MechE needs Elec.Cir advice 2

[maxchief]

Hello all,
To anyone who may be interested.
I wish to demonstrate a P/I Transducer which when fed an input
of 4-20mA will output to a pneumatic actuator an air signal of
3-15psig. I need to be able to generate a signal of 4-20mA.
I seem to recall that 10volts and a 500ohm resistor will produce 20mA but how do I feed this to my transducer?
It does not need to be a smooth modulation(although that would
be nice) but I would be willing to make four set-ups one each
for 5mA, 10mA, 15mA, and 20mA if that is easier.
Possibly there is a ready made device which I can purchase for
under a gazillion dollars.
Does a rheostat through some sort of resistance device make
any sense?
Thanks for any help and Happy Thanksgiving,

 

[ScottyUK]

If you have any basic circuit-building skills, the image in this link

http://cr4.globalspec.com/thread/2905/4-to-20-MA-Source-Circuit-Diagram

might be of some use to you.

For a slightly more professional ready-made unit which you can connect a potentiometer to, try

http://www.prelectronics.co.uk/filer/5111uk.pdf.

Cost is about £100 but it has everything you'll need (except the potentiometer) built in including power supply. PR in the UK will supply it factory configured F.o.C if you tell them what config you want at time of order.

Does you I/P converter have any details on it? Specifically look for a resistance. If you know that resistance then it would be possible to calculate the additional resistance required so you can use (e.g.) a 12V battery with a series resistor to drive the desired current through the I/P. To do calculate the external resistance we'll need the coil resistance of the I/P. Can you borrow a multimeter and measure it?

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Sometimes I only open my mouth to swap feet...

 

[SteveHuck]

Take a look at the Fluke 705 Loop Calibrator. It will do what you need but will no doubt be pricy!

 

[OperaHouse]

You can just put a pot in series with a battery, but you seldom know the resistance of the device you are controlling. A common device to make a constant current source with is a 7805 three terminal voltage regulator. Use two 9V batteries to supply the power. Look up the 7805 application notes to see if you are comfortable with assembling 5 parts.

 

[itsmoked]

I would just put a pot in series with a 12V battery in series with a DMM(set to mA), in series with the I/P.

Set the pot visually to maximum resistance.

Close the circuit.

Turn down the pot.

Note the current increase and the I/P change.

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If you need some protection. Set the pot to achieve 22mA. Then read its resistance. Procure a resistor that is the measured value or slightly less. Add this in series with everything else. Now if the the pot is run all the way down to zero you can't exceed 20mA plus a few mA.

###############

Definition: You have stated that you send 4-20mA to a device and it puts out a corresponding pressure.

This makes it an "I to P" not a "P to I".


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com