Analog, Voltage to Current conversion

There is a real reason I am asking this, but wont go into it right now.

Question is, can you simply take a 0-10V output from a device, to a device with a 4-20ma input, then install a 500 Ohm resistor across the + and - at the 4-20ma device and call it good?

What u describe works the other way round, resistor across a V in driven with a 4-20 mA loop. Have seen some discussion on this forum about V out to A in, but the resistor is in series..

padees said:

There is a real reason I am asking this, but wont go into it right now.

Question is, can you simply take a 0-10V output from a device, to a device with a 4-20ma input, then install a 500 Ohm resistor across the + and - at the 4-20ma device and call it good?

Click to expand...

No you can't.

A 4-20mA Input will measure the voltage across an internal resistor, usually 250 Ohms, but could be anything the designer chose.

Adding your own resistor is just putting it in parallel to that internal resistance, and that will affect the voltage the input sees.

daba said:

Adding your own resistor is just putting it in parallel to that internal resistance, and that will affect the voltage the input sees.

Click to expand...

If the V out can push enough mA, it wont effect the current through the input at all.... still x V across a fixed resistance..

Lemming said:

If the V out can push enough mA, it wont effect the current through the input at all.... still x V across a fixed resistance..

Click to expand...

You are correct, but i don't think that was what the OP was asking...

0-10V applied to a current input (with an internal 250 ohm load) will give 0-40mA, not 4-20mA

The elevated "zero signal" is the problem, a 0-10V can never give the derived voltage across an internal resistance to give 4mA at the "signal low" level.

Yeah, but if the mA in can do 0-20, and the internal resistance is 250, an additional 250 in series will give 0-5 v (ish).. Not advocating it, but will work..

there are a number of suppliers out their that sell din rail
converters
Phoenix / automation direct

daba said:

You are correct, but i don't think that was what the OP was asking...

0-10V applied to a current input (with an internal 250 ohm load) will give 0-40mA, not 4-20mA

The elevated "zero signal" is the problem, a 0-10V can never give the derived voltage across an internal resistance to give 4mA at the "signal low" level.

Click to expand...

Your assuming the 0 to10v output can provide that much current. Check the specs.

twisted question

Voltage outputs have a limited current capability. Most require a minimum high impedance (resistance) in the sensing device to prevent over current and damage to the transmitter. Your scheme would probably cause the transmitter to shut down.

4-20 mA transmitters usually have a maximum impedance that is set by the limit of the voltage available to push the current through the circuit. If the total circuit is within those limits you can use a resistor to convert the mA to voltage.

For your case the best solution is a signal conditioner as recommended by IanT.

I'm checking on the same thing... 1-10v to 4-20 mA, for feedback from a drive to a SCADA. The Yaskawa V1000s have a 1-10v feedback, and the SCADA needs a 4-20 mA signal. I'm looking at this conditioner:

http://www.automationdirect.com/adc...Voltage,_Current_or_Potentiometer_Input/FC-33

Does anybody have any other recommendations?

Brian

You can use a converter for this,
a lot of these current inputs work with a resistor (external or internal)
If this is case (check your input, if there is a resistor across the input to ground like 250 ohms or 500 ohms.
if so, take it out, now the input will be a voltage input and you can connect it. if resistor is 250 ohms, you will need 2 resistors of 10kohm in series to get your signal down to 5 volt.
This way i use on 1-5Volt inputs a current signal of 20 mA.
dont mind the difference as you can adjust the input or output with calibration.

You can also look if output can handle 20 mA, normally max is around 5, but it is in the documentation.

padees said:

There is a real reason I am asking this, but wont go into it right now.

Click to expand...

You've received some good answers to your question, but I'm still curious about the "real reason" that you asked it?

We usually use converters for situations like this as proposed. I like the modules from Weidmuller, but phoenix is probably cheaper .

I am hesitant to installing "do it yourself" solutions like resistors, but that is personal preference I guess.

Thanks for the feedback!

No, this is not my brainchild. Just trying to figure out what this other controls company was thinking when they rigged this up. And No, it don't work! Claims were made that it did when it was first tested...

The 0-10V output is driving a valve that requires 4-20ma to control the valve for process control, you get the picture.

There was a falling out of parties on this project. Controls were never completed, and plenty of bugs and ghosts in this facility.

We were asked to take this over, but none of us has ever seen an analog rig set up like this, and therefore the question. I would never do it, but didn't know if this was a reliable work around before we tell them ya need new stuff! They did claim it worked for a bit, so there is a conflict here, but all are dead now, so they get the wrenches and manually position the valves.

Tom - Lots of bio-bugs here and it has an odor all about, but not as bad as a treatment plant. These valves keep the bugs happy so they can turn the squirrel cage and reduce your need for candles.