage to set the desired current.
A more elegant solution is to use a current mirror, wich provides some isolation from the load to the control circuit.

Hey, sorry meant to update this. when i got home my computer just died and now is a giant paperweight but thats problem for another thread i suppose. So yes its like you said, and i just misinterrpreted the details.
i just have to be sure to provide 4-20mA output for a up to 500 ohm load(not the transmitter). so that would be a 0-10V output

im looking through several products online at the moment that will do this for me. if anyone who has more knowledge in this area who would like to make a recommendation that would be helpful

 

[PFakhir]

What's the need for a DAC? If it really is a voltage to current converter that you want, then there's a simple circuit attached. A low offset opamp is required.

im going to be using it to send a signal from a micro control and would like to convert it to a analog signal. from my understanding this reduces the distoration over the length of a wire.

im just working on this 1 piece of the project, would of been better if they given me better details of what they need.

 

[unclejed613]

current sources are by definition high impedance devices. a hypothetical perfect current source would have infinite output impedance and infinite compliance voltage. real current sources are designed for as high an impedance as possible (usually anywhere from 10k-250k), and are limited in their compliance voltage to a little bit less than whatever your supply voltage is.

 

[dougy83]

im going to be using it to send a signal from a micro control and would like to convert it to a analog signal. from my understanding this reduces the distoration over the length of a wire.

You will have no distortion if you transmit and receive the signal digitally. With the 2ms max time between values that you've stated, it's easy to transmit the value kilometers...

im just working on this 1 piece of the project, would of been better if they given me better details of what they need.

If you don't have sufficient information to complete your part, then you should ask for the information that you need.