AC vs. DC control voltage

I've designed and executed countless successful PLC based control systems over the years but have yet to gain a clear understanding of one issue... what are the variables governing the choice of 115vac vs 24vdc control voltage?

Manufacturers offer solenoid coils, I/O points, etc. for both voltages, but what variables should drive the decision of which to specify for a project?

24 VDC is safer, cheaper in many cases, smaller and doesn't want to leak as badly. Where 120V has advantages is in dirty environments, where wires are long, and with devices that consume considerable power. Noise is also a factor, it takes a lot to disturb a 120V signal. The AC stuff tends to be slower and less versatile. You also get more wire faults in my experience. A foundry is a good place for 120 V control as fumes and dust are common and contacts are more reliable with the higher voltage. I personally prefer 24 VDC for reasonable environments.

Regards

cww

John,

Like you, I have often had to justify one or the other. I prefer to use 24DC if I can because it's safer (less of a shock hazard) and I have 24 VDC for my instrumentation. However, when you start using large coils, e.g. large solenoids or motor starters, they are not available in 24 DC. (AC has an advantage over DC for large coils.) Motor starters usually have individual 120 V control circuits that need to be kept separate from each other dictating isolated discrete output modules.

120 VAC 2 wire proximity switches that work with 120 VAC input modules are not as common, usually the leakage current is enough to turn on the input, 24 VDC switches present less of a problem.
With 24 DC I can often save on wiring cost with 4-20mA instrumentation and discrete field devices in one multi-pair cable.

As far as I can see there is no one answer, I quite often use a combination of 120 for motor circuits and 24 for field devices trying to keep the wiring apart wherever possible, e.g. separate modules and wire duct.

Hope this helps,
Roy

Responding to Just John's 25-Jan (00:10) query... this topic has been covered before. Search the List Archives, starting with thread # 984585006!

Regards, Phil Corso ([email protected])

I am a system integrator based in colombia. normally i prefer to use:
24Vdc for input purpose because there are many more options for this voltage than for 120 vac. This options are cheaper. In other hand is more safety to use this voltage for sensors and pushbottons, etc. Normally the operator have some touch for this parts.

120Vac for oputput purposes because i have to use a direct comand forom the PLC to actuators (solnoids, contactors, lights, etc).