So, what is AC power and how is it different from DC power? AC and DC are electrical formats. Alternating current repeatedly changes direction, alternating between the positive and negative poles within a circuit. This is the format used to deliver mains electricity. By contrast, direct current does not change direction. DC is used to transmit electricity along electrical pylons, and it also powers telecommunications equipment, vehicle batteries, and electrical devices with rechargeable batteries.
AC adapters typically feature a boxy central unit containing power-switching circuitry. This draws AC power from the mains and converts it into DC at the voltage required by the device. A power cord then feeds this power from the adapter to the device to charge its battery or allow it to run.
Like most electrical devices, AC adapters are built for compatibility with different levels of power, typically expressed in Volts. They also feature a variety of plugs for use with different socket designs. The UK standard power plug is different from that used in Europe, for example, and different again from the standard US plug.
Most AC/DC adapters are not universal and can only be used to power compatible devices. However, some models are designed to work as replacement chargers for a wide range of devices. These universal power supply adapters are compatible with multiple voltages within a range - for example, 100-240 Volts - and some even feature plugs of different sizes. The latter is also known as star connectors or X connectors.
TDK-Lambda Americas Blog
Application Information
DC-DC Converters
September 4, 2007
The answer is yes, sometimes.
Many standard AC-DC switch mode power supplies (most of Lambda’s products) specify a high voltage DC input range in addition to the more common AC input range of 90-264VAC. We receive many questions about how and where to connect the DC input to an AC-DC supply that is spec’d to operate off of DC as well as AC inputs.
Where and why is high voltage DC power used? It turns out that many power generation facilities provide a high voltage DC to power the plant’s equipment rather than the regular 115VAC or 208VAC power grid. This high voltage DC (typically 120 or 130-330VDC) can be easily used with batteries to provide a secure source of power rather than using expensive centralized or local UPS systems.
Now back to the subject. The topology of many switch-mode power supplies actually lends itself to operation from either AC or DC input. Important Note: Always check your power supply’s Operations Manual or spec sheet to confirm that it is designed to operate from either an AC or DC input.
Referring to the simplified power supply schematic below:
When powered by an AC sine wave, during the first half cycle the current flows from the Line terminal through the input filter and charges capacitor C1 through diodes D1 and D3. During the second (negative) cycle, current flows from the Neutral terminal and capacitor C1 is charged through diodes D2 and D4.
When powered from a high voltage DC source, the polarity of the connection is not critical as far as the operation of the power supply is concerned. If the positive connection is made to the Line terminal, C1 is charged through diodes D1 and D3. If the positive connection is made to the Neutral terminal, then C1 is charged through diodes D2 and D4. An important note of caution to insert here is about the protective fusing of the power supply. Internally most power supplies have a fast acting AC rated fuse in series with the Line terminal. It is recommended that a DC rated fuse be installed external to the power supply. If one side of the high voltage DC buss is connected to ground, then the fuse is usually positioned in series with the “hot” side (the ungrounded side). It is recommended that you consult with your local safety engineer to be sure.
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