This page provides some basic information on testing semiconductor junctions using the diode setting on digital multimeters (DVM). Only a few silicon devices will be tested here.
Ohmmeters are not really suitable for testing semiconductor junctions but they will indicate if a diode, for example, is conducting or not.
Two, inexpensive, digital multimeters were used for the data on this page. Meter #1 has a 3 volt battery supply and Meter #2 has a 9 volt battery supply.
The 9 volt supplied meter can indicate junction voltages a bit higher higher than the meter with the 3 volt supply. Other meters may be able to indicate even higher junction voltages.
For checking devices with higher junction voltages, such as many types of LEDs, a circuit like the equivalent circuits above could be made with the DVM being used as a voltmeter.
The device tested in the example below was a 1N4002 rectifier, 1 amp diode. The results can be applied to most types of general purpose diodes.
Other diode types, such as Schottky or LEDs, will have different forward bias voltages. A meter with a low test voltage such as Meter #1 can't be used to test many types of LEDs.
Many general purpose silicon diodes have a stated forward bias voltage of 0.7 volts. This voltage is at the rated full load current for the device and at a specified temperature. For testing and in many actual circuits these conditions are not met so the voltages measured are lower.
If a diode indicates zero volts or a very low voltage when forward biased, the diode is likely shorted and should not be used.
When a diode is reverse biased (voltage reversed), the meter should indicate an open circuit voltage. If a lower voltage is observed, the diode is damaged and should not be used.
All semiconductors are sensitive to temperature. In most cases the junction voltage will decrease as its temperature increases. (Diode junction voltage is used to measure temperatures in in some thermometer circuits.)
All semiconductors junctions are sensitive to light in one wavelength region or another, some visible, some infrared and some ultraviolet. In phototransistors, the junctions are not shielded from light and will change their conductance's based on the amount of light falling on the junction.
Electrically, phototransistors are actual bipolar transistors but in many devices, like those in "T" style packages, the base junction does not have an external connection and is ignored. Some phototransistors do have an accessible base terminal but they are not often used.
Some phototransistors have packages that block or pass certain wavelengths to limit their sensitivity in various light conditions. Dark coloured packages are often used to block visible light while passing infrared light.
Phototransistor sensitivity in actual circuits is also a function of the current flowing through them. The test current supplied from the multimeter is typically much larger that the current in the actual circuit. (770uA for Old Faithful versus approximately 25uA for the 8 Photo-Detector Circuits at this site.)
The greater the current flowing through the phototransistor, the less sensitive the device will appear to be. At higher currents, the amount of light needed to produce a specified voltage drop across a transistor will be greater than at low current flows.
Phototransistor are sensitive to light, obviously, but the light can vary in intensity and wavelength(s), therefore, testing phototransistors using the diode setting can only be used to indicate that a device is functioning electrically. Measuring the actual parameters of a photosensitive devices requires much more sophisticated techniques than are presented here.
Reverse biasing a phototransistor will indicate the same as a reversed diode.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.
06 March, 2019