# Questions about measuring Shottky diode...

#### be80be

Joined Jul 5, 2008
2,070
I figured this was put to bed LOL

Are you not using a DMM now some meters are backward on ohms too some are not.

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#### ebp

Joined Feb 8, 2018
2,332
Almost all simple power semiconductors (diodes, transistors, SCRs, triacs, FETs, etc) fail short-circuit, at least at first. If there is enough energy available, the short may then lead to the device going open-circuit, but that isn't too common except in power circuits. Often if the part shorts then goes open circuit due to high current the package will be visibly damaged - broken in two for small axial diodes, the front cracked or even blown open for bigger plastic packaged devices. I've blown lots of TO-247 packages apart in doing power supply design work.
A "short" won't necessarily be zero ohms, but usually the resistance isn't more than a few ohms, and it will be the same no matter what the polarity of the applied test voltage or current from a meter is.

Some semiconductors do degrade in performance over time if the circuit mistreats them. For example, the current gain of a bipolar transistor will drop if the base-emitter junction is subjected to repeated reverse breakdown voltage but the current is low enough that gross failure does not occur. This isn't something easily detectable with just a meter. There are additional failures other than shorts, but they are much less common.

You can make a very simple tester for diodes that are out of circuit. Connect a 1.5 volt dry cell in series with a 1 k ohm resistor and the diode. Measure the voltage across the resistor with any meter - even a "20 000 ohms per volt" simple VOM will do. The voltage is of course one volt per milliamp of current through the diode and the resistor, remembering that the actual resistance may be a bit less due to the meter's resistance in parallel with the 1 k resistor. In one direction there will be about 1 volt across the resistor for a schottky diode with forward bias, about 0.8 V for a silicon PN junction diode. With the opposite polarity on the diode the voltage across the resistor should be zero or just a tiny bit above for a big Schottky diode. If you have a meter with high input resistance you can measure the voltage across the diode (you can do it with any meter, but you now really must consider what conduction through the meter is doing). You could also measure the current in the circuit. Obviously there are all sorts of variations - higher test voltage, higher or lower test current, etc.

Power Schottky diodes tend to have fairly high reverse leakage current. The MBR20150 (very high reverse voltage rating for a silicon schottky!) is specified for reverse leakage current of typically around 20-30 µA at a few volts at 25°C (see curves in Diodes Inc datasheet). When measuring reverse-biased resistance don't expect the resistance to be "infinite." (Reverse leakage in small Schottky diodes can be an issue if they are used in precision circuitry, but then so can the leakage of most fast small PN junction diodes.)

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"Analogue meters are counter intuitive"
Analogue meters don't have counters.