Power Conversion: Frequently-Asked Questions

 

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What is a power supply?

A power supply is a device that converts wall power (120 VAC) to battery power (DC).

Why do I need a power supply?

A power supply is required for devices that are designed to operate from direct current. Devices such as semi conductors cannot perform reliably with alternating current (AC) and are not typically designed to handle the higher power input or the pluses and minuses in the alternating energy form. They run more effectively off of low voltage dc current.

What types of power supplies are there?

Typically, there are two kinds of power supply technologies. There are linear power supplies and switch-mode power supplies. For most applications today, switch-mode technology is appropriate.

What are Linear Power Supplies?

Linear Power Supplies have a bulky steel or iron laminated transformer. This transformer has two purposes. It provides a safety barrier for the low voltage output from the AC input and reduces the input from typically 115VAC or 230VAC to a much lower voltage around 30VAC.  The low voltage AC output from the transformer is then rectified by two or four diodes and smoothed into low voltage DC by large electrolyte capacitors.

What are Switch-mode Power Supplies?

Switch-mode power supplies are a lot more complicated. The 115 or 230 VAC voltage is rectified and smoothed by diodes and capacitors resulting in a high voltage DC. That DC is then converted into a safe, low voltage, high frequency voltage using a much smaller ferrite transformer and FETS or transistors. The voltage is then converted into the DC output voltage of choice by another set of diodes, capacitors and inductors. Corrections to the output voltage due to load or input changes are achieved by adjusting the pulse width of the high frequency wave form.

What shapes and sizes do they come in?

Power supplies are available in many different configurations to suit each unique application. Power supplies are mainly available in desktop, rack-mount or open frame for behind the scenes applications.

What DC Voltages are available?

Power Supplies are available in a variety of configurations. You'll see them most commonly in 3.3, 5, 7.5, 9, 12, 13.5, 15, 18, 24, 27 and 48 VDC.

For the majority of the applications that you would uncover, requirements for 12 and 24 VDC are typical.

How much power do I need?

In a power supply, we determine its current rating by amps. This is the amount of power output that the unit can put out. Your customer will usually have a handle on how much power they require and the necessary voltage as well. Each time that you increase the output voltage the current rating drops by ½. For example, say you have a 12VDC power supply @ 12.5 amps of current. If you were to choose the 24VDC equivalent, the current rating would only be 6.5 amps.

Contact TESSCO at 800-472-7373 or email us with any questions. We look forward to doing business with you!