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 Connect Diodes

How to Connect Diodes

Diodes are semiconductor materials that behave like switches. They are open to electricity in one direction and short-circuited in the other, meaning that current is allowed to flow in only one direction. Silicon and germanium are two materials they are often made from.

The symbol for a diode is an arrow to show the direction current flow is allowed. A diode placed in a circuit this way is known as forward-biased. A diode with current flow in the wrong direction is called reverse-biased. Ordinary reverse-biased diodes will finally allow current when there is too much current flowing the wrong way. When this happens, they are destroyed.

Things You'll Need

Silicon diode, such as a 1N4001

Resistor

Multimeter

Breadboard

Jumper wires

Battery

Instructions

Measure the forward-biased voltage on the diode by placing the multimeter on the diode setting. Put the positive or red lead on the anode side of the diode, which is unmarked. Place the negative or black lead on the cathode side of the diode, which is marked by a stripe. An undamaged silicon diode will read 0.5 to 0.7 when it is forward-biased.

 Forward-bias measurement.

Measure the reverse-biased voltage on the diode by switching the multimeter probes. Place the positive or red lead on the marked or cathode side of the diode. Place the negative or black lead on the unmarked or anode side of the diode. You should get a reading indicating an overload or no current flow.

 Reverse-bias measurement.

Attach a 3- to 9-volt battery to the circuit board. Add a 330 ohm to 1k ohm or more resistor to the positive lead. Connect the anode side of the diode to the free end of the resistor and the other end to the negative terminal of the battery. Note that the diode is in series with the battery and resistor.

 A forward-biased circuit.

Measure the voltage across the resistor by placing the multimeter on a low DC voltage setting, and by placing a lead across each side of the resistor.

Calculate the current in the circuit using Ohm's Law I = V/R, where V is the voltage from the battery, I is the current, and R is the value of the resistor. This mathematical model assumes the diode is ideal and behaves like a switch, so that the load is only across the resistor.

Measure the voltage across the diode. Note that the diode behaves like a 0.7 V battery.

Why Do I Need a Blocking Diode on My Solar Panel?

Why Do I Need a Blocking Diode on My Solar Panel?

Blocking diodes in photovoltaic systems serve two functions. They can prevent batteries from draining at night, and can isolate damaged or under-performing solar cells. The latter can be solved with either a blocking or a bypassing diode.

Function
A diode is a semiconductor device, a one-way valve for electricity. Diodes are rated in the maximum current they can pass through while on and the maximum voltage they can block when off. When the correct conditions are met, the diode will stop current from flowing.

Shaded Panels
Solar panels contain several solar cells wired together in series. If part of the panel is shaded, all of the cells become less efficient.

Bypass Diodes
A blocking diode in the panel "averages out" any shaded cells. The diode allows the loss of voltage from a shaded cell to be shared among the others, so that the panel functions more normally.

Battery Drain
At night, dusk and dawn, the battery outputs a higher voltage than the solar cells do. Without a blocking diode, the battery will send power to the panels instead of the other way round. Over time, this can drain the batteries.

Blocking Diodes
When the battery voltage is higher than that of the solar panel, a blocking diode shuts off flow. During the day, the blocking diode can also prevent drain from the battery caused by damaged cells. In a function similar to bypass diodes, the bad cells are isolated from the others.