Diode Testing Using Ohmmeter

1. When an ohmmeter is used to test a diode, a very low resistance (but not zero) in one direction means that?

the diode is a. open b. forward c. shorted d. reversed bias
2. for which region the diode curve look like an open circuit
a. diode voltage less thatn the barrier potential
b. diode voltage greater than the barrier potential
3. for the region of the diode curve greater than the diode's barrier potential
a. the curve is essentially horizontal
b. the diode forward resistance approaches an open circuit
c. the diode voltage increases rapidly
d. the diode current increases

1/ b) forward biased diode appears as a low resistance
2/ a) when the diode voltage is less than the barrier voltage, the diode doesn't conduct (appears as an open circuit
3/ d) After the barrier voltage is exceeded, the diode behaves like closed switch - it's current increases dramatically.

Think about the I-V characteristic curve for a diode. At voltages close to zero, the current is zero. At some voltage (.5V - 2.2V depending on the device), the current will increase until it appears to be a sloped line.
Of course, when voltage increases beyond the device's maximum rating, the diode will explode and current will drop to zero. To demonstrate, try putting 120V across a 1n... diode , but be sure to wear safety glasses.

Ohmmeter $70.1 High Quality Content by WIKIPEDIA articles An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current. Microohmmeters (microhmmeter or microohmmeter) make low resistance measurements. Megohmmeters (aka megaohmmeter or in the case of a trademarked device Megger) measure large values of resistance. The unit of measurement for resistance is ohms ( ). Author: Surhone, Lambert M./ Tennoe, Mariam T./ Henssonow, Susan F. Binding Type: Paperback Number of Pages: 88 Publication Date: 2010/08/15 Language: English Dimensions: 6.00 x 9.02 x 0.21 inches

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Laser Diode Microsystems $301.11 Laser Diode Microsystems provides the reader with the basic knowledge and understanding required for using semiconductor laser diodes in optical microsystems and microoptical electromechanic systems. This tutorial addresses the fundamentals of semiconductor laser operation and design, coupled with an overview of the types of laser diodes suitable for use in Microsystems, along with their distinguishing characteristics. Emphasis is placed on laser diode characterization and measurement as well as the assembly techniques and optical accessories required for incorporation of semiconductor lasers into complex microsystems. Equipped with typical results and calculation examples, this handon text helps readers to develop a feel for how to choose a laser diode, characterize it and incorporate it into a microsystem. Author: Zappe, Hans P./ Zappe, H. Series Title: Microtechnology and Mems (Hardcover) Binding Type: Hardcover Number of Pages: 355 Publication Date: 2003/10/10 Language: English Dimensions: 9.21 x 6.14 x 0.81 inches

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Testing Solenoids and Alternators with a Compass

Apple A1021 AC Adapter Supply Outputs

Depending on the Apple ac adapter type of equipment, there may be a single output of several outputs from the Apple A1021 AC Adapter (http://www.usadapter.com/Laptop-Apple-Adapter/Apple-A1021-AC-Adapter.html ) supply. A failure of one of these can result in multiple systems problems depending on what parts of the equipment use what supply. Check for bad fuses in the secondary circuits - test with an ohmmeter. Try a new fuse of the same ratings. If this one blows immediately, there is a fault in the laptop ac adapter supply or one of its loads. See the section: About Fuses, IC Protectors, and Circuit Breakers. The use of a series current limiting resistor - a low wattage light bulb, for example - may be useful to allow you to make measurements without undo risk of damage and an unlimited supply of fuses.
Dried up electrolytic capacitors will result in excessive ripple leading to hum or reduced headroom in audio outputs and possible regulation problems as well. Test with a scope or multimeter on its Apple A1021 AC Adapter scale (but not all multimeters have DC blocking capacitors on its laptop AC adapter input and these readings may be confused by the DC level). If ripple is excessive - as a guideline if it is more than 10 to 20% of the DC level - then substitute or jumper across with a good capacitor of similar uF rating and at least the same Apple ac adapter voltage rating.
Once the Apple A1021 AC Adapter line input and primary circuits have been found to be good (or at have continuity and a resistance) that is reasonable, the problems is most likely in the secondary side - fuses, rectifiers, filter capacitors, regulator laptop ac adapter components, bad connections, excess load due to electronic problems elsewhere.
Locate the large electrolytic filter capacitor(s). These will probably be near the Apple A1021 AC Adapter power transformer connections to the circuit board with the power supply components. Test for voltage across each of these with power on. If they are in pairs, this may be a dual polarity supply (+/-, very common in audio equipment). Sometimes, two or more capacitors are simply used to provide a higher uF rating. If you find no Apple A1021 AC Adapter voltage on one of these capacitors, trace back to determine if the problem is a rectifier diode, bad connection, or bad secondary winding on the Apple ac adapter power transformer.

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