How to Check if a Diode Is Bad

How to Check if a Diode Is Bad

Diodes are semiconductor devices that conduct current in one direction only, and are commonly made from silicon or germanium. Diodes have two terminals -- an anode and a cathode -- with the cathode being marked by a line painted on the body of the diode. Current is allowed to flow from the anode to the cathode, but is blocked in the other direction. This property is used most commonly in rectifier circuits, which change alternating current to direct current. Diodes are also used to protect components in a circuit if the power is connected the wrong way around, blocking the flow of current to stop damage being caused. Although diodes rarely fail, it can happen if they are exposed to voltage or current above their rated limits.

Things You'll Need
Digital multimeter
Soldering iron
Desoldering braid
Pliers


Instructions
Unsolder one leg of the diode if it is part of a circuit, otherwise the other components in the circuit may affect the results of the test. Heat the solder pad around the diode leg until the solder is molten, and then gently pull the leg through from the other side using pliers. Soak up any excess solder with desoldering braid, leaving the hole clear.

Set the multimeter to its diode test mode, denoted by the circuit symbol for a diode which looks similar to an arrow. If the multimeter does not have a diode mode, set it to the lower end of the resistance range.

Place one probe from the multimeter on to one of the diodes legs, and the other probe on to the other leg. Note the reading obtained, and then swap the positions of the probes and note the new reading.

Interpret the results. If you get an open circuit in one direction indicating the current is blocked, and a low resistance reading in the other direction, the diode is good. If there is an open circuit in both directions, the diode has failed with an open circuit. If there is a low resistance in both directions, the diode has failed with a short. In both cases the diode should be replaced.

How to Identify Diode Markings

How to Identify Diode Markings

Diodes are electronic components that permit the flow of electric current in one direction only, shutting current off when it reverses. These handy devices have dozens of important uses in circuits, including as power supplies and radio signal detectors. Because diodes have polarity, their packaging carries distinct markings that help you connect them correctly in a circuit.

Signal Diodes
The smallest discrete diodes are designed to handle about 100 milliamps of current. These devices are typically packaged in tiny glass cylinders with a connecting lead at each end. A stripe on the cylinder marks the diode’s cathode, making the opposite side the anode. With some diodes, you may need a magnifying glass to see the stripe clearly.

Mid-Power Diodes
Diodes used as rectifiers in small power supplies are usually rated between 1 and 5 amps of current, maximum. These components usually have a cylindrical body made of dark epoxy or ceramic with a white or silver stripe marking the cathode side.

High Power Diodes
Diodes designed to carry more than a few amps of current may get very hot during operation, so they come in standard metal packages that bolt to a heat sink for cooling. Although the manufacturer may stamp a diode symbol on the package, indicating the anode and cathode connectors, you can also determine the device’s polarity by the package itself. Component catalogs and specification sheets indicate the case type and how to connect the diode.

Surface Mount Diodes
In recent years, electronics equipment manufacturers have increasingly moved from traditional leaded components to surface-mount devices, or SMDs. They are less expensive than traditional components, lower in cost and are compatible with the high-speed, pick-and-place robots now common in electronic circuit assembly. Like their cylindrical counterparts, SMD diodes have a white stripe that marks the cathode end of the device.

Light-Emitting Diodes
Light-emitting diodes have similar electronic characteristics as plain diodes; they have a cathode and an anode, and they block current in the reverse direction. They are compact, rugged and efficient at emitting light, making them very useful in such applications as simple on-off indicators, numeric displays, video screens and room illumination. Traditional LEDs have a clear or colored epoxy dome with a flat spot indicating the cathode side. Additionally, the cathode’s lead is shorter than the anode’s. LED arrays and other complex packages follow industry standards. However, if the package is not clearly marked, you may have to look up the part’s specifications in a manufacturer’s catalog.

How to Choose the Right Diode

How to Choose the Right Diode

Diodes are electronic one-way valves, letting current flow in one direction but not the reverse. If you're designing a circuit that uses them, you'll need to know they have limits. They can handle a rated maximum current, and if you exceed this limit you'll destroy the part. They also have a reverse voltage limit, past which they will start conducting, possibly with damaging results. And diodes come in several different physical case styles, with leads or in a surface-mount (SMD) case.

Things You'll Need

Pencil and paper

Calculator

Circuit schematic

Semiconductor catalog

Instructions

Evaluate the schematic at the point where the diode will be located. Determine the maximum current that will be flowing through that point and write the figure down. Determine the maximum voltage that the diode will have to sustain. Voltages are differences in potential between two points, so if one side of the diode is at 25 volts and the other at 5 volts, 25-5 = 20 volts. Write the voltage value down.

Multiply the voltage value by 1.25 and write it down. Do the same for the current value. These will be your minimum ratings. Multiply the voltage value by 2.5 and write it down, and do the same for current. These will be your maximum ratings. The diode you use can exceed the maximum ratings if you cannot find one that's smaller than it. For example, if your maximum circuit voltage is 15 and the smallest diode value you can find in the catalog is 100 volts, it's perfectly safe to go with 100. Do not use a part rated for less than your minimum value.

Determine how you'll be building the circuit. If the diode is rated for much more than 5 amps of current, it may need to be mounted in a metal heat sink. If you're using surface-mount components, you'll be looking for that style of diode package.

Open the catalog and search through the diode section, beginning with the lowest voltage ratings that match yours, then look for current ratings that match. The catalog will list diode voltage ratings as either Peak Inverse Voltage (PIV) or Peak Reverse Voltage (PRV). It lists current ratings as Average Forward Current, Recurrent Forward Current and DC Forward Current. If the diode is rectifying 60-cycle AC, use Average Forward Current. If the diode will be handling recurring current pulses, use Recurrent Forward Current. Otherwise, use DC Forward Current to find the right diode. Make sure you select a physical package that fits your overall design in terms of leads or surface mount.