Analog, Voltage to Current conversion

[Lemming]

They did claim it worked for a bit

If it were a butterfly valve, the position probably wouldn't need to get more than 30% in a flow control situation.

To drive that would only need 8.8mA @ 1.1V (assuming 125Ω total..)

The 1769-OF8V will do 10mA, so likely other modules similar??

The theoretical position relative to the actual valve position wouldn't be even close...

I'd believe it might have survived a while like this..

 

[shooter]

If it is just for a valve you only need a simple transistor, and some sort of power supply of 11 volt or so.
the output of the controller being max 10 volt is going to the base of the transistor.
choose any NPN (like a bd139), put the top (emitter) on the power supply.
put the bottom (collector) direct to the valve and it will work fine.
you can not break anything as the valve can have 10 volts, and the current is limited by the internal resistor.
It is not exactly accurate, however it will control just fine.
Many pneumatic valves done this way, as original was getting to hot.

 

[padees]

Thanks again for all of your replies, very much appreciated!

We had a guy go and upload all of the programs, customer did not have a copy.

We were given a list of problems they are having that they wanted us to investigate. They claimed all this worked before. It's really quite simple now. There is absolutely no logic in the PLC programs for these issues they are having, including the valves. Most of the subroutines are empty and a number of other things have been bypassed so they do not function with PLC Control.

We won't make any assumptions here, but there was a falling out of involved parties.

 

[James Mcquade]

most 0-10 volt signals are currnt limited while a 4-20 ms signal will produce what ever voltage it takes to get to 20 ma, as much as 32 volts.

this has been my experience and i can be wrong, so if i am, someone please post the correct answer so everyone can learn.

thanks,
james

 

[daba]

There is no correct answer, you have to check the specs of each and every device to see if they can offer any sort of compatability.

You cannot rely on Ohm's law in these cases.... Just because a card will deliver 0-10V, does not mean it can do it into any load impedance (resistance). A nylon carpet can produce voltages in excess of 30,000V, but don't try and draw even a few mA from it.

In general, as has already been stated, a Voltage output will usually only be able to drive a relatively HIGH impedance load, lower the load impedance and the output stage starts to "sit-down", and your signal out is not representative of the process signal sent to the output by the PLC.

A current output will usually only be able to drive a relatively LOW impedance load, increase the load impedance and the output stage starts to "sit-down", and your signal out is not representative of the process signal sent to the output by the PLC.

You can see from those statements that a Voltage Output driving a Current input, may, might, possibly, just about, wing-and-a-prayer, work, but I doubt it will be ideal, and in my book, it's a KLUDGE from start to finish. We have a saying "horses for courses".....

 

[daba]

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..

"ish", "not advocating", "will work".... maybe, just maybe ..

who's to say the 0-10V output can deliver 20mA ??? Most can't, they demand (by design) a higher impedance load than 250 ohms.