Hameg HM 2005 Manual 

Hameg HM 2005 Manual  Manual pdf 31 page image

31Subject to change without noticegiving an ellipse-shaped display. The position and opening widthof the ellipse will vary according to the impedance value (at 50Hz)of the component under test. A horizontal ellipse indicates a high impedance or a relativelysmall capacitance or a relatively high inductance. A vertical ellipse indicates a small impedance or a relativelylarge capacitance or a relatively small inductance. A sloping ellipse means that the component has aconsiderable ohmic resistance in addition to its reactance.The values of capacitance of normal or electrolytic capacitorsfrom 0.1μF to 1000μF can be displayed and approximate valuesobtained. More precise measurement can be obtained in asmaller range by comparing the capacitor under test with acapacitor of known value. Inductive components (coils,transformers) can also be tested. The determination of thevalue of inductance needs some experience, becauseinductors have usually a higher ohmic series resistance.However, the impedance value (at 50Hz) of an inductor in therange from 20 Ohm to 4.7kΩ can easily be obtained orcompared.Testing SemiconductorsMost semiconductor devices, such as diodes, Z-diodes,transistors, FETs can be tested. The test pattern displays varyaccording to the component type as shown in the figuresbelow. The main characteristic displayed during semiconductortesting is the voltage dependent knee caused by the junctionchanging from the conducting state to the non conductingstate. It should be noted that both the forward and the reversecharacteristic are displayed simultaneously. This is a two-terminal test, therefore testing of transistor amplification isnot possible, but testing of a single junction is easily and quicklypossible. Since the test voltage applied is only very low, allsections of most semiconductors can be tested withoutdamage. However, checking the breakdown or reverse voltageof high voltage semiconductors is not possible. More importantis testing components for open or short-circuit, which fromexperience is most frequently needed.Testing DiodesDiodes normally show at least their knee in the for wardcharacteristic. This is not valid for some high voltage diode types,because they contain a series connection of several diodes.Possibly only a small portion of the knee is visible. Zener diodesalways show their forward knee and, depending on the testvoltage, their zener breakdown forms a second knee in theopposite direction. If the breakdown voltage is higher than thepositive or negative voltage peak of the test voltage, it can notbe displayed. The polarity of an unknown diode can be identifiedby comparison with a known diode.Testing TransistorsThree different tests can be made to transistors: base-emitter,base-collector and emitter-collector. The resulting test patternsare shown below.The basic equivalent circuit of a transistor is a Z-diode betweenbase and emitter and a normal diode with reverse polaritybetween base and collector in series connection. There are threedifferent test patterns.For a transistor the figures b-e and b-c are important. The figuree-c can vary; but a vertical line only shows short circuit condition.These transistor test patterns are valid in most cases, but thereare exceptions to the rule (e.g. Darlington, FETs).With the COMPONENT TESTER, the distinction between a P-N-P to an N-P-N transistor is discernible. In case of doubt,comparison with a known type is helpful. It should be noted thatthe same socket connection (COMP. TESTER or ground) for thesame terminal is then absolutely necessary.A connection inversion effects a rotation of the test pattern by180 degrees round about the center point of the scope graticule.In-Circuit TestsCaution!During in-circuit tests make sure the circuit is dead. Nopower from mains/line or battery and no signal inputsare permitted.Remove all ground connections including Safety Earth(pull out power plug from outlet). Remove all measuringcables including probes between oscilloscope and circuitunder test. Otherwise both COMPONENT TESTER leadsare not isolated against the circuit under test.In-circuit tests are possible in many cases. However, they arenot well defined. This is caused by a shunt connection of real orcomplex impedances - especially if they are of relatively lowimpedance at 50Hz - to the component under test, often resultsdiffer greatly when compared with single components.In case of doubt, one component terminal may be unsoldered.This terminal should then not be connected to the ground socketavoiding hum distortion of the test pattern.Another way is a test pattern comparison to an identical circuitwhich is known to be operational (likewise without power andComponent Texter