digital output of 24 V dc vs. 230 Vac

  • Thread starter ECG, Freddy del Toro Leyva
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ECG, Freddy del Toro Leyva

hi list, what is the advantage and disadvantage between the digital output of 24 V dc and those of 230 Vac??? regards, Freddy del Toro Leyva. PCE

Hakan Ozevin

1. 24 V dc outputs are fed by transistors and cheaper than 230 V outputs which are fed by either relays or triacs. 2. 24 V level is safer than 230 V. 3. It is easier to have an integrated short circuit protection in 24 V dc 4. You can command other electronic devices (on/off control of VSD, PID controller) by 24 V dc, but 230 V input is not allowed for such devices. 5. You have a higher output current in 24 V dc. 6. 24 V dc causes less electromagnetic interference, however more effected from EM. We usually advise our customers to prefer 24 V DC, unless he has already chosen the actuators and contactors. Best regards Hakan Ozevin

George Robertson

Principally, the advantage of the AC output is the reduced wire diameter required to carry power to a relay coil, for example. A much more interesting question would be AC vs DC inputs to a PLC. I already have a strong opinion (how surprising!) and would be very interested to hear the rest of you weigh in. George G. Robertson, P.E. Manager of Engineering Saulsbury E & C (915) 498-6080 ext 232

Alan B. Rosean

I agree with the other gentlemen but would like to add one consideration: speed. An AC signal or output is essentially useless 50% of the time as it is crossing zero whereas the DC signal or output is either zero or 24 VDC (neglecting slew rate). Many triac and SCR outputs will turn on only immediately after zero crossing (requiring zero-crossing switches). High-speed solenoid valves, for example, are all DC, not AC, for the above reason.

Steve Myres, PE

I work a lot with large machinery, where wire runs can be several hundred feet, and I find one disadvantage with 24VDC is power transmission capacity for long distances to outputs (like a large solenoid coil requiring 25W, or a three phase pump contactor). You've multiplied the current by a factor of 5 (in the case of 24 vs.120), while also reducing the tolerance of the system for voltage drop by a factor of 5 as well, making the wire sizing 25 times more critical! Also, the large three phase starters are much more readily available in 120VAC coils, if the customer needs a replacement unit in a hurry. Also much cheaper, both for me and for my customer. Note that all these factors are only relevant to outputs driving a physical device. Inputs require much less power, typically in the range of <1W, so power transmission is not an issue. Additionally, there is a MUCH greater selection of sensors for 24VDC switching than 120AC or 230AC solid state units (and mechanical switches will accept either anyway). Therefore my standard is 24VDC for inputs and 120VAC for outputs (240VAC in Europe). When I used to do automation machines, though, (about 10' x 10'), and the largest output load was a small air valve or ice cube relay, 24VDC outputs worked just fine. Steve Myres, PE [email protected]

Kinner, Russ

The US OSHA regulations are being interpreted by a growing number of companies to require protection of personnel who are working on "live" panels operating on voltages over 50 volts. This includes use of insulating panels (normally clear plastic) in front of PLC terminals (such as the Allen-Bradley swing arms) if they are operating on 120 VAC and/or use of insulating gloves for electricians. Many electricians I know will scoff at using gloves for 120 VAC, but more companies are requiring this protection regardless of the hassles that maintenance staff must deal with. By using 24 V (AC or DC), no special protection is required. We are often being asked to design our controls using 24 VDC for everything except circuits inside MCCs. The only downside we've seen as of yet is the voltage drop in circuits with long runs and higher currents on 24 Volts and the need to specify contacts on switches that will reliably work on the lower voltage for 10-20 years. There is a small additional cost for the power supplies that we don't have using 120 VAC, however, we are seeing some costs reductions in other parts so the differential is quite small. Russ Kinner AVCA Corporation Maumee, OH USA
Responding to Steve. Myres, PE, Thu, Mar 15, 4:57pm message: Although the merits of higher voltage AC systems have been covered, there is a more insidious side... AC devices are not very forgiving of momentary power supply interruptions. Looking at it from a susceptibility standpoint: DC devices will tolerate 15 to 20 milliseconds of interruption, AC devices will drop out for interruptions of only a half a cycle. They also are responsible for many spurious, unwarranted outages, or trips. Today, it is possible to substitute 'Intrinsically-Safe' Solenoid operated valves. Their operating power is nominally 1 to 1.5 Watts. Looking at it from a reliability standpoint: high voltage, high wattage, AC devices, have failure rates up to 2 orders of magnitude higher than low voltage, low wattage, DC devices. Regards, Phil Corso, PE (Boca Raton, FL)
I am curious as to how many people out there require instrument techs or electricians to use gloves or other personal protective equipment(PPE) when working on 120 VAC instrument circuits? If so, what regulation or code are you basing your decision on? Bill Mostia ======================================================= William(Bill) L. Mostia, Jr. PE Independent I &E Consultant WLM Engineering Co. P.O. Box 1129 Kemah, TX 77565 [email protected]

Johnny Feathers

Hello there!

24VDc Digital Outputs are safer than 230VAC, especially in Harsh environments.

Also When connecting Remote I/O (Field Wiring) back to your PLC with Multicore cable, you will probably have 24VDc inputs on the same cable as your 230VAc. This could be considered poor practice as you could always have the possibility of shorting one of your 24VDc inputs from your 230Vac Outputs and blowing the input card on your PLC.