Are low-voltage direct current supplies a safe choice?

We've previously written about power distribution for control panels generally, so in this article I'd like to specifically look at DC power supplies for industrial control panels, automation systems and instruments.

One of the main things to consider in terms of power supplies is the tradeoffs that come with employing high or low voltage. Higher voltages result in lower current flows, which means thinner gauge wire can be used, however, lower voltages are safer for workers, require less insulation and allow for the use of more compact devices.

With that in mind, I want to focus on 24VDC power supplies, their popularity for use in industrial control panel signalling voltage, and how these power supplies can be implemented for reliable control circuits.

What are common control voltages?

Control voltages are often chosen based on what is common for an application or industry. Basic hardwired controls such as on/off switches and lights may use 120Vac because it is a readily available everywhere in North America. A large number of motor control panels use 480 Vac for hardwire control, some older applications utilize 277Vac. 

These potentials were originally selected because they are available directly from the utility with no need for a transformer needed. . On-vehicle applications favour 12Vdc and other battery-friendly voltages and building HVAC systems traditionally use 24Vac supplied by small transformers.

While some automation devices can work with many different voltages, designers will find that most control system components such as PLCs, HMIs, smart relays, I/O modules and instruments tend to all accommodate 24Vdc. This makes 24Vdc power supplies and distribution a convenient choice for control system designs, but there are other reasons why there are so widely employed. 

• Regulations: NFPA 70E and OSHA consider 50V and above as hazardous voltage, and often stipulate that power supplies are below this threshold.
• Safety: Electrical shock hazards are greatly diminished at 24Vdc, making these power supplies much safer for installers, operators and maintainers in the industrial environment.
• Power delivery: Most automation devices have relatively low power requirements and draw small reasonable currents at 24Vdc.

How is 24Vdc control voltage generated?

Simple and passive transformers are used to transform one AC voltage to another. However, power supplies are needed to rectify AC voltage to a DC voltage. There are also devices called converters for converting one DC voltage to another, but these are far less common.

What types of 24Vdc power supplies are available?

Power supplies fall into one of two main families - linear and switching. Linear is an older, bulkier and more expensive technology, however it features very stable DC voltage output with little or no electrical noise or ripple. Linear power supplies are very good for delicate electronics and other high-sensitivity applications.

Switching power supplies, the standard for automation purposes in North America, convert AC to DC using pulse-width modulation (PWM). Early designs had high output electrical noise, but modern switching power supplies provide an excellent output in a small form factor with high efficiency, so they are currently the most popular form of DC power supply.

How are DC power supplies specified?

DC power supplies are rated by the nominal input AC and output DC voltages, the output current in amperes (A) and the effective power rating in watts (W). All power supplies generate heat when operating, therefore the input watts are always higher than the output watts, with the difference being the heat dissipated.

This heat in watts can be readily converted to BTUs for thermal calculations if needed to size panel cooling components. DIN rail mounting options are popular, as are panel-mount, especially for larger versions, and many versions are available in a touch-safe housing. Some DC power supplies must be mounted in a specific orientation maintain cooling and proper operations.

Additionally, high output power supplies may require de-rating if used above a threshold temperature, which means users must utilize at lower output current to prevent overheating.