Meter check of a diode
Being able to determine the polarity
(cathode versus anode) and basic functionality of a diode is a very
important skill for the electronics hobbyist or technician to have.
Since we know that a diode is essentially nothing more than a one-way
valve for electricity, it makes sense we should be able to verify its
one-way nature using a DC (battery-powered) ohmmeter. Connected one way
across the diode, the meter should show a very low resistance. Connected
the other way across the diode, it should show a very high resistance ("OL"
on some digital meter models):
Of course, in order to determine which
end of the diode is the cathode and which is the anode, you must know
with certainty which test lead of the meter is positive (+) and which is
negative (-) when set to the "resistance" or "Ω" function. With most
digital multimeters I've seen, the red lead becomes positive and the
black lead negative when set to measure resistance, in accordance with
standard electronics color-code convention. However, this is not
guaranteed for all meters. Many analog multimeters, for example,
actually make their black leads positive (+) and their red leads
negative (-) when switched to the "resistance" function, because it is
easier to manufacture it that way!
One problem with using an ohmmeter to
check a diode is that the readings obtained only have qualitative value,
not quantitative. In other words, an ohmmeter only tells you which way
the diode conducts; the low-value resistance indication obtained while
conducting is useless. If an ohmmeter shows a value of "1.73 ohms" while
forward-biasing a diode, that figure of 1.73 Ω doesn't represent any
real-world quantity useful to us as technicians or circuit designers. It
neither represents the forward voltage drop nor any "bulk" resistance in
the semiconductor material of the diode itself, but rather is a figure
dependent upon both quantities and will vary substantially with the
particular ohmmeter used to take the reading.
For this reason, some digital multimeter
manufacturers equip their meters with a special "diode check" function
which displays the actual forward voltage drop of the diode in volts,
rather than a "resistance" figure in ohms. These meters work by forcing
a small current through the diode and measuring the voltage dropped
between the two test leads:
The forward voltage reading obtained with
such a meter will typically be less than the "normal" drop of 0.7 volts
for silicon and 0.3 volts for germanium, because the current provided by
the meter is of trivial proportions. If a multimeter with diode-check
function isn't available, or you would like to measure a diode's forward
voltage drop at some non-trivial current, the following circuit may be
constructed using nothing but a battery, resistor, and a normal
voltmeter:
Connecting the diode backwards to this
testing circuit will simply result in the voltmeter indicating the full
voltage of the battery.
If this circuit were designed so as to
provide a constant or nearly constant current through the diode despite
changes in forward voltage drop, it could be used as the basis of a
temperature-measurement instrument, the voltage measured across the
diode being inversely proportional to diode junction temperature. Of
course, diode current should be kept to a minimum to avoid self-heating
(the diode dissipating substantial amounts of heat energy), which would
interfere with temperature measurement.
Beware that some digital multimeters
equipped with a "diode check" function may output a very low test
voltage (less than 0.3 volts) when set to the regular "resistance" (Ω)
function: too low to fully collapse the depletion region of a PN
junction. The philosophy here is that the "diode check" function is to
be used for testing semiconductor devices, and the "resistance" function
for anything else. By using a very low test voltage to measure
resistance, it is easier for a technician to measure the resistance of
non-semiconductor components connected to semiconductor components,
since the semiconductor component junctions will not become
forward-biased with such low voltages.
Consider the example of a resistor and
diode connected in parallel, soldered in place on a printed circuit
board (PCB). Normally, one would have to unsolder the resistor from the
circuit (disconnect it from all other components) before being able to
measure its resistance, otherwise any parallel-connected components
would affect the reading obtained. However, using a multimeter that
outputs a very low test voltage to the probes in the "resistance"
function mode, the diode's PN junction will not have enough voltage
impressed across it to become forward-biased, and as such will pass
negligible current. Consequently, the meter "sees" the diode as an open
(no continuity), and only registers the resistor's resistance:
If such an ohmmeter were used to test a
diode, it would indicate a very high resistance (many mega-ohms) even if
connected to the diode in the "correct" (forward-biased) direction:
Reverse voltage strength of a diode is
not as easily tested, because exceeding a normal diode's PIV usually
results in destruction of the diode. There are special types of diodes,
though, which are designed to "break down" in reverse-bias mode without
damage (called Zener diodes), and they are best tested with the
same type of voltage source / resistor / voltmeter circuit, provided
that the voltage source is of high enough value to force the diode into
its breakdown region. More on this subject in a later section of this
chapter.
- REVIEW:
- An ohmmeter may be used to
qualitatively check diode function. There should be low resistance
measured one way and very high resistance measured the other way. When
using an ohmmeter for this purpose, be sure you know which test lead
is positive and which is negative! The actual polarity may not follow
the colors of the leads as you might expect, depending on the
particular design of meter.
- Some multimeters provide a "diode
check" function that displays the actual forward voltage of the diode
when it's conducting current. Such meters typically indicate a
slightly lower forward voltage than what is "nominal" for a diode, due
to the very small amount of current used during the check.
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