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How to Choose a Power Supply

The Power

Jameco Buyer's Guide: Understanding Power Supplies

By Robert Cong

There's the old saying, "Use the right tool for the job!" But sometimes there are multiple "right tools" for the job, so how do you know which one to use? In order to select the right power supply there are some important basics to understand.

The power line of products from Jameco Electronics includes a wide selection of power supplies. They provide all your power source needs from wall adapters and table-top power supplies to open/enclosed AC-to-DC power supplies and DC-DC converters/DC-AC inverters. Whichever tool you choose as your power source, you can rest assured that you will be getting great quality products that are right for the job.

Power Supply Terms

First off, let's clarify some terms which often confuse people but are important in selecting the right wall adapter power supply. AC-to-DC "switching" power supplies vs. "linear" power supplies are often misleading terms to those who aren't familiar with them.

Linear power supplies take an AC input (typically 120VAC or 240VAC), step down the voltage with a transformer, then rectify and filter the input into a DC output.

A switching power supply takes an AC input, but rectifies and filters into DC first, is converted back into AC at some high switching frequency, steps down the voltage with a transformer, then is rectified and filtered into a DC output.

The difference between the linear and switching processes is that they allow for different components to be used. The linear power supply is typically less efficient, uses a bigger and heavier transformer as well as bigger filter components. The switching power supply implies higher efficiency due to the high switching frequency, enabling it to use a smaller, less-costly high-frequency transformer as well as lighter, less-costly filter components. Switching power supplies contain more overall components, therefore are usually more expensive.

There is a difference between "switching" on the input side and "switching" on the output side. What we just discussed referrs to switching on the output side. When talking about the input side, there are 2 types of "switchable" power supplies:
1) Switching - Automatically switches between AC inputs and frequencies, or
2) Switch-selectable - There is a manual switch on the supply that changes the AC input range and frequency.

Sumarizing although the linear process seems to be more efficient due to it shorter process, a switching power supply is in fact more efficient.

Linear Power Supply
Astec ACV15N4.5 – 15V, 4.5A Linear Power Supply
Size: 7.0"L x 4.8"W x 2.7"H

Switching Power Supply
Mean Well PS-65-15 – 15V, 4.2A Switching Power Supply

Size: 5.0"L x 3.0"W x 1.7"H

Many questions also arise when speaking of "regulated" vs. "unregulated" power supplies. These terms deal with the control circuit of the power supply.

In an unregulated power supply, the switching transistor stays at a constant duty cycle, so there is nothing to control the output. Outputs do not stay at a definite value; instead, they fluctuate slightly when different loads are applied. Only a very low voltage will cause the supply to shut down.

In a regulated power supply, the output is kept very close to its rated output by changing the duty cycle to compensate for changes in the load. This provides for better protection of your devices and more accurate outputs.

The main differences between regulated and unregulated power supplies are protection and price. Regulated power supplies provide better efficiency and protection, but unregulated power supplies are substantially cheaper in cost.

Regulated Linear Wall Adapter
Jameco ReliaPro 12V, 1A Regulated Linear Wall Adapte

1-Unit Price: $14.95

Unregulated Linear Wall Adapter
Jameco ReliaPro 12V, 1A Unregulated Linear Wall Adapter

1-Unit Price: $9.95

Now that you know what to look for, make sure you have all the parts you need. If for some reason you can't find what you need, just write us a note and we'll do our best to find it for you.

Still have questions? Write us at [email protected]
Jameco relies on our customers. We know you have all the experience, tips and tricks. We'd love to hear from you on power stories, purchasing ideas or even product information. In turn, we'll share it with the world. Send your thoughts to: [email protected]

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