Help with 4-20mA splitter circuit design

DLMUK

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Hi,

I want to make a simple 4-20mA splitter for use when testing PLCs and save on wiring multiple signal conditioners into input modules.

The problem is i'm not really sure where to start! Does anybody have any tips they can share?

What I need to do is:

1. Using a 24VDC power supply
2. Take a single 4-20mA signal into my circuit
3. Get two 4-20mA signals out which mimic the input signal

The logic to me seems like I would need to amplify my current to double it, and then drop it over two equal resistors afterwards?

Thanks
 
Im sorry. My response was wrong anyway. I see that you are wanting to make a test tool.

Probably the way I would do it is use banks of resistors to simulate 4 ma, 12ma, and 20 ma. Then use a 3 position switch to swap between banks.


6000 ohms at 24 volts gives you 4 ma
2000 ohms gives you 12 ma
1200 ohms gives you 20 ma

bring 24 volts to your 3 position switch, and then feed out each leg of the switch to a bank of resistors that then goes out to the card you are testing.

I think you could fit this in a pretty small footprint.
 
i'm not sure I follow you correctly.

you have a 24 volt power supply.
are you wanting to bring a 4-20 ma signal into the analog input of the plc and create two 4-20 ma signals like the input?

or

are you wanting to use the power supply to cerate two 4-20 ma signals?

regards,
james
 
Sorry, to clarify what I want to do.

I have single 4-20mA signal. I need to split this into two 4-20mA signals.

The original signal conditioner providing 4-20mA input has a 24V power supply, therefore I have a 24V power supply to power any electronics I use to split my signal.

Exactly what Bernie Carlton has suggested, other than the fact I do not want to buy one off the shelf. I want to learn how to do it :)

Thanks
 
This is what I have so far.

The problem I am having is that the input resistance of the circuit it is attaching to (simulated by R2 and R4) is having an effect on the output current shown on XMM2 and XMM3.

Splitter.jpg

https://www.dropbox.com/s/ufnlroxfcil35wg/Splitter.jpg?dl=0[
 
Spend a few bucks to purchase a genuine 4-20 mA signal generator and then wire it in series through all of the analog current inputs that you wish to test in a batch.

Sometimes, it's just not worth the time and effort to reinvent the wheel.
 
Bit_Bucket_07 said:
Spend a few bucks to purchase a genuine 4-20 mA signal generator and then wire it in series through all of the analog current inputs that you wish to test in a batch.

Sometimes, it's just not worth the time and effort to reinvent the wheel.
Click to expand...

I do get what you're saying, but I am trying to learn at the same time. This isn't for learning how to test PLCs more to learn the electronics behind the simulators.
 
DLMUK said:
Hi,

I want to make a simple 4-20mA splitter for use when testing PLCs and save on wiring multiple signal conditioners into input modules.

The problem is i'm not really sure where to start! Does anybody have any tips they can share?

What I need to do is:

1. Using a 24VDC power supply
2. Take a single 4-20mA signal into my circuit
3. Get two 4-20mA signals out which mimic the input signal

The logic to me seems like I would need to amplify my current to double it, and then drop it over two equal resistors afterwards?

Thanks
Click to expand...
One method is demuxer 74HC4051. (better chip is MC14051BCP, it have wider voltage levels)

First you need change 4-20mA to volts with resistor, then with clock pulse multplex volts to different outputs

Last you can change volts back to mA with operation amplifier LM324 and couple resistors.

Or use arduino
 
Last edited:
Start here:

https://www.allaboutcircuits.com/te.../chpt-8/voltage-to-current-signal-conversion/

In your case, it will probably be easier to use a V to I converter, and at the input side of it convert your current to volts.

To do this, put a 250 ohm resistor in your current loop and pick your 1 - 5 volts off of each end of the resistor. This will become the input to your V to I converter.

And as long as the input impedance of the Op-Amp is high enough, you won't experience much error.
 
I know it's not your objective here, but more often than not you'll find that splitting the signal is not the main concern when someone is looking for this device. If you read the description from Berni Carlton's link, you'll see that isolation comes before the signal splitter.

For this reason, perhaps you should also look into opto-couplers on how they can help with the isolation.
 

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