Here’s an usual sight. This meter shows a DIODE beside the 2K ohm resistance range. However, there is no “select” to switch between the two measures.
It just so happens these features are combined into one setting!
If you give measure either resistor or a diode in this mode, you will read on the screen either resistance in kiloohm, or the voltage drop across the diode junction.
Is it magic?
If you remember in the last blog, a digital multimeter measures resistance by producing a constant current through the test leads, causing a voltage drop across the resistor. The meter then internally calculates the ratio of voltage drop to supplied current, because ohms law states R=V/I.
The same method is used in the diode mode. A low current is sent out through the leads, and the diode junction responds with a voltage drop across the diode’s P-N junction. The meter reports that voltage.
With just the right current, the universe of the resistor aligns perfectly with the universe of the diode. That current happens to be 1 mA (or 0.001 A). This is similar to that special -40 degrees which is the same for both Celsius and Fahrenheit.
Example
If you are measuring a 510 ohm resistor in the 2K range of this multimeter, it will report “.510” on the screen. Then switching leads to a silicon diode, forward biased, it may read “.510” which is the diode voltage drop. In either case, the multimeter measures the 0.51 volt drop across the input jacks in response to 1 mA delivered. The multimeter doesn't care if it is a resistor or diode!
Some people prefer to troubleshoot a circuit without constantly switching between resistance and diode mode. This type of multimeter might be a suitable option. However, this one doesn’t combine a continuity beep with the result, which is also a typical wish.
Can the 2K range always be used?
If the meter doesn't have a diode printed beside an Ohms range, it may or may not work as well as one that does. First of all, the voltage the multimeter provides must be higher than the P-N forward bias voltage of the diode you are checking.
Further, the value on the screen might be less sensible. The voltage drop across the diode in resistance mode might be less accurate because the output current is not guaranteed to be 1 mA. You'll just have to give it a try with your meters to see what happens.
Have fun in your testing universe!
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