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Thread: PSU 101: Electrical Basics

  1. #1
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    Default PSU 101: Electrical Basics

    We hope this helps some of you out there. If anyone has any input, please feel free to comment. We are not necessarily experts, but simple hobbyist with a thirst for knowledge and a desire to teach others. -- jon

    • Electrical Basics

    a. Voltage:

    Voltage, measured in Volts, is the electrical potential for a circuit to do work. If a circuit has voltage, it has electrons ready to flow.

    b. Current:

    Current, measured in Amps or Amperage, is the flow of electric charge. When electrons flow, the movement of those electrons is measured in amperage.

    When used in the context of a computer power supply; components at work within a computer require a certain amount of current of a particular voltage to operate correctly. A power supply is capable of putting out certain voltages up to a certain amount of amperage, or current.

    c. Resistance:

    Resistance, measured in Ohms, is a measure of the degree to which an object opposes an electrical current through it. Think of resistance as “friction”. In the mechanical world, friction can cause a part to slow or wear down. In the electrical world, resistance caused by a thin gauge wire, a long length of wire, several connections on a wire or a large increase in current for example, can cause voltages to drop. The mathematical equation for calculating voltage with consideration for resistance, which is also known as “resistive loss”, is V = I * R.

    With computer power supplies, we have to be aware of this because multiple connectors coming from one lead or a heavy load on one connector can cause resistance and this resistance can result in a drop in voltage. The voltage may be within specification as it comes out of the power supply, but may be lower by the time it gets to the end of the connector.

    d. Wattage:

    Wattage, measured in Watts, is the rate at which energy is transferred by an electrical circuit. Typically wattage is measured by multiplying Amperage by Voltage. Of course, resistance could be taken into consideration as no circuit can truly have zero resistance. But for the sake of conversation, we’ll use the simple formula of V * I = W (“I” is the variable for current, or amperage, in electronics and physics. It stands for “Impetus”.)

    Example: 5A of 12V is essentially 60W (5 x 12 = 60) and 12A of 5V is also 60W. If you wanted to factor in a known measure of resistance, Wattage would equal I^2 (I squared) times resistance.

    Computer power supplies are often categorized (read: bought and sold) by their total wattage capability, which is essentially an additive formula of how much current each voltage output of the power supply is capable of delivering.

    e. DC (Direct Current):

    Direct current is also called continuous current. It is the constant flow of electric charge in the same direction. Computer components typically operate off of direct current of different voltages.

    f. AC (Alternating Current):

    Alternating current is an electrical current whose magnitude and direction vary cyclically with time. The outlets in your house provide alternating current.

    g. Efficiency:

    The ratio between the useful output of an energy conversion device and the input is called efficiency. With computers, the term is often used to describe the ratio of AC power inputted into the power supply unit and the DC power coming out. The difference is exhausted as heat from within the power supply.

    h. Power Factor:

    Power factor, or “PF” for short, is the ratio of the real power to the apparent power. Real power is the capacity of the circuit for performing work in a particular time and is measured in Watts. Apparent power is the product of the current and voltage of the circuit and is measured in volt-amperage (or “VA”.) Now, it sounds as if Watts and VA are the same thing, and in DC they are (240W DC is equal to 240VA DC, for example) but because energy stored in the load of a device using alternating current (AC) is returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can actually be greater than the real power.

    Poor power factor does not typically affect efficiency as far as how much DC power is converted to AC power, but can create losses in the power distribution system. In the European Union, it is now mandatory that all power supplies of 75W or greater have some form of power factor correction.

    i. Power Factor Correction (or “PFC”):

    Poor power factor can be corrected by adding some form of power factor correction to the AC input of the power supply. Power Factor Correction comes in two forms: Active Power Factor Correction, or APFC, and Passive Power Factor Correction.

    Computer power supplies can create harmonics of the same frequency as the input current, due to the non-linear load caused by the bridge-rectifier doing the AC to DC conversion, and typically have poor power factor (typically 0.55 to 0.65).

    Passive Power Factor Correction uses a filter that kills any harmonic current and passes current only at line frequency (typically 60Hz in the U.S.) The filters typically come in the form of large, high-value inductors.

    Active Power Factor Correction is done by using a boost converter in between the bridge-rectifier and main input capacitors. The boost converter attempts to maintain a constant output voltage while drawing a current that is always in phase and at the same frequency as the line voltage.

  2. #2
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    Thanks for the part on PFC. I just completed a super simple linear PSU of my own, and I was trying to figure out what the hell PFC actually does...This helped a lot.

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