Question
Using a multimeter to measure the voltage between T1 and T2 in the circuit below, what reading will you get?
Background
Hardware engineers should pay attention to details in circuit design. Even a very simple circuit can produce confusing results if details are overlooked. In the circuit above, several colleagues gave different answers about what voltage would be measured between T1 and T2: some said the diode is reverse-biased so nothing would be measurable, some said 5V, and others were uncertain.
Principle and Measurement Setup
Measuring instrument: multimeter UT61E. According to its datasheet, the input impedance on the DC voltage range is 10 MΩ.
With the input impedance known, the circuit can be modeled as the following equivalent:
Assume R1 has 5V across it. The current through R1 is then 0.5 uA. In other words, the diode D1 must have a reverse leakage current of at least 0.5 uA to allow the multimeter to read 5V. If D1's reverse leakage is below 0.5 uA, the meter will not read 5V.
Measurements
Example 1: WSR4003AL (SMA package)
Using the UT61E, a 5V reading is obtained. This device is a Schottky diode with low forward drop and is a relatively high-power diode rated for up to 3A. Its reverse leakage current is relatively large (see marked region in the datasheet).
Example 2: 1N4148WS (SOD-323 package)
Using the UT61E, the meter does not show 5V; the display reads below 0.1V. This is a common small-signal PN diode used for high-frequency switching. Its reverse leakage is very small: 25 nA at 20V, which is far below 0.5 uA, so it cannot supply the meter the needed current to display 5V.
Conclusion
Whether a multimeter reads 5V between T1 and T2 depends on the diode type, specifically its reverse leakage current. Designers often focus on forward current, reverse breakdown voltage, and forward drop, but may overlook reverse leakage current, which can determine what a high-impedance meter will read in this kind of circuit.
Practical Implication
Consider a common dual-power arrangement: VCC_3V supplied by an LDO, and a backup coin cell. When the system is powered, VCC_3V supplies the load; when the system is off, the coin cell supplies the load. If the selected diode has significant reverse leakage, that leakage can discharge the coin cell during the off state.
