Loop Powered device not turning on


Thread Starter


I have a metso nd9000 positioner that is loop powered by a 4-20mA signal from a AD Click PLC.

It doesn't power on, the specs say that 9.7vdc is required to power on. How can I change my existing setup to do this or do I need a different PLC, what spec should I look from on AO side of the PLC?

Much obliged,

Curt Wuollet

One of the easiest ways is to put a current meter in the loop and see if you have 4-20 mA. If not, I'd review how the loop is powered.

The Metso ND9000 spec says:
Load voltage: up to 9.7 VDC/20 mA, (corresponding 485 ohms)

So when you look at the PLC's analog output (AO) spec, you look for the load resistance that the AO can drive.

Most active, powered, sourcing analog outputs will drive a loop resistance of at least 600 ohms, if not greater resistance. For example, the spec for PAC AO on my bench is "Load resistance: 750 ohms max"

The Metso's 485 ohms is rarely, if ever a problem for an AO, unless you are split ranging two positioners and trying to do so from the same 4-20mA source, because many AO's will not drive a 485 * 2 = 970 ohms load.

Things other people have done when their positioner doesn't light up:

- it isn't a loop powered 4-20mA positioner, it's a Profibus or Foundation Fieldbus model (which have (+) and (-) terminal connections, just like HART/4-20ma devices). It pays to check the model number.

- demand/control signal wired to positioner's analog output retransmission terminals. The loop control signal cannot drive a positioner's output.

- wired to the right terminals but wired backwards with respect to the AO. Or, junction box wiring issues.

- I/S barrier wiring/functionality issue

- demand/control signal wired to a limit switch - a short across the AO doesn't hurt it, but the wiring won't power the positioner.

- attempting to drive the positioner with a 24Vdc power supply while testing the positioner on the bench. This can burn up the positioner quite rapidly because the analog output is supposed to supply a regulated loop current; the positioner is not the loop's current regulator. So the positioner's electronics rapidly overheat and burn-up. A positioner is NOT a 2 wire loop powered transmitter (that regulates the loop current); it is a load on an analog output.

It's a smart positioner, it's gotta light up when you power it through an analog output.

If your plant uses electro-pneumatic positioners, someone has a 4-20mA loop simulator or calibrator that is battery powered and can source the DC power and current to a positioner. Those units all have a display that shows zero loop current when the circuit is open, and some current value when the circuit is complete. Find one and connect it to your positioner. See if the positioner powers up. If it doesn't then something is wrong with the positioner.
OK, so I looked the spec for the AO and it says that it can drive a Load Impedance of 250 ohms and there is this info:

Load Power Supply:
DC 18V: 600maximum
DC 24V: 900maximum
DC 30V: 1200maximum

So this plc AO cannot drive my positioner? What would that list be for, is it for 4-wire loop?
250 ohms is a low load value for an analog output. It only takes 5Vdc to drive 20mA through 250 ohms. If that's the active output capability of your AO, then, yes, that's why your positioner won't light up. 5Vdc < 9.7Vdc minimum required.

The Load Power Supply ratings that you provided infer that the analog output can be passive, that is, it can be loop powered by an external power supply. The greater the power supply voltage, the more load resistance the power supply can drive.

The advantage of a passive output is that the external power supply can be picked to perform the task necessary, like split ranging 2 positioners which would require 970 ohms, more than 24Vdc, probably a 30Vdc supply. If an analog output is only active, with it being internally powered, then the AO drives what it drives, and you're stuck if the load is more than the output can handle. The downside is an external power supply is needed to make the current flow.

I wasn't aware of PLC AO's having passive AO's, but anything's possible in this global world of controls.

If the AO can be either passive or active, you'll have to read up on how to make it passive so that you can use a stand alone 24Vdc power supply to power your loop (18Vdc will do the job, but the convention is 24Vdc). The means of doing either active or passive is likely a DIP switch, a rotary switch, or some jumpers; some kind of hardware selector.

Whose analog output (brand and model number) are you using?