Article
When I first strapped on my electrician's tool belt, control power 
was relatively simple. Many times, the control power came directly from 
line voltage. For example, take one leg of the 480 VAC incoming power on
 a starter and voila … a 277-volt control circuit.<br><br>As long as the coil on the starter was rated for 480, no problem.

Control Power Options and Solutions

Steve Maurer, IME
When I first strapped on my electrician's tool belt, control power was relatively simple. Many times, the control power came directly from line voltage. For example, take one leg of the 480 VAC incoming power on a starter and voila … a 277-volt control circuit.

As long as the coil on the starter was rated for 480, no problem.

The hardest thing was to understand how to route it properly for your applications. For simple on-off circuits, it wasn't a big deal.

Add in some start/stop circuitry and auxiliary contacts, and many of my buddies scratched their heads.

Of course, line voltage control has its problems. One issue was employee safety.

When using a direct line voltage, it meant that the controls at the worker's operating station were full voltage. The risk of electric shock and even electrocution was very real.

Enter … the step down transformer.

Most are configurable for various input and output voltages. They could take an input of 480 or 277, or even 120 VAC and step it down to a safer voltage.

With a 24-VAC coil in a starter, for example, you can step the voltage down to 24 VAC to send to the pushbuttons and switches at the operator station.

The output is determined by the winding ratio of the primary and secondary coils. The induced voltage produced for the output is determined by the input voltage, factoring in that ratio.

Many are totally enclosed, others are open wound, and some are encapsulated for protecting the coils and cores from moisture.

Often the encapsulated and open coils have components to hold fuses for both the incoming and outgoing circuits, mounted directly on the transformer.

They still work great for many control power applications.

Even so, the output voltage remains alternating current. And with the rise of electronics for machine operation, something else is necessary in many cases.

Electronic power supplies solve that issue.

One of the first things you may notice when installing a power supply is that, in most cases, it's much lighter than a transformer.

Because it uses electronic circuitry instead of heavy cores and coils to "transform" the voltage, it's not as heavy.

In fact, many are light enough to install in a control panel on a din rail. And because of their compact, enclosed design, several can be ganged on one din rail to provide multiple control voltages or even redundancy in the same cabinet.

The other, and probably most important, feature is that the incoming AC voltage is easily converted to a DC output voltage. Many electronic components require DC for control. The electronic power supply makes that possible.

LEDs on the component's case provides a visual indication of the status of the power supply. Very helpful for monitoring and troubleshooting.

There are many other features that may make the power supply over standard transformers. For instance, features like wide operating temperature range, power boost capability, and adjustable output voltage ensure reliable operation in the harshest industrial environments.

I like the output voltage adjustment. With a transformer, you get out what's in direct proportion to what goes in.

With a power supply, you can fine tune the output voltage for the specific range you need.
So, both transformers and power supplies are good options for control power requirements. What you'll need depends on what you're controlling and how precise the voltage needs to be.
Photo courtesy of Emerson Electric Company
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