Testing Assemblies with Diodes
Test programs sometimes include instructions to assess components connected to your device-under-test. These component instructions evaluate wires, resistors, capacitors, twisted pairs, and diodes.
For wires, resistors, and capacitors, the tester will return measured values. In the case of diodes and twisted pair, the results will verify if the component does or does not exist.
While the tester uses different methods to test each component, this article will focus on diodes. How does the tester decide if a diode exists or not? How do you get the most accurate results? How do you test different types of diodes from LEDs to Zener diodes?
Diodes in General
A diode is a semiconductor device with two electrodes that allows electrical current to flow only in one direction. The electrodes are known as cathode and anode. Current will flow when the potential on the anode terminal is greater than the potential on the cathode terminal.
The potential on the anode and cathode terminals is voltage. The difference must be from about 0.3 volts to somewhere around 0.7 volts depending on the diode type. This voltage difference is referred to as the Forward Voltage and some applications are very particular about this value.
When the voltage on the cathode is more positive than that on the anode, the diode should prevent current flow or not conduct. However, there are limits. The Peak Inverse Voltage is the maximum positive voltage the diode can withstand on the cathode with respect to the anode. If this value is exceeded, the diode will fail. The Reverse Breakdown Voltage, which is a higher voltage than the Peak Inverse Voltage, will conduct and could be damaged if exceeded.
Varieties of Diodes
Diodes are used in many applications.
- A typical rectifier diode could have a Forward Voltage of about 1.1 volts and a Peak Inverse Voltage of 1000 V.
- A Schottky diode will have a very low Forward Voltage and is found in high-speed applications or applications requiring low heat dissipation.
- An LED is a diode and it behaves like one, but its primary purpose is illumination. An LED’s Forward Voltage might be high and variable while its Peak Inverse Voltage could be low.
- A Zener diode makes use of a precise Peak Inverse Voltage by allowing it to conduct when the voltage on the cathode is at or above a specific voltage known as the Zener Voltage. You will find Zener diodes in surge arrestors or transient voltage suppressor arrays.
When you are testing assembled circuits that contain diodes, the tester will look for errors. The diode could be missing. It could be backward. It could be the wrong type. It could be short to other circuits.
When Cirris Easy-Wire® executes the diode test instruction,
- The tester applies its internal current source to the test point connected to the anode, then sinks the test point connected to the cathode to the ground potential.
- A voltmeter is placed across the current source and ground to measure the voltage. This is the Forward Voltage.
- The source is moved to the cathode point and the tester sinks the anode point. The voltage is measure again with the expectation to see an open-circuit voltage.
The tested does not measure the diode; rather, the tester looks for the diode. The test focuses on what can go wrong. It can determine that the diode is present in the assembly, that it is facing the right way, that it is the right type, and that its connections are good.
Diodes and Cirris Testers
Technology has come a long way since the Signature 1000 series tester. To the 1000 series testers, a diode was a device that had somewhere between 0.5 and 1.0 Forward Voltage and open circuit reverse. The more current stand-alone Signature 1100 series can do little more.
Easy-Wire software brought the ability to set limits on the Forward and Reverse Voltage; however, the level of current required for an accurate test of the Forward Voltage is often more than we can safely apply. While the test conditions are not such that you could support a corrective action on the supplier for a bad diode, it is enough to flag a questionable assembly and pass a good assembly.
While LEDs are technically diodes, they do not meet the diode model programmed into the stand-alone Signature 1000 and Signature 1100 testers. Often the forward voltage of an LED is as high as 2.5 volts while the reverse is a high resistance but somewhat short of an open circuit.
If Easy-Wire software is controlling the Signature 1100 series, the tester can test LEDs since the expected forward and reverse voltage limits are user-defined. Without Easy-Wire, the stand-alone models can be made to verify LEDs using a custom component script when the tester is equipped with the optional scripting SCPT-1R or SCPT-1H. With a script, the tester can even light the LED.
The proper operation of a circuit containing a Zener diode depends on everything verified about a standard diode plus a measurement of the Zener voltage.
When the Zener voltage is specified to be less than 4.5 volts, your Easy-Wire controlled benchtop tester will test this as a diode instruction with the Zener voltage specification set as the reverse voltage. Keep in mind that the Zener voltage is a function of test current applied. While the test conditions could not support a corrective action on the supplier for a bad Zener diode, it is enough to flag a questionable assembly and pass a good assembly.
For Zener voltage above 30 volts, your Cirris benchtop high voltage analyzer can prove that the right diode is in place. Under control of a custom component script, the tester uses its high voltage supply to verify the Zener voltage.
Suppose the Zener voltage specification is 35 to 38 volts. This means that with less than 35 volts on the cathode, the diode must not conduct. With greater than 38 volts, the diode must conduct. The script is programmed to establish 34 volts as the must isolate voltage and 39 volts as the must conduct voltage. Using the high voltage supply of the tester, the script places the must isolate voltage on the cathode and measures the current flow. If the current is below the user-defined threshold, you will see a pass test flag. If the current is too high, you will see a fail test flag.
Next, the script applies the must conduct voltage level to the cathode and again measures the current through the supply. If the current is too low, you will see the fail test flag which means the diode is not conducting. If the current is high, you will see the pass test flag which means the diode is conducting.
When both tests produce pass test flags, the diode has passed the test. This script can also test reverse leakage current.
The maximum current through the high voltage supply is 6.5 mA, so make certain that the diode’s maximum reverse current specification is greater than 6.5 mA before applying this method.
Again, this test method will not win any arguments with the diode supplier, but it will let you know that the diode is not reversed, it is not leaking, and that the diode bin in the stock room is not contaminated with mixed parts.
Precision Zener Voltage
The Cirris CH2 tester has a unique architecture and powerful software for controlling external instruments. While external instruments alone are not suited for multiconductor tests, the CH2 can perform the necessary tests.
Connect a Keithley Source Meter to the Cirris CH2 test point matrix. Easy-Wire software will switch each circuit to the source meter and report the desired measurements. Source meters can apply precise currents and measure voltages that range from millivolts to hundreds of volts. With the external precision current source, you have an industry-accepted test method for Zener diodes.