Can 24V AC common with 0-10V DC?

flyers · Sep 12, 2011

Hi guys,

We've an application to control a few units of valve actuators. These modulating actuators are running 24V AC. Control from PLC is 0-10V DC.

Attached is the wiring diagram for the actuator, can we short the 24V AC Common with the 0-10V Common as shown in the wiring diagram?

Tharon · Sep 12, 2011

Yes, you can. It's no different than a device being powered by 24VDC, sharing a common with a 0-10VDC input/output on it.

andybutcher · Sep 12, 2011

Hi Flyers,
I have not seen this type of arrangement before with AC and DC sharing a common terminal. I cannot see why it won't work as long as the AC is fed from an isolated secondary of a transformer, so there is no possibility of ground loops affecting the analogue output of the PLC. It might mean that you will require a transformer for each actuator.

Tharon · Sep 12, 2011

andybutcher said:
Hi Flyers,
It might mean that you will require a transformer for each actuator.

I don't think this is the case. It's probably an HVAC or similar unit, using a 24VAC transformer/power supply to get its control power. I don't think ground loops will be an issue.

flyers · Sep 12, 2011

Hi guys,

Thanks for prompt reply. Referring to the attached file below, my power source 24V AC is coming from MCC panel (step down transformer). The signal is coming from PLC panel (Analog output module). There will be a few actuators to be control by the PLC panel. The Ampere (24V AC) from MCC panel is sufficient. The small actuator only consume <0.2A/unit.
There is no space for additional transformer in the PLC panel, thus we have to tap from the MCC panel. Is there any problem with such wiring method?

keithkyll · Sep 12, 2011

One side of 24VAC must be grounded for safety. Commons joined at actuator #2 form a ground loop, because commons are joined at first terminal block. Just run a single wire for the common.

flyers · Sep 12, 2011

keithkyll said:
One side of 24VAC must be grounded for safety. Commons joined at actuator #2 form a ground loop, because commons are joined at first terminal block. Just run a single wire for the common.

Hi Keith,

Your reply "One side of 24VAC must be grounded for safety." means the common of the 24V AC in the PLC panel to be wired to ground? Correct me if I'm wrong. Thanks.

JesperMP · Sep 12, 2011

I think the actuator is a bit quirky.

AC Neutral ("N"), and DC ground ("Gnd"), are both almost universally tied to a common earth right after the transformer (for the AC), and the PSU (for the DC). But after this point they are normally not mixed.
Not sure if it is EN60204-1 allow to mix AC and DC circuits.
If you mix AC and DC, you have an issue what color the wiring should be.
Normally AC control = red, and DC control = blue.
Also, since the analog signal is 0-10VDC, if the actuator is far away from the 0-10V source, it will be sensitive to common mode noise or potentials.

If I had to use it, I would place an isolation amplifier between the DC controls (PLC ?) and the actuators 0-10V input. And I would wire a good fat N/Gnd back to the control panel.
If the 24V AC isnt used for anything else, maybe the isolation amplifier can be omitted.

leitmotif · Sep 12, 2011

I suppose it would work just fine. I would not do it. I like keeping things as "separated" (??) as possible. What are we talking about here 10 foot of wire?

Dan Bentler

keithkyll · Sep 12, 2011

24 AC common should be grounded at the source - inside the MCC box.
Mixing AC and DC is okay. Blue and red are for hot leads. Returns can be tied together.
At 200mA per unit, noise shouldn't be an issue. If it is, a small cap at each actuator (across 0-10V input) should cure it.

flyers · Sep 12, 2011

JesperMP, I'll check at site if the 24V AC Neutral is grounded. The DC power supply at the PLC panel is grounded. Mixing is only for the Common, NOT the Live (red - AC) and (bleu - DC) wire, as per required by the wiring diagram. The cable we use from MCC to PLC and PLC to Actuator is 2.5mm sq.

leitmotif, your reply "keeping things as seperated as possible" means? Not required to ground the Neutral and DC common wire? Please explain more. The cable length from MCC to PLC is about 100 meter. PLC to Actuator is about 100 meter to 200 meter.

keithkyll, Thanks for your input, I'll check if the 24V AC common is grounded. We shall address it as 24v AC Neutral (black) & 24 DC (Common) right?

Thanks guys.

leitmotif · Sep 12, 2011

leitmotif, your reply "keeping things as seperated as possible" means? Not required to ground the Neutral and DC common wire? Please explain more. The cable length from MCC to PLC is about 100 meter. PLC to Actuator is about 100 meter to 200 meter.

I would run one pair of wires for all 24AC and another completely separate pair for 10 VDC. The AC should have one side bonded to a ground point of couse to comply with USA NEC and maybe also one side of DC - is it not also a separetly derived source also?

Granted bonding one side of both DC and AC ties them together which can be an argument that a common for grounded AC and DC should be OK. I just dont like the "mix n match" of the common - may be I am just not understanding things correctly and am being overly conservative out of ignorance.

Dan Bentler

Johnster · Sep 12, 2011

I'm with Leif...I think =)

I'd give each voltage its own common for
1) redunancy, you can always spice them together later if one breaks
2) reliability - if one wire goes, only 1 voltage goes away... not both.
3) colors - having a seperate common lets you color or mark it differenly (if this is an option in the 1st place)

-John

keithkyll · Sep 12, 2011

White for 24VAC return. White with blue stripe for DC return.

bce123 · Sep 12, 2011

If the ac and the dc come from two seperate panels maybe yellow should be used to show that the power suorce comes from a different panel and the panel should be labeled as such. lots of things to think about aint there a ul thing about this? but that might apply in amk. maybe those other safety standards guys would have a say so one this one. Doe the ac that feeds the dc power supply and the ac that feeds the ac xfmr come from the same source? and they each should be grounded at their respective location. just my thoughts on the matter. MY self I like to keep my ac away from my dc so things dont get cr-a-zy. Steve

Can 24V AC common with 0-10V DC?

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