4-20 ma simulation

J

Thread Starter

J Hoke

Does anyone have a cheap and dirty design to simulate 4 to 20 ma 2 wire into an analog input in a PLC? Thanks.
 
M

Michael R. Batchelor

There's a very good chance your 4-20 ma is really a 1-5 VDC signal developed across a 250 ohm resistor. )OK, maybe 2-10VDC, check and see.)

If it is, then just use a potentiometer and pick a voltage off the wiper arm for the input. Free if you have one in the junk box, 25 cents at radio shack. Any value will work, just use a pot and not a rheostat.

Michael
--
Michael R. Batchelor - Industrial Informatics, Inc.
Contribute to society: http://www.distributed.net/ogr/
 
R

Robert Scott

How about a 24-volt battery, a 5K potentiometer, and a 50ma milliammeter connected in series?

-Robert Scott
Real-Time Specialties
 
T

Thomas Hergenhahn

Use a Battery and a potantiometer.
If the battery provides 9V, you'll need 450 Ohms for 20 mA, 2250 Ohms for 4 mA.
Take a 2.5 or 2.2 kOhms Poti in series with a 390 Ohms resistor, and you'll be able to go savely through the full range.
 
M
The simplest design I have is a 470 ohm 2 watt resistor in series with a 10K ohm 1 watt 10 turn pot. The fixed resistor limits your maximum current to 51mA on a 24vdc supply. If you need to read the actual current, put your meter in series with this.

Now, if you do this a lot, get a bunch of resistors and pots, mount them in some kind of panel (we just bent and painted 16ga steel), and add terminals and decently long twisted pair cables.
 
M
There is one missing piece of information in your question - resistance of
PLC input (I assume 250 ohm).
If so, a voltage source higher than 5 VDC (say four AA batteris) and a
potentiomeneter (wired as rheostat) can do.
I leave the calculations to you.
Meir.
 
> I use an "Altek" signal generator. The model I use does 4-20 or 0-10. <

Would you tell me what kind or maybe a part number?
Thank you
 
I have a very good simple design based on a 317 Voltage regulator
It uses 5 switches and gives 4-20 mA in 1 mA increments.
It's very accurate and independent of loop resistance.

Send me an e-mail

roy_matson [at] yahoo.ca
 
If you're only trying to light up the PLC inputs to verify wiring from a field terminal, you could hook up a 10k potentiometer to the existing 24VDC bus. It will max out at 5mA and will trigger the PLC inputs. This would <b><i>NOT</b></i> be a good way to calibrate something, but i use this cheap potentiometer in lieu of a very expensive process meter to ensure the field terminals are on the right PLC inputs. =)
 
B
I don't get your math here.

A pot can go all the way down to zero Ohms, so the current in the loop would only be limited by the input resistance of the analog card. A fair number of analog input cards have 50 ohm dropping resistors, so a zero ohm external resistance to the power supply positive on a 2 wire circuit would give you almost 500 mA of current, not 5 mA.

I have run across a few AIN cards that use a 250 Ohm dropping resistor. Even so, the max that would give you would be something like almost 100 mA.

--
Bob
 
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