# Isolated 4-20mA output

Discussion in 'Electronic Basics' started by selbakan, Oct 12, 2006.

1. ### selbakanGuest

Dear Sirs & Ladys

I have touchpoint4 four channel gas detector controller equipment.This
controller is giving Isolated 4-20mA output.How can I measure with
multimeter.

2. ### BanGuest

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

3. ### John PopelishGuest

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.

4. ### Jenalee K.Guest

John Popelish a écrit :
Why the diode?

Thanks,
Jenalee K.

5. ### ChrisGuest

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 = 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

6. ### John PopelishGuest

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

7. ### Bob EldGuest

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

8. ### JamieGuest

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.

9. ### Jenalee K.Guest

John Popelish a écrit :
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.