If the meter shows the expected forward resistance the meter lead polarity is not reversed. Which meter is yours? A simple resistance test on a known good diode can be used connect the black -ve lead to the cathode and the red +ve lead to the anode. In this case the method described for digital meters should be followed. NOTE: the above paragraph refers only to true analogue meters, many modern "analogue" models tend to be digital meters with an analogue display. The most suitable analogue range will normally be indicated in the user instructions, but as with digital meters the actual voltage used on the testing range should be checked to to understand its effect on the expected forward and reverse resistances. Some analogue meters have a specific diode testing range, but most analogue meters will be quite suitable for diode testing. This means that the black lead should be connected to the anode and red to cathode to measure the FORWARD resistance of the diode. Therefore when measuring resistance of a diode with an analogue meter on any range the BLACK lead is positive and the RED lead is negative. If an analogue meter is used for testing it must be remembered that because zero on the resistance and voltage scales are reversed, due to the inner workings of the meter, the polarity of the probes when using analogue meters for resistance measurement, is also reversed compared to digital meters. However very low or zero ohm readings may indicate a short circuit diode (the most common fault with diodes) making it worthwhile, if no other obvious reason for the very low reading can be seen, to remove at least one end of the diode from the circuit and re-check the diode's forward and reverse resistance. Therefore readings will be lower than those indicated above. If the diode is already in a circuit, the resistances measured, always with the circuit switched off, will be affected by any parallel paths. With the meter leads reversed, an out of range (infinity) or open circuit reading (usually indicated by a display something like ‘1.’ on a digital meter, as shown in Fig. When measuring a good silicon diode (not connected to any circuit), a reading in the forward direction of about 500Ω to 1kΩ could be expected, similar or slightly less with germanium diodes. The actual value of resistance will depend on the slope of the forward characteristic of the diode at the voltage applied by the meter, and so will vary from device to device and from meter to meter, so a precise value cannot be given. Therefore in the forward direction (meter positive lead to the diode anode, and the negative lead to the cathode) the diode's resistance can be measured. If the diode range is used, the test voltage applied by the meter will be high enough in most cases to overcome the forward junction potential and the diode will conduct. This would give an indication that the diode was open circuit (very high resistance). The normal voltages used by the meter on other resistance ranges may not be high enough to overcome the diode´s forward junction potential and so will not make the diode conduct, even in the forward direction. The reason for this is that the meter tests the diode by applying a voltage across the diode junction. This range should always be used when testing diodes or any other semiconductor device. Most digital multi-meters are suitable for diode testing, and in many cases will have a special ‘diode test’ range usually marked with a diode symbol. There are however a number of points to remember when testing diodes. It is normally the resistance of the diode in both forward and reverse directions that is tested. 2.8.1 Digital Meter Multimeter Diode Testingĭiodes can be tested using a multi meter.
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