Download (direct link):
^ Determine added costs for duty, taxes, and shipping when buying internationally.
Okay, so now you know what to do. Hereís what you should avoid whenever possible:
Donít buy from a source unless you feel very comfortable about sending money to them.
Donít give your credit card number over e-mail or on a Web page order form unless you know youíre using a secure communications link. When
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374 Part VI: The Part of Tens
youíre using a site with security features, a little key lock appears on the status bar of your browser. This shows you that the communications between you and the site are encrypted with a code that thieves have a hard time breaking.
^ Donít use a credit card to pay for goods from a company that you havenít yet dealt with if you can just as easily send a check or money order. This way, you limit the exposure of your credit card accounts to possible Internet fraud.
^ Donít send money to foreign companies unless youíre positive theyíre safe bets. While youíre checking them out, be sure that they ship to your country.
New or Surplus?
Surplus is a loaded word. To some, it means junk that just fills up the garage, like musty canvas tents, or funky fold-up shovels that the U.S. Army used back in the 1950s. To the true electronics buff, surplus has a totally different meaning: Affordable components that help stretch the electronics-building dollar.
Surplus just means that the original maker or buyer of the goods doesnít need it any more. Itís simply excess stock for resale. In the case of electronics, surplus seldom means used, as it might for other surplus components, such as motors or mechanical devices that have been reconditioned. Except for hard-to-find components ó such as older amateur radio gear ó surplus electronics are typically brand new, and someone still actively manufactures much of this equipment. In this case, surplus simply means extra.
The main benefit of shopping at the surplus electronics retailer is cost: Even new components are generally lower priced than at the general electronics retailers. On the downside, you may have a selection limited to whatever components the store was able to purchase. Donít expect to find every value and size of resistor or capacitor, for example.
ł Remember that when you buy surplus there is no manufacturerís warranty. Sometimes that lack of warranty is because the manufacturer is no longer in business. Though most surplus sellers accept returns if an item is defective (unless it says something different in their catalog), you should always conó sider surplus stuff as-is, with no warranty implied or intended (and all that other lawyer talk).
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Ten Electronics Formulas You Should Know
In This Chapter
Putting Ohmís Law to work ^ Calculating resistance and capacitance ^ Finding the time (and power) to calculate energy units ^ Laying down some time constants ^ Introducing frequency and wavelength
ormulas take the guesswork out of electronics. Instead of dumping a m bunch of components on the table and plugging them in any which way, the seasoned electronics experimenter builds new circuits with the help of a handful of formulas. These formulas help you determine specific values for things like voltage when you are designing electronic circuits.
You use these same formulas when modifying existing circuits. For example, you can apply the basic Ohmís Law formula for direct current (which you can find in the section titled ďCalculating Relationships with Ohmís LawĒ) to select a resistor so that a light emitting diode shines brighter or dimmer, as your prefer.
This chapter summarizes many of the more commonly-used electronic formulas that you encounter in your electronics work. The electronics world has used quite a few of these formulas for many, many years, but they still work just fine.
Calculating Relationships With Ohm's Law
Ohmís Law calculates the relationship between power, voltage, current, and resistance. Table 18-1 gives you the formulas you use to find these values.
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376 Part VI: The Part of Tens
Table 18-1 Formulas for Ohm's Law
Unknown Value Formula
Voltage, in volts (<) V = IR
Current, in amps (I) I = V I R
Power, in watts (P) P = VI
Resistance, in ohms (R) R = V
Note that in Table 18-1:
V = voltage (in volts)
I = current (in amps)
P = power (in watts)
R = resistance (in ohms)
Check out this example: To find the power in a circuit consuming 100 volts at ten amps, multiply volts by amps (100 x 10 = 1000). So, you get the answer of
1,000 watts. You might use this figure to judge how big a fuse you can add to your circuit without damaging it, or how big an electric bill youíre going to have at the end of the month.
Hereís another example: To calculate the resistor that you need to handle a given amount of current through an LED, you use Ohmís Law like this: