4-20mA Sensor with 12V adapter

[Derby]

Hey all,
planning to use an instrument that gives a 4-20 mA analog output. I am not sure whether to use a SMPS based power source (which is cheaper) or a regulated power source for the DC power supply? Any ideas?

 

[alec_t]

Welcome to ETO!
An SMPS is a regulated power source.

 

[Derby]

Can we use it with 4-20mA circuit or not ?

 

[Pommie]

You can use a AAA battery if you want. If it'll work depends on the instrument(?)/circuit.

More details may help.

Mike.

 

[unclejed613]

**broken link removed** a 4-20mA circuit is a current loop. the most common power supply for 4-20mA loops is 24V, but can use anywhere from 12V to 36V. for digital data, 4mA is a "0", 20mA is a "1". current loops are often used for connecting industrial equipment to a controller, but they have been used for teletype lines, and a standard telephone works on a similar principle, except the variations in current carry analog signals. the power supply for current loop communications is usually a "floating" supply, and there is no signal ground.

 

[large_ghostman]

**broken link removed** a 4-20mA circuit is a current loop. the most common power supply for 4-20mA loops is 24V, but can use anywhere from 12V to 36V. for digital data, 4mA is a "0", 20mA is a "1". current loops are often used for connecting industrial equipment to a controller, but they have been used for teletype lines, and a standard telephone works on a similar principle, except the variations in current carry analog signals. the power supply for current loop communications is usually a "floating" supply, and there is no signal ground.

And finally i know why they use current balance relay on telephone stuff.

 

[KeepItSimpleStupid]

The ripple on an SMPS power supply is higher, but thats only a part of the equation. The A/D may or may not respond to that ripple. There also may be front end filters. Then ther is the power supply rejection ratio and even the resolution. So, it's not a simple answer with the info given. My answer: probably

 

[gophert]

Can we use it with 4-20mA circuit or not ?

If you put a 250 ohm resistor across your sensor's output, it will generate a 1 to 5 V signal (see ohms law for reference).

If you want a different voltage range, use a different resistor.

If you need an offset, like 0-5 volts, that you can't get with a simple resistor, add an op amp and a voltage reference then wire it as a differential amplifier.

 

[JimB]

If you put a 240 ohm resistor across your sensor's output, it will generate a 1 to 5 V signal (see ohms law for reference).

I think you mean a 250 Ohm resistor ?

JimB

 

[gophert]

I think you mean a 250 Ohm resistor ?

JimB

Ha, thank you. Fixed.

 

[Derby]

I am able to know about so many suggestion but i still have a huge doubt ....
if we are using resistor of 250 ohms to solve the problem of 4-20mA ouptut to easily generate in 1-5 V and using external 24VDC power then what is the importance of using 4-20mA current loop Recievers modules ?
Does these modules strictly requires pure DC voltage or we can provide them power supply with AC to DC voltage adapter of 5v?

 

[schmitt trigger]

Very simple, just look at the feature set, that you don't get with a simple resistor.

Features

• 4 Channel 4-20 mA Current Loop Receiver
• 4-Channel Input with 16-Bit Resolution
• 4-20mA Industrial Sensor Monitoring
• INA196 Current Shunt Monitoring
• Ideal for Sensor and Instrument Monitoring
• ADS1115 16-Bit Analog to Digital Converter
• Onboard 16V Voltage Supply (Boosted from 5V)
• Ideal for Industrial Automation 4-20mA input

I would also add that these modules very likely have extensive fault protections.

 

[Reloadron]

Can we use it with 4-20mA circuit or not ?

Yes, I use 24 volt switch mode power supplies all the time with sensors having a 4 to 20 mA output. You can use a SMPS or a Linear regulated DC power source. Here is an example of a SMPS (SOLA 24 VDC) power supply powering four each temperature sensors with each having a 4 to 20 mA output. Then as Gopher mentions each 4 to 20 mA loop is passed through a 250 Ohm 1% precision resistor providing 1 to 5 VDC which is passed to an ADC.

Ron

 

[KeepItSimpleStupid]

I'm confused. If you look at the IN196 datasheet, Vin is MISSING, so I have NO idea how this thing is supposed to work.

EDIT: Misssed "Onboard 16V Voltage Supply (Boosted from 5V)" So that's how it works!

The general idea with 4-20 mA or 0-20 mA is that ground loops are eliminated. Local voltage (1-5, and 0-5 typically) outputs or inputs can be converted to currents with just a resistor. On the source side the resistor is at the source. On the receiving side, the resistor is at the receiver.

With current loop, you generally don't need isolation and the ground loops are eliminated.

Isolated is even better. I had to control 7 power supplies, but there was no output for voltage or current. I think we used isolated current out for control and isolated voltage in for measure. What happens in a traditional DC power supply is that the control reference is referenced to the positive output voltage and each supply could be different, so that really wrecks havoc on things.

Current monitoring tapped into a sense resistor in the power supply, this the input was isolated voltage to the A/D.

Voltage monitoring (max 40 V) was done with a voltage divider into an isolated voltage input.

In another set-up, a multiplexed non-isolated pseudo differential A/D was used for the outputs of the nstruments and current loop was used for the inputs with a resistor at the instrument.

One instrument allowed you to connect the gnd reference back to the "computer" for voltage programming, This was unusual.

I did mess up with a "in house"instrument converted to computer control. We initially had a manual 6-channel mass flow controller (MFC) interface with 3 power supplies.
Each MFC was normally programmed with a resistor that developed 0-5 V relative to the remote MFC. Getting the output relative to the remote MFC was easy with a pseudo differential A/D. I made changes to make the instrument easily calibratiable from he front panel. A switch gave you a 5V output and you just dialed the calibrate pot. Decimel selection was internal with a DIP switch.

The input turned out to be a problem that I did not address initially address and management would not allow me to later. I did not use current loop outputs and I had no way to return the reference to the "PC" to eliminate ground loops.

I lobbied HARD for a separate acquisition system for a MAC II when there were LOTS of "interface cards". I was turned down. When the NuBus MAC;s went to 3 slots, it wasn't my problem. A used MAC ended up being used.

I think the project was eventually scrapped, but I have no idea. I knew nothing about PLC's at the time.
I would have needed an difference amplifier for all of the channels. Management chose to by a 4-channel readout.

 

[unclejed613]

in a loop, all the devices (power supply, transmitters, receivers, etc...) are wired in series. that's why there's no "Vin" or "Vout". the transmission medium is the current,

 

[Derby]

Very simple, just look at the feature set, that you don't get with a simple resistor.

Features

• 4 Channel 4-20 mA Current Loop Receiver
• 4-Channel Input with 16-Bit Resolution
• 4-20mA Industrial Sensor Monitoring
• INA196 Current Shunt Monitoring
• Ideal for Sensor and Instrument Monitoring
• ADS1115 16-Bit Analog to Digital Converter
• Onboard 16V Voltage Supply (Boosted from 5V)
• Ideal for Industrial Automation 4-20mA input

I would also add that these modules very likely have extensive fault protections.

Ok so you mean to say that these will help me to use less gpio pins will make my sensors much more power efficient ?

 

[KeepItSimpleStupid]

Efficient is not a good word to use. Less GPIO because of the I2C.

Elimination of ground loops is probably #1 and #2 is reduction in wiring because the 4 mA can be used to "power" some sensors.