Creating a 4-20 ma source to troubleshoot Water Valve

I should know this , but we had a modulating water valve
go bad this morning. I would like for the guys to test this on the bench. How can I take a bench Adjustable DC Power Supply and create
a 4-20 ma source to troubleshoot this valve. Like a range of 4.00,
8.00 ,16.00 ,and 20.00 ma.

Thanks Guys,

I'm SURE that the other members will have better ideas, but just to get you started, I've used two 9-volt batteries and a couple of 5K "volume control" potentiometers from Radio Shack for a makeshift rig ... we actually wrapped it up in Duck Tape ... a guy working on a tugboat in Charleston harbor needed one in a hurry - to test out the hydraulics on the boat's rudder control system ...

4 to 20 mA ... the stuff is everywhere ...

Coachman said:

I should know this , but we had a modulating water valve
go bad this morning. I would like for the guys to test this on the bench. How can I take a bench Adjustable DC Power Supply and create
a 4-20 ma source to troubleshoot this valve. Like a range of 4.00,
8.00 ,16.00 ,and 20.00 ma.

Thanks Guys,

Click to expand...

For future reference,

our field technies are supplied with these

I use this.... http://www.actionio.com/products/slimpak2/wv438_spec.html

100k pot input - 4-20 or 0-10 output then all you have to do is turn the pot and see the valve move, turn it 50% the valve should move to 50%

PS Ron's idea is too much thinking... my brain will hurt

My mind is going blank again. What is happening in the following components ?

The PLC Output:
- In the circuitry ,it is usally about 1 to 6 vdc ,what is creating
the varible current ?

The Valve, Freq Drive or any other device:
- What is the Device seeing? Is it going thru a current sensing
component ,and then going to a setpoint?

Sorry for sounding stupid.

PS Ron's idea is too much thinking...

Click to expand...

I actually got the "emergency" request for this rig late one Friday afternoon when I worked for an electrical distributor ... a fire had put a tugboat out of commission - and the hydraulic rudder system was being repaired ... the "controls" guys weren't due in for a day or so, but the "hydraulics" guy needed to do a quick test to see if his part of the job was finished ... seems he had a rush job for another boat in Texas ... I told him we could order what he needed - the price tag was NOT an issue ... but he really needed to catch a plane that afternoon ... I told him what to get and he picked up a sack of parts from Radio Shack (a lot more parts than we needed - but he wanted to be "sure") ... I heated up the soldering iron while he was on the way to the office ...

ten minutes was all it took to kludge it together ... basically just a wad of duck tape - with red and black gator clip leads hanging out of one end - and the knob for the pot on the other ... he called me about an hour later - a very happy man on his way to Texas ... he said the rudder stroked back and forth just fine ...

sometimes a techician's gotta do what a technician's gotta do ...

Ron Beaufort said:

I'm SURE that the other members will have better ideas, but just to get you started, I've used two 9-volt batteries and a couple of 5K "volume control" potentiometers from Radio Shack for a makeshift rig ... we actually wrapped it up in Duck Tape ... a guy working on a tugboat in Charleston harbor needed one in a hurry - to test out the hydraulics on the boat's rudder control system ...

4 to 20 mA ... the stuff is everywhere ...

Click to expand...

Ron, sounds like a real MacGyver contraption. I'd be interested in knowing what you could make if I gave you a paper clip, surgical rubber tubing, a shoe lace, and a turkey leg (cooked).

geniusintraining said:

I use this.... http://www.actionio.com/products/slimpak2/wv438_spec.html

100k pot input - 4-20 or 0-10 output then all you have to do is turn the pot and see the valve move, turn it 50% the valve should move to 50%

PS Ron's idea is too much thinking... my brain will hurt

Click to expand...

Mine's a little older.

If you have a bench top power supply most of them have current regulation knobs. Couldn't you just adjust those?

Just a thought, never tried it, maybe wouldn't have enough resolution to adjust accurately enough.

I have used a battery and a pot as mentioned before.
You may want to also check out a Fluke 789 process meter for future
reference.
http://us.fluke.com/usen/products/Fluke+789.htm?catalog_name=FlukeUnitedStates&category=LOOPCAL(FlukeProducts)

Coachman said:

- In the circuitry ,it is usally about 1 to 6 vdc ,what is creating
the varible current ?
.

Click to expand...

Just to answer this part of your question.

OHMS LAW.... I=V/R..... I= 6/500

If you have 6 volts and it is being fed to a 500 ohm resistor then your answer is 12 mA
Most 4 to 20 mA loops are dropped across 250 ohms or 500 ohms impedence.
Co incidentally the instrument is most likely reacting to that derived voltage internally of 1 to 5 volts or 2 to 10 volts. to drive the electronics.

The PLC Output:
- In the circuitry ,it is usally about 1 to 6 vdc ,what is creating
the varible current ?

Click to expand...

The PLC will have an Analog Output Module. The module takes the DC supply voltage, runs it through an effective variable resistor [probably switching transistor] to produce a current that is proportional to a process feed-back variable. This output current is controlled by instructions in the PLC. The most common is a PID instruction, which calculates the level of the output based on the input parameter.

In the case of your water valve, the PLC probably has an input from a water flowrate transmitter downstream of the valve. The PLC analog output 4-20 mA current signal will be proportional to this input.

The Valve, Freq Drive or any other device:
- What is the Device seeing? Is it going thru a current sensing
component ,and then going to a setpoint?

Click to expand...

The Device will be seeing (in your case) a varying DC current with a range of 4 to 20 miliamperes. The valve controller will have a signal-follower circuit that will probably be calibrated to make the valve go closed at 4 miliamps, go 50% open at 12 miliamps, 100% open at 20 miliamps, and proportionally open at all points in between. Or it could do the inverse, depending on the application.

Coachman said:

I should know this , but we had a modulating water valve
go bad this morning. I would like for the guys to test this on the bench. How can I take a bench Adjustable DC Power Supply and create
a 4-20 ma source to troubleshoot this valve. Like a range of 4.00,
8.00 ,16.00 ,and 20.00 ma.

Thanks Guys,

Click to expand...

The easiest way to hook it up to a variable power supply is to use a fixed value resister in series with the valve.
Using a multimeter measure the voltage drop accross the resistor and watch the multimeters displayed voltage to control your current flow through the loop.
Using a 1K ohm resistor would be easiest with each Volt displayed representing a milliamp. Range 4-20V
With a 500 ohm resistor the voltage will be from 2-10V with each half volt being another milliamp.
With a 250 ohm resistor the volatge will be 1-5V with each quarter of a volt being another milliamp.

On the bench it is simple:
put a mA meter in series and cranck up the voltage thats all.
if only one this is fine.
if more get a good processimulator yes i do have a fluke 725

i did have found one problem when using RTD when current is 2 mA instead of only one the meter goes bezirk.
bye