24v Power Supply

Just what does it mean to discuss a 24V power supply?

Does its power come from the building's AC wiring, at 110 or 220 volts? Or is it a portable battery unit?

Is the output 24V DC or AC?

The answer to any of the above questions may be "yes," because the "24V Power Supply" is a versatile phrase. A wide variety of items use 24 volt power, so choosing the right 24V power supply clearly starts with knowing what your application needs.

Some ham radio sets need 24V DC. The external power supply unit would plug into the home's AC wiring.

Polycom's SoundPoint IP Telephone system uses 24V AC, fed through a power supply unit which is plugged into the office's AC circuits.

Small aircraft have 24V DC wiring. The power supply is a battery, recharged in flight by the engine's alternator. Clearly, this is a very portable power supply unit.

Some "replacement" power adapters for laptop computers are even more versatile. They: travel with the user; plug into a building's AC outlets (whether 110 or 220V AC); supply 12V to 24V DC; have a variety of plug tips to adapt to a range of laptops; and allow a choice of positive or negative polarity as needed. The maximum output might be 90 watts.

Output current may range from 0.5A (one-half ampere) to 40A to operate industrial equipment. A 24 volt output at .5A is a mere 12 watts; whereas 40A output provides 960 watts.

Narrow the Field

This article will ignore laptop power supplies, since laptops use a variety of input voltages. The article also excludes 24V DC inverters, which produce 110V AC output.

It will focus on 24V power supply DC output.

Even with this focus, further choices present themselves. Should you buy an open frame or an enclosed chassis power supply? Which of the many plugs does your device need? Do you need a DIN rail power supply? Will it be installed outdoors - does it need to be water resistant?

Power Ranges

It is convenient to categorize power supply units by power output: low power is up to 70 or 75 watts; a middle range from 75 to 325 watts; and the high range above 330 watts.

A low-power unit, such as the Mean Well PM-15-24 by TRC Electronics, is rated at 15 watts. This small unit - 3X2X0.9 inches - complies with medical safety standards. It has protections against over-voltage, overload, and short circuit conditions. It is cooled by free air convection.

A mid-range unit may serve a hospital bed's right-angle gear motor. TRC's NXT-100-1006 power supply is rated at 100 watts, also has a medical certification, but is cooled by forced air.

An example of a high-range unit is TRC's SE-450-24. Rated at 450 watts, it uses a built-in fan for cooling. It has protections against over-temperature, as well as over-voltage, overload, and short circuit conditions.

Safety Certifications

There is no excuse to buy a 24V power supply which lacks a safety certification. Look for a UL (Underwriters Laboratories) or CSA-US (Canadian Standards Institute for products marketed in the US)

Special circumstances, such as hospital use, require further certifications.