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| Multimeters |
| Written by David Jackson | ||||
| Thursday, 22 March 2007 | ||||
Page 1 of 2 Watt matters in modding
When over-clocking and performing circuit level mods, a multi meter is an invaluable tool. Even when it’s a simple lighting mod, a good meter can save innumerable headaches. In this special three-part Tool Time we start with some basic electrical knowledge, then move on to the use of multi meters. To understand the use of a multi meter, you must first study electricity basics and to know what it is you are measuring. Electric Current Without going deep into atomic theory, we can say that a body that has an excess of free electrons will be negatively charged, while a deficiency of electrons causes it to be positively charged. If a wire connects two differently charged bodies, electrons will flow to the one having the lesser charge. This is called an electric current. An electric current is the rate of flow of electrons. If 6,240,000,000,000,000 electrons pass a given point in one second, the current flow is one ampere, or one amp. There are two basic types of current flow. If the current flows in the same direction and is consistent in magnitude, it is called Direct Current or DC. If the current is continually varying in magnitude and periodically reversing direction, it is called Alternating Current, or AC. Whether it’s DC or AC, the movement of electrons is always from negative (-) to positive (+) 
The circuits in our Macs use DC current because it is directional and flows at a constant rate. The power supply uses transformers to change the AC current flowing from our household outlets to the DC our computers require. Resistance and Conductance There is no perfect conductor. Every substance has at least some opposition to the flow of current, and this is called resistance. Resistance is measured in a unit called an Ohm, which is represented by the Greek letter Omega Ω. The higher the resistance, the lower the current flow. A resistor is a component that is inserted into a circuit that has a relatively constant value of resistance. Resistors are commonly made of Carbon or Carbon compounds. Resistors are normally located on drawings in two ways. Sometimes a number is followed by Ω (omega) as in 150Ω, which represents one hundred fifty ohms. In other diagrams you may find “R” used in place of a value such as R4 or R231, etc. Resistors are important to us when over-clocking many Macs, as Apple seems to like using resistors instead of jumpers to set clock and bus speeds. For instance, my iBook G3 uses 0Ω resistors to set clock speed. And any Star Trek fan knows that at 0Ω, "resistance is futile." |
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