Control voltage, 24VDC or 230VAC


Thread Starter

Bill Sturm

I am designing a panel where I really do not need 110VAC power. I have incoming 230VAC and I have 24 VDC. It is somewhat difficult to find contactors with 24VDC coils. I also have a future need to drive some coolant solenoid valves. As much as I hate higher voltages, I am leaning towards using a MCR contactor with a 230VAC coil. I would spec coolant valves with 230VAC coils also. Is this likely to cause a problem?

I have found that putting large coils on my 24VDC circuits to be problematic. The 24VDC is powering several microprocessor based devices,
and I have seen spikes and noise from the contactors reset the electronic devices. It makes sense to me to keep them on separate circuits.


Bill Sturm

Mayur Agravat

you can make use of redundant 24VDC power supply, and also siemens make 24VDC contactors available
which draws less power and does not impact on the circuit.

Bouchard, James [CPCCA]

All our machines now use 24 VDC for control. We do not have problems finding solenoids and contactors for this voltage until you get above
size 3 ( all the IEC contactors seem to be available with 24 VDC). Our approach is to use 2 power supplies one for coils and such and the other for inputs. In my experience 230 is going to be harder to find ( except maybe in Europe but they have been using 24 volts for some time ) also
more hazardous.

James Bouchard

James Ingraham

Where are you that you have 230VAC but not 115VAC? Sounds vaguely European, in which case I would think no one would let you use the higher voltage unless absolutely necessary. If you are in North America, I would say go with 24VDC for the same reason the Europeans do; it's safer! I absolutely agree that it's a good idea to have your controllers on a seperate power supply from your coils, contactors, solenoids, and sensors. That's easy; a 230VAC to 24VDC power supply is cheap, small, and simple.

Sage Automation, Inc.
I have built several panels for European installation where the supply was 380/220 and have stuck to 24 volt controls where ever possible, except on the large motor starters. It seems to me that the general trend is towards lower voltages. You say however that you do not really need 110V, and yet you are wondering about the contactors and solenoid coils. If you dont want to use 24v it sounds like you should rethink your need 110 after all. Machine tool transformers are dirt cheap.

If you do use 24vdc, as far as voltage spikes from inductive devices are concerned, either use seperate power supplies or put a freewheeling diode (a diode with the cathode connected to +) across the coil.

Ranjan Acharya

We always try to use 24VAC. The only case where we have had any real trouble is with some hydraulic solenoids that are better suited to AC voltages - be it 120VAC or 230VAC / 240VAC.

Faster, safer, easier et cetera.

If you're in the US and subject to NEC, you'll want to look at NFPA79 Section 9.2.1:
"Alternating-current (ac) control voltage shall be 120 volts or less, single phase. Where the supply voltage is greater than 120 volts, the control voltage shall be provided from a transformer with an isolated secondary winding." Some exceptions are then listed, but they *probably* don't help you.

Good luck!
Why not use a 24 Vdc interface control relay, mounted in/at the starter, to switch the 230 Vac contactor coil? Of course, regs/stds/ap's covering wire/circuit/source/location must be applied.

Phil Corso, PE
(Boca Raton, FL)
We always use 24Vdc for control circuitry, and use 110Vac for contactor coils etc. (24Vdc coils generate so much heat when energised). Panels are designed so that 110Volt equipment is located together and away from 24vDC to alleviate interference and for safety.

NFPA 79 does not necessarily apply to this situation. NFPA 79 specifically applies to industrial machinery. NFPA 79 specifically cross-references to NEC 670 on Industrial Machinery, and NEC 670 is the ONLY place where it refers to NFPA 79 as a reference standard. Refer to NEC 670-2 for a description of covered equipment. The scope of NEC 670 clearly re-iterates the scope is industrial machinery.

Richard Williams

Under no circumstances should you use your 24vdc supply for coils. The back emf from the de-energising coils could destroy your electronics. Yes, you could put diodes across the coils, but why risk a diode failure and the subsequent destruction of sensitive equipment.

I quess you are in the US as you seem to have problems getting 230vac coils. I think your best solution would be to add a small 230-24 volt step down transformer just for controls purposes. That way you also are able to seperatly fuse your control voltage. The added advantage is that you could then use safe 24vac soilenoid coils.

Good luck
Richard Williams
Your condemnation of diodes is unfounded, at least in my experience. I have used diodes as transient suppressors for DC coils for at least 4 decades. NEVER HAD A FAILURE. Of course, they must be properly rated! However, I do warn my customer base that a high rate of coil
interruption will negatively impact the diode's useful life expectancy.... like the proverbial "straw that broke a camel's back"!

A major advantage of DC coil-operated devices over AC, is that DC devices are less susceptible to momentary power supply interruption. Most AC coil-operated devices will dropout in less than 8 or 10 ms.... depending on line frequency. DC coil-operated devices will "ride-thru" as much as a 15-20 ms interruption.

My paper "Probabilistic Risk Assessment of Safety Systems" references the problem of sudden, inexplicable, and unwarranted plant-trips caused by AC device dropout, ESPECIALLY if powered from an UPS!

One last point. Even AC coils should be equipped with transient suppressors to reduce transient overvoltages (although to a lesser degree than DC) from being coupled into high impedance control, alarm, and trip circuits.

Phil Corso, PE
(Boca Raton, FL)