What is a Fuse ? And how to test it.Written by Jestine Yong
A fuse is a very thin wire, which either melts or vaporizes when too much current flows through it. The thin wire may be made of aluminum, tin-coated copper or nickel. The resulting open in circuit stops current flow. In electronic equipment, most fuses are cylindrical glass or ceramic type with a metal cap at each end! The current rating also can be seen in one of two metal end caps. There are two popular physical sizes: 1 – ¼ X ¼ - inch and 5X20mm. The 1 – ¼ X ¼ - inch size is used in many automobiles. You’ll find both sizes in many electronic equipment, but smaller 5 X 20mm has become more common. Fuses are available with current ratings from 1/500 Ampere to hundreds of amperes.
Purpose of Fuses
The purpose of a fuse is to open an electronic circuit when current flow exceeds a certain amount, determined by rating of fuse. Opening a circuit under high current conditions can save electronic components from damaged and prevents overheating, which could cause a fire.
Types of Fuses
There is two basic types of fuses: fast acting and slow blow. The fast acting type will open very quickly when their particular current rating is exceeded. This is important for analog meter movements, which can quickly be destroyed when too much current flows through them, for even a very small amount of time. Slow blow fuse have a coiled construction inside. They are designed to open only on a continued overload, such as a short circuit. The purpose of coiled construction is to prevent fuse from blowing on just a temporary current surge.
Do not use a slow blow fuse in place of a fast acting fuse. It may not open fast enough to prevent components damage under a high current condition. It’s not harmful to replace a slow blow fuse with a fast-acting fuse, but it will probably open up unnecessarily every now and then when equipment is first switch on. A blown fuse can tell you something about your service problem. Often glass case of fuse appears clear, and you can still see broken pieces of fuse element. This means you have kind of problem that causes a slow, gradual overload on power supply. Some fuses even die of old age. But if inside of glass fuse is discolored, and there is no trace of fuse element (the center connector), you know that center connector was destroyed quickly and violently, using a lot of heat. The fault was a short circuit or other problem that caused a lot of current to flow very quickly.
What is a crystal ? And how to test it.Written by Jestine Yong
Crystal are use to keep frequency of clock from drifting. If signal from this clock stops, or is weak, or pulses begin to vary, electronic equipments might show intermittent faults or might stop altogether. The microprocessor pins that hold crystal are usually called OSC IN and OSC OUT as shown in Figure 1 and frequency is marked on crystal. Typical examples of crystal oscillator frequency are 3.58MHZ, 4MHZ, 8MHZ, 24MHZ etc.
Testing Crystal Crystals are quite fragile components because of their construction. Unlike a resistor or capacitor, if you drop one on ground from a decent height, its 50-50 bet whether it will work again. Testing crystal is not a breeze either. You cannot just take out your trusty multimeter and plug crystal in it. In fact, there are three right ways to test a crystal: -
(a) Using Oscilloscope A crystal produces a sine wave when excited. It is appropriate then, to see a waveform representative of a sine wave on clock pins. If clock is not functioning properly, replace crystal. In most cases this should solve problem since microprocessors are usually very reliable. Check crystal with power on.