How to check diodes with a multimeter

Tuesday, May 13, 2014

Trouble Junction

Q. In my diode lab lesson it says to use the multimeter to check the diodes. It says that the IN4149 diode should measure 16 ohms when it is forward biased and when it is reverse biased. It also says the IN34 diode should read 80 ohms and the 1N4002 should read 15 ohms when they are forward biased. I read infinity both ways for all the diodes. Are all my diodes bad?

A. No, you diodes are probably all good. This happens with a digital meter. A digital meter works on a different principle than an analog meter, which is what we used when preparing this lesson. To measure resistance, both kinds of meters send a test current through the resistance you are measuring. In an analog meter, the meter movement basically displays the resistance according to how much current flows through it. (See figure below.)

ohms circuit

 The test current comes from the battery inside the meter. In a digital meter, a current regulator sends a fixed amount of current through the resistance being measured. The meter then measures the voltage across the unknown resistance to display its value. From Ohm’s Law, E=IR. If the meter uses a test current of, say, lmA, then a 100 ohm resistor will measure 0.100 V, a 1000 ohm resistor will read 1.000 V, a 3900 ohm resistor 3.900 V, and so on.

All the meter maker has to do is move the decimal point in the display, so instead of showing the voltage (which it is really measuring) the resistance value is there instead. Different ranges are obtained by using different amounts of test current, such as l0mA or 100 uA, and the decimal point is moved accordingly.

The current regulators in most digital meters use very small currents. This is because the meter is usually set up for a full-scale reading of 0.2 Volts when the resistance ranges are selected. The voltages produced by the current regulators are often too small to overcome the barrier potential of the diode, and no current flows.

Your digital meter has a special setting for checking PN semiconductor junctions. On most meters this is indicated by a diode symbol. This range uses enough voltage to overcome the barrier potential and turn the junction on when forward biased. It will probably read the voltage across the junction (about 0.6 V) instead of a resistance value. This is because semiconductor junctions are non-linear resistances. Their resistance varies according to the test current. Your digital meter uses a different test current than the analog meter around which the lesson is written. It is also calibrated to read resistance values differently. This is why your results, even on the meter’s diode check setting, will differ from those in the book.

Leave your comment