Isolated 4-20mA output

Put resistor 100 ohm between out and gnd, measure 2V=20mA.

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ciao Ban
Apricale, Italy

A common method is to add a silicon diode in series with the current on some convenient terminal strip. Test by connecting a milliamp meter across the diode. If the meter has a resistance low enough that its total voltage drop, at

20 mA is less than about .3 volts, essentially all the current will be diverted through the meter, and none will pass through the diode. This uses up about the minimum voltage possible, leaving the maximum for the other resistances in the loop.

John Popelish a =E9crit :

Why the diode?

Thanks, Jenalee K.

A current source has a voltage limit based on its power supply. For instance, if you add a 1K resistance in series with your current loop, and the power supply of the current source is only 12V, it will only be able to source 12mA (12mA * 1K =3D 12V). Placing a diode in parallel with the ammeter ensures that:

1) The added voltage burden on the current loop will never be more than one forward diode drop (0.7V), and 2) In the event of an open fuse on the ammeter or inadvertent open circuit (errant test leads), the gas detector will still work.

Mr. Popelish speaks from experience. Particularly if the gas detector performs a safety function, it's good form to ensure the test won't interfere with equipment operation.

Cheers Chris

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It carries the loop current while limiting the voltage drop=20 to less than a volt, when you are not measuring the current=20 with the meter.

A 4 to 20mA detector is a two wire device. It's output is simply the current that it draws from a power source. The power source is usually 24VDC. All you have to do is measure the current it draws. You can do this with a current sense resistor in series with the controller and ground and measure the voltage across it. Or, you can place the multimeter, in a current mode in series with the controller and ground. Either way, the sensed current is the output. Bob

you can get a low level DC clamp meter but the simplest thing to do is use your DMM in series with the connection in AMP mode or get our self a stand alone current and monitor source unit supply designed to test and align the units.

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Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

John Popelish a =E9crit :

Oh, I see. You mean that the diode is there all the time and just provides a test point so that you can measure the current without breaking the loop, right?

Thanks, Jenalee K.

Yes.