BRIDGE-TYPE MEASUREMENTS

1-16 Figure 1-11.—Safety shorting probe. When using the general-purpose safety shorting probe, always be sure first to connect the grounding clip to a good ground connection (if necessary, scrape the paint off the grounding metal to make a good contact — paint can be replaced, lives can't). Then, while holding the safety shorting probe by the handle behind the protective shield, touch the end of the metal rod to the points to be discharged. Touch each point several times to ensure that the circuit is completely discharged. Be extremely careful that you do not touch any of the metal parts of the safety shorting probe while touching the probe to the exposed "hot" terminal. Don't develop a nonchalant or routine attitude about these procedures. It pays to be safe; use the safety shorting probe with care. Large capacitors, dormant in storage, can also develop a large static charge. This charge is caused by environmental conditions such as a close proximity to an rf field. An easy way to avoid this condition is to short the stored capacitor's terminals with a piece of wire before putting it in storage. Remember to remove the wire before installing the capacitor. If you receive a large capacitor that is not shorted, short the terminals together. Remember, CHARGED CAPACITORS CAN KILL. Q-12. Charged capacitors can kill. True or false? BRIDGE-TYPE MEASUREMENTS Capacitor tests involving quality and value must be made in the course of everyday troubleshooting. You must make the important decision of whether to reject or continue to use a certain capacitor after it has been tested. Capacitance measurements are usually accomplished by either a bridge-type or a reactance-type capacitance meter. The bridge-type capacitance meter is much more accurate than the reactance-type meter. You may want to review rcl bridges in chapter 1 of NEETS, module 16, before reading further. Capacitance tolerances vary more widely than resistance tolerances and are dependent upon the type of capacitor, the capacitance value, and the voltage rating. The results of capacitance tests must be evaluated to determine whether a particular capacitor will fulfill the requirements of the circuit in which it is used. The power factor of a capacitor is important because it is an indication of the various losses attributable to the dielectric, such as current leakage and dielectric absorption. Current leakage is of considerable importance, especially in electrolytic capacitors. Figure 1-12 is a simplified schematic of a capacitance bridge. As you can see, a capacitance bridge is very similar in construction to a resistance bridge with the exception of the standard capacitor (C_S) and the unknown capacitor (C _X). Because current varies inversely with resistance and directly with capacitance, an inverse proportion exists between the four arms of the bridge. The following expression shows the inverse proportion between resistors A and B and capacitors C_S and C_X: