Control Circuits: xformer vs power supply

[controlsgirl]

Long ago, it was often a customer spec to always use a transformer for isolation of the control circuit from the rest of the equipment. For 24vdc circuits you would use a transformer to go from 480 to 120 and then a power supply from 120vac to 24vdc. I am seeing the use of 480VAC to 24VDC power supplies more and more often.

Does a transformer still provide a layer of isolation and that power supplies do not? Are there any other benefits to using a transformer prior to stepping down to 24vdc? Is there something in the technology of power supply construction that gave them a level of performance that made the use of a xformer obsolete if 120VAC was not needed?

When I started in the industry, 24VDC circuits were just starting to make a name for themselves. It was still common to see HMIs and PLCs powered with 120VAC even if IO cards were 24VDC.

 

[jaden]

I've worked on some machines that had 480/24 VDC power supplies, but it seems sooner or later, there becomes a need for a 120Vac power source. I still use the transformer 480/120 for any future needs that may arise.

 

[Jeev]

You'll find a lot of modern 24V DC power supplies are capable of being fully isolated (Depending on what you do with your DC- terminal), as well as include a number of protection and over-boost features. Things have come a long way since the ol' transformer, rectifier, and big capacitor

 

[parky]

Most PSU's are switchmode types now, they have a transformer & it is generally built for isolation, the mains supply is rectified, chopped at a high frequency then fed to the transformer this being quite small as the frequency of the switched supply is high so fewer turns are required so in essence most 415-120v to 24V DC are isolated, they also offer far better crowbar protection against peaks & oveloads.

 

[lfe]

I remind you that it is not correct for the control circuit supplies to be isolated, that is, floating with respect to ground, since large voltage differences can be generated between them and ground, even thousands of volts.

One of the output terminals of the power supply or transformer must be connected to 0V, either the neutral of the installation or the neutral that you have created connected to ground.

 

[strantor]

Does a transformer still provide a layer of isolation and that power supplies do not?

Technically, yes. It adds a layer of isolation. Not to be confused with an added layer of protection. You could feed a 24V supply from a transformer that is fed from a transformer that is fed from a transformer that is fed from a transformer, for 5 total layers of isolation (the supply itself counts). None of those transformers would offer any benefit though.

Are there any other benefits to using a transformer prior to stepping down to 24vdc?

It affords the opportunity of installing an outlet for plugging your laptop into. The outlet will usually remain functional for the first day or two of commissioning of the system, after which it will have blown fuses from someone using it to power a hammer drill, floor fan, or shop vac.

Is there something in the technology of power supply construction that gave them a level of performance that made the use of a xformer obsolete if 120VAC was not needed?

Yes. High speed (switch mode) level transformation is more energy efficient than mains frequency transformation and costs less than big iron, and intelligent protection features can be designed into the supplies for cents per unit that result in less downtime than systems which blow fuses in response to every non-ideal situation they encounter.