Friday, July 30, 2010

How to test capacitors

Our latest videos, just uploaded to YouTube, a two part series on how to test capacitors. Enjoy.








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Thursday, July 29, 2010

Philips 23PF5320 LCD TV repair

Presenting Preher-Tech's latest videos from YouTube. In these two videos we do a live repair on a broken Philips 23PF5320 LCD TV.











Don't forget to check out this 195 page best seller from Preher-Tech, Only $25.00.

"Troubleshooting and Repairing LCD TVs"


Sunday, July 18, 2010

Tuesday, July 13, 2010

FPD2275W , turns on with black screen, repaired.



A Gateway FPD2275W was dropped of at the shop with the complaint of no video just a black screen, started out with the video going dim until eventually the monitor turned on with the black screen only. Immediately I suspect the PSU or inverter board. After opening the monitor which is usually the most difficult part of repairing LCD monitors, I could immediately see a puffed and vented electrolytic capacitor with a value of 1000uF @25 volts, location C862 on the voltage supply line to the inverter circuitry. I checked the rest of the capacitors on the PSU/Inverter board with an ESR meter and they were all well within tolerance. I did find some questionable solder connections while looking over the board with my optic visors and re-soldered them. After replacing the capacitor and reassembling the monitor, it worked great once again.

Interested in learning more about electronics and LCD repair? Check out these top selling e-books from Preher-Tech electronics.




Varistors



Do you know what a varistor is? If you work on power supplies you have surely noticed them used for transient voltage suppression normally located next to the fuse and before the EMI filter starts connected across the line and neutral. Read the following article to learn more about varistors and MOV(Metal Oxide Varistors).


Varistor


Schematic symbol

A varistor is an electronic component with a significant nonlinear current–voltage characteristic. The name is a portmanteau of variable resistor. Varistors are often used to protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from the sensitive components. A varistor is also known as Voltage Dependent Resistor or VDR. A varistor’s function is to conduct significantly increased current when voltage is excessive.

Note: only non-ohmic variable resistors are usually called varistors. Other, ohmic types of variable resistor include the potentiometer and the rheostat.



Metal oxide varistor
The most common type of varistor is the Metal Oxide Varistor (MOV). This contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and a large current flows. The result of this behaviour is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

Follow-through current as a result of a lightning strike may generate excessive current that permanently damages a varistor. In general, the primary case of varistor breakdown is localized heating caused as an effect of thermal runaway. This is due to a lack of conformality in individual grain-boundary junctions, which leads to the failure of dominant current paths under thermal stress.

Varistors can absorb part of a surge. How much effect this has on risk to connected equipment depends on the equipment and details of the selected varistor. Varistors do not absorb a significant percentage of a lightning strike, as energy that must be conducted elsewhere is many orders of magnitude greater than what is absorbed by the small device.

A varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its "clamping voltage". If a transient pulse (often measured in joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This (catastrophic) failure occurs when "Absolute Maximum Ratings" in manufacturer's datasheet are significantly exceeded. Varistor degradation is defined by manufacturer's life expectancy charts using curves that relate current, time, and number of transient pulses. A varistor fully degrades typically when its "clamping voltage" has changed by 10%. A fully degraded varistor remains functional (no catastrophic failure) and is not visibly damaged.

Ballpark number for varistor life expectancy is its energy rating. As MOV joules increase, the number of transient pulses increases and the "clamping voltage" during each transient decreases. The purpose of this shunt mode device is to divert a transient so that pulse energy will be dissipated elsewhere. Some energy is also absorbed by the varistor because a varistor is not a perfect conductor. Less energy is absorbed by a varistor, the varistor is more conductive, and its life expectancy increases exponentially as varistor energy rating is increased. Catastrophic failure can be avoided by significantly increasing varistor energy ratings either by using a varistor of higher joules or by connecting more of these shunt mode devices in parallel.

Important parameters are a varistor's energy rating (in joules), response time (how long it takes the varistor to break down), maximum current and a well-defined breakdown (clamping) voltage. Energy rating is often defined using 'industry standard' transients such as 8/20 microseconds or 10/1000 microseconds. MOVs are intended for shunting short duration pulses. For example, 8 microseconds is a transient's rise time; 20 microseconds is the fall time.

To protect communications lines (such as telephone lines) transient suppression devices such as 3 mil carbon blocks (IEEE C62.32), ultra-low capacitance varistors or avalanche diodes are used. For higher frequencies such as radio communication equipment, a gas discharge tube (GDT) may be utilized.

A typical surge protector power strip is built using MOVs. A cheapest kind may use just one varistor, from hot (live, active) to neutral. A better protector would contain at least three varistors; one across each of the three pairs of conductors (hot-neutral, hot-ground, neutral-ground). A power strip protector in the United States should have a UL1449 2nd edition approval so that catastrophic MOV failure would not create a fire hazard.


Continue reading more about varistors by clicking here.

Monday, July 12, 2010

Simple Home Made FM Radio Receiver





Hello Everyone, thought you all would enjoy this simple FM radio receiver I made recently. This circuit is simple and works well.

Find more info on the basis for this circuit and the "one transistor radio" from Andy Mitz on his website by clicking here.




Click image for expanded view.

Parts List:

C2, 3-36pF variable capacitor
C3, 1nF ceramic disc capacitor
C4, 20pF ceramic disc capacitor
C5, 4.7nF ceramic disc capacitor
C6, 220nF polyester film capacitor
C7, 2.2nF ceramic disc capacitor
C8, 220uF electrolytic capacitor
C9, 100uF electrolytic capacitor
L1, see note**
L2, see note**
L3, 24uH choke, see note**
R1 and R8, 1k ohm carbon film resistor 1/4 Watt
R2, 10k ohm carbon film resistor 1/4 Watt
R3, 15k ohm carbon film resistor 1/4 Watt
R4 and R5, 470k ohm carbon film resistors 1/4 Watt
R6, 180 ohm carbon film resistor 1/4 Watt
R7, 34 ohm carbon fil resistor 1/4 Watt
Q1, MPF102 N channel JFET
IC1, 431AZ adjustable precision shunt regulator
D1, 1N4001 general purpose rectifier diode
D2, 3mm green LED
SPK1, small speaker or head phones
SW1, SPST switch
note**:
L1 and L2 were made by wrapping three tight side by side loops around a sharpie marker, I also made L2 by wrapping about 8 inches of 35AWG magnet wire around a ferrite core, careful to make the windings side by side and glued it in place with a hot glue gun, you can use any 20-30 uH RF choke though, you don't have to make your own.
C1 was removed from the circuit after doing some experimenting so that is why the capacitors part numbers start at C2.

Download a wonderful 269 page PDF on wave propagation, transmission lines and antennas by clicking here.


You may also be interested in our LCD TV Repair Guide. Click here to read more.