Tech Forum

Multimeters, particularly digital VOM's, and some high impedance analog types, have a Diode test postion on their scale selector. This position presets the full scale range, typically 1000 - 2000 ohms FS, to measure the front to back ratio of a diode and also the forward resistance.  Many DVM's do not provide enough voltage (up to 3 volts) in their 'normal' resistance/ohm scales to cause a forward 'breakdown' of the junction to measure it. (Especially in auto-ranging units). Also, this pre-set range allows one to test diodes rapidly without waiting for the unit to count down.

The diode setting measures the forward drop of a diode at a current of a few milliamperes. To ensure the diode is conducting, the potential across the meter probes is a few volts. On my aged Radio Shack DVM, the open-circuit potential is 2.88 volts, sufficient to "turn on" most diodes, including red, yellow and green LED's, which glow dimly. Note that blue, ultraviolet and white LED's, as well as Darlington transistors and some high-voltage diodes (that are, internally, series-connected diodes) show as open circuits, since they need a higher voltage to conduct.

The resistance setting is intended not to "turn on" a diode, so, on that setting, the voltage across the meter probes is only a few tenths of a volt. That allows accurate measurement of a resistor in parallel with a diode or across other semiconductor device, except perhaps Schottky diodes, which may have a forward voltage drop of 0.2 volts.

See [url=http://en.wikipedia.org/wiki/Diode]http://en.wikipedia.org/wiki/Diode[/url] for more information, but the list below is a rough guide to forward voltage drop at a few mA:

• Ge Schottky diode:   0.2 V
• Ge signal diode:        0.3 V
• Si rectifier:                0.6 V
• SiC Schottky diode:   0.6 V
• SiC junction diode:    0.6 V
• Red GaAsP LED:      1.3 V
• Blue InGaN LED        3.0 V

Using the diode setting, you can usually identify the type of a diode or junction transistor, as well as determining if it's open or shorted.

BTW, thanks for asking that question, which inspired me to check the open-circuit voltage and current draw of my DVM diode and resistance settings. Now my measurements should be a bit more meaningful.

The diode selection on a multimeter is for continuity and diode checking. Digital multimeters do not produce enough voltage to forward bias a diode ordinarily so this position outputs several volts at low current and sounds a buzzer if current is drawn. If the diode is good, the forward voltage will be displayed in one polarity and open circuit will be indicated in the other.

The "diode" range on digital multimeters is used to "measure" diodes (any type) to see if they are good or not. To use the range, connect the BLACK meter lead to the diode wire with the "stripe" on the diode body (cathode lead) and the RED meter lead to the other diode wire (anode lead). The following are "typical good" forward-bias readings for various diodes:

• "Silicon" diodes: "0.6-0.7"
• "Germanium" diodes: "0.3-0.4"
• "Tunnel" and "Schottky" diodes: "0.2-0.3"
• Zener diodes: similar to silicon diodes
• "Selenium" rectifier: "0.9-1.2" (approx)
• "Power" (i.e., 10A rating) and "high-voltage" (i.e., kilovolt rating) diodes: "0.7-0.8"
• LEDs: "1.6-1.8"

Reverse the leads (reverse-biasing) will show "OL" - any "numerical" reading usually indicates the diode is "shorted" in reverse-bias. This range is also useful for checking the "polarity" of BIPOLAR (PNP, NPN) transistors, if you don't know what kind you have:

• The emitter-base drop will read "0.5-0.6"
• The collector-base drop will read "0.7-0.8"
• The collector-emitter junction will read "OL" ("anything less" indicates a blown transistor)
• Reverse-bias the emitter-base and collector-base junctions and you'll read "OL" ("anything less" indicates a blown transistor)

NOTE: The BLACK lead will tell you which lead(s) are the "N" lead(s) for determining "PNP" or "NPN" polarity (use the voltage drop difference to  determine collector from emitter). It's also useful for determining the polarity of "unmarked" diodes: BLACK is cathode ("stripe") and RED is ANODE. DO NOT use this range to measure FET (field-effect transistors) as modern FETs are "static-sensitive" devices and will be probably be destroyed from the action of taking the reading. SCRs, triacs (i.e.,"thyristors") and similar "exotic" junction-type devices cannot be accurately "tested" on this range. Finally, you can measure LOW-VALUE resistors (<2000 ohms) and the range usually has a "buzzer" associated with it for audible continuity readings (i.e., wire tracing, testing fuses).

The diode function on a digital multimeter is part of the Ohms function. This function doesn't autorange. It provides a constant current to forward bias the diode under test and displays the voltage drop. You should measure about .5 to .6 volts for a standard silicon diode. Other junctions will read higher or lower depending on the construction of the device.