ELECTRONIC TEST EQUIPMENT

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VOLTAGE, CURRENT, AND RESISTANCE By Joe Grand

Voltage, current, and resistance are 3 staple quantities that you’ll encounter with anything that has electrons running through it. This article explores voltage and current, then resistance, and at the end shows how they all tie together.

CHARGE, CURRENT, VOLTAGE, AND POWER

Electricity starts with charge, which means electrons. Charge is measured in coulombs (C), where one coulomb equals 6. 25 10¹⁸ electrons, a very large number. Electric charge can be static, as held in a capacitor, or it can flow, as through a wire. Flowing charge is current, denoted with the symbol I. The rate of flow is measured in amperes, or amps (A), where 1 ampere equals 1 coulomb flowing through a given point on a circuit per second.

Voltage, also known as potential difference, is the amount of work (energy) required to move a positive charge upstream in a circuit from a more negative point, which has lower potential, to a more positive point, which has higher potential. Think of it as an electrical pressure, or force. Voltage is represented in equations by V, E, or U, and its unit of measure is the volt (V).

Power is a measurement of the energy that’s expended by a flowing charge over some amount of time. One watt of power (W) is equal to the work done in 1 second by 1 volt moving 1 coulomb of charge. You can calculate the power consumed by a circuit with the simple formula:

DIRECT CURRENT (DC) AND ALTERNATING CURRENT (AC)

Direct current (DC) flows in one direction through a conductor, as either a steady signal or as pulses. The most familiar form of DC supply is a battery. Most electronic circuitry uses DC, except for power supplies or motor circuitry.

Alternating current (AC) flows in both directions, which makes it more complicated to work with than DC. But it travels better, which is why AC is the form of electricity delivered as house current. In the United States and Canada, AC power outlets provide 120V AC at 60Hz (cycles per second). Other parts of the world use different standards.

Peak-to-Peak Voltage

Peak Positive Voltage

RMS Voltage

Peak Negative Voltage

P = V I

where P = Power ( W)

V = Voltage (V)

I = Current (A)

To differentiate between voltage and current, we refer to voltage as going between or across 2 points in a circuit, and current as going through a device or circuit connection. You say, “The voltage across the resistor is 1.7V,” and “The current through the diode is 800mA.” When referring to the voltage at a single point in a circuit, it is defined with respect to ground (typically 0V).

Several terms describe an AC signal: Peak voltage (V_PEAK) — the maximum positive and negative voltages of the AC signal, measured from the center. Peak-to-peak voltage (V_PP) — the total voltage swing from the most positive to the most negative point of the AC signal. Root-mean-square (RMS) voltage (V_RMS) — the most common term used to describe AC voltages, calculated by taking the square root of the average of the integrated square over 1 cycle. This calculation sums both positive and negative voltage, to measure total useful energy.

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