3.3. DIODE RATINGS107OFFCOMAVAVAMFigure 3.13:Ohmmeter equipped with a low test voltage, too low to forward bias diodes, doesnot see diodes.which is negative! The actual polarity may not follow the colors of the leads as you mightexpect, depending on the particular design of meter.• Some multimeters provide a “diode check” function that displays the actual forward volt-age of the diode when its conducting current. Such meters typically indicate a slightlylower forward voltage than what is “nominal” for a diode, due to the very small amountof current used during the check.3.3Diode ratingsIn addition to forward voltage drop (Vf ) and peak inverse voltage (PIV), there are many otherratings of diodes important to circuit design and component selection. Semiconductor manu-facturers provide detailed speciﬁcations on their products – diodes included – in publicationsknown as datasheets. Datasheets for a wide variety of semiconductor components may befound in reference books and on the internet. I prefer the internet as a source of componentspeciﬁcations because all the data obtained from manufacturer websites are up-to-date.A typical diode datasheet will contain ﬁgures for the following parameters:Maximum repetitive reverse voltage = VRRM , the maximum amount of voltage the diodecan withstand in reverse-bias mode, in repeated pulses. Ideally, this ﬁgure would be inﬁnite.Maximum DC reverse voltage = VR or VDC, the maximum amount of voltage the diode canwithstand in reverse-bias mode on a continual basis. Ideally, this ﬁgure would be inﬁnite.Maximum forward voltage = VF , usually speciﬁed at the diode’s rated forward current. Ide-ally, this ﬁgure would be zero: the diode providing no opposition whatsoever to forward current.In reality, the forward voltage is described by the “diode equation.”Maximum (average) forward current = IF(AV ), the maximum average amount of currentthe diode is able to conduct in forward bias mode. This is fundamentally a thermal limitation:how much heat can the PN junction handle, given that dissipation power is equal to current (I)multiplied by voltage (V or E) and forward voltage is dependent upon both current and junctiontemperature. Ideally, this ﬁgure would be inﬁnite.