# Tutorial 13

1. What is Ohm's Law?

Ohm's Law states that the current (electrons) flowing through a conductor, or resistance, is in linear proportion to the applied potential difference (volts).

2. What is the formula for Ohm's Law?

The formula for Ohm's Law is voltage is equal to the current multiplied by the resistance (V = I R).

3. What is the difference between AC and DC?

AC is alternating current in which the voltage varies from positive to negative. DC is direct current in which the voltage stays the same all the time.

4. What instrument is used to measure the various components of electricity?

The instrument used to measure the various components of electricity is VOM-Volt-Ohm Meter (sometimes referred to as DVOM or Digital Volt-Ohm Meter).

5. How do you test for continuity?

Continuity is a term used to indicate whether or not there is a connection from one point to another. It is used to determine the presence of breaks in wires and electrical circuits. If no continuity setting is available, use the resistance setting. If the multimeter measures infinite resistance, then there is no continuity, indicating a break in the circuit. If the multimeter shows little or no resistance, then there is continuity and the circuit is complete.

6. What is "AC Ripple?" How do you test for it?

A power supply converts AC to DC voltage. When working properly, a pure DC signal will be produced. Sometimes, however, as the power supply ages, its ability to produce pure DC falters. A power supply uses capacitors to filter or smooth the voltage after being converted from AC to DC. These capacitors are second only to fuses as the part of a power supply most likely to fail. When a capacitor begins to fail, it allows more and more AC voltage to pass through. This AC voltage is superimposed on top of the DC voltage and is called noise or ripple.

To test for ripple, set a meter to read AC. Then connect a .1mfd capacitor to the red lead. With the power turned on, measure the DC voltage to ground. Any ripple present will be displayed as an AC voltage.

7. Describe ESD and how to prevent it.

Just as high voltages generated by electrical and electronic equipment can do severe damage to humans, high voltages generated by humans can do damage to computers. We have all seen what a short circuit can do to electrical equipment (smoke, fire, and destruction). Electrostatic discharge (ESD) is an unseen (and sometimes unheard) force, created by humans, that is just as deadly to a computer.

The key to ESD prevention is to keep all electronic components and yourself at the same electrical potential. This usually means ground potential or zero volts. Maintain a habit of "grounding" yourself to the computer chassis whenever you attempt a repair. An ESD wrist strap is the tool most commonly used by technicians to prevent ESD.

8. What is a latent failure? What makes it especially troublesome?

This type of ESD problem occurs when a transistor junction becomes weakened. A transistor in this condition may pass all quality tests but, over time, will generate poorer system performance and eventually fail completely.

9. What is a catastrophic failure?

Catastrophic failure is the destruction of a part because of the heat generated during the mishandling and misapplication of a power source, cable, or test instrument.

10. When working with a computer, when is it acceptable to use an AC power supply that is not grounded?

It is never acceptable to use an AC power supply that is not grounded.