How to Tell Which Way Round a Diode Should Be
Friday, October 2, 2009
, Posted by Admin at 12:56 AM
A diode is a two terminal electronic device which conducts current in one direction and blocks current in the opposite direction. A diode is also called a rectifier. Rectifiers convert AC to DC.
An electrical engineer views current as abstract positive charges; physicists and technicians view current as physical negative charges (electrons) moving in the opposite direction of the engineer's positive charges. A diode permits the engineer's charges to flow in the direction of the arrow, while the actual electrons flow in the opposite direction, and are blocked in the direction of the arrow.
The lead connected to the arrow is called the anode (the sink for electrons) and the lead connected to the "T" is called the cathode (the source of electrons). These same conventions are applied in the schematic symbols for the various transistors.
Steps :
- There are two methods for determining the direction of current flow in a diode: 1) examine markings on the device, and 2) test the device with an ohmmeter.
- Method 1
- Small diodes are cylinders that look like resistors except they are encapsulated in black plastic or glass. There is a white band on one end of a black plastic diode, and a black band on one end of a glass diode.
- An engineer's positive current (which is also the layman's conception of current) flows from the terminal farthest from the band to the terminal closest to the band, and is blocked in the opposite direction. The band corresponds to the crossbar on the "T" in the schematic symbol.
- Larger rectifiers come in plastic TO-220 packages or are packaged as threaded studs. Both packages may be easily mounted on heat sinks.
- On a TO-220 package, the two outside leads are marked with the symbols "+" and "-" molded into the plastic. Positive current flows from the "+" lead to the "-" lead. The center lead may be absent, or it may be connected to the metal tab which screws to the heat sink. The heat sink tab may be electrically isolated from the terminals, or it may be electrically bonded to one of them. Which is the case may be determined by examining the manufacturer's data sheet (search for the part # online) or by measurement with an ohmmeter. The bonded lead will allow current to flow both to and from the tab, while the other lead imposes the diode between the lead and the tab.
- A stud rectifier usually has the schematic symbol for a diode printed on its body. Most often, the stud's threaded end is the cathode and the solder terminal is the anode.
- Method 2
- Set up your meter for reading resistance.
- Attach the red (positive) lead to one lead of the diode.
- Attach the black (common, or ground, or negative) lead to the other lead of the diode.
- If the reading is some fraction of an ohm, then the red lead is attached to the anode and the black lead is attached to the cathode. If the reading is larger than a few hundred kilo-ohms, then the red lead is attached to the cathode, and the black lead is attached to the anode.
- Reverse the ohmmeter leads. The reading should be the opposite of the reading in step 4).
- Most modern rectifiers will show a reverse impedance of many mega-ohms. Older selenium rectifiers may have a sub mega-ohm reverse impedance.
- If the measurements do not fall in the ranges given, the device is either not a diode, or it is defective. Any modern rectifier with a reverse impedance of less than a mega-ohm is defective.
- When the reverse leakage current is extremely low (the reverse impedance extremely high) the reading some meters show may either drift about, or indicate an open circuit.
- The forward conducting impedance may drift about. A diode is a non-linear device, and this drifting is just the diode and the meter electronics looking for a mutually stable operating point. In any case, the reading will be low.
- There are many specialty diodes which may give peculiar measurements. These include zener diodes, gunn diodes, varactor diodes, and schottky barrier diodes.
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