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Have you ever come across power problem where once you switch on the power supply and the fuse blow? You have checked all the components in the power (primary) and secondary section and all the components seems to be ok! Where is the fault? A fuse blown are usually caused by a shorted bridge rectifier, defective posistor, power transistor or FET, shorted primary winding of switch mode power transformer, shorted secondary diode and etc. But in this article I’m gone to show you another secret of electronic troubleshooting tips.

I got frustrated as to where is the cause of power problem. Every time when i switch on the power supply the fuse blow immediately (the fuse became dark color and this indicate that there is a major short circuit in the power supply). I have checked all the components in the power supply and can’t find the culprit! What i do is i desoldered all the suspected parts one by one and replaced with a known good component. I eventually found the caused of the power supply problem. Guess what? It was the main filter capacitor (220 microfarad 400 volt). After replacing the filter cap the power supply worked perfectly fine. I begin my detective work to find out why this capacitor can caused the fuse to blow even though i already confirmed it ok with my meters.

The meters that i used to check the filter cap were analog meter, digital capacitance meter and esr meter. In this article i will not explain about how to check capacitor or testing capacitor and how capacitor work. I believed most of you know how to check capacitors and also generally using this type of meters. Measured with analog it showed capacitor charging and discharge, with digital capacitor tester it showed around 220 microfarad and with esr meter it showed low esr reading!

This proved that the bad capacitor breakdown when under full operating voltage. Then, how do i confirm that this filter capacitor is faulty? By using an analog insulation tester. When i connect the faulty cap to the meter and press the go button-it showed a very low resistance and this is the proved of short circuit between the plate when voltage applied! There is nothing to do with bad electrolyte. A good capacitor will just showed a charge and discharge in the insulation meter just like you are checking a capacitor using analog multimeter. In the market there is quite a number of ranges that you can buy. It has the range of 50v, 100v, 250v, 500v, 1000v and even 5000v! If you want to test a capacitor of 100 microfarad 160v then you have to select 100v. If you select 250v, it will blow your capacitor that is under test.

If you have the SENCORE TEST EQUIPMENT such as the sencore lc meter LC102 OR LC103, these meters have the capabilities of checking any type of capacitors with four tests:

-testing for capacitor values

-checking for leakage

-equivalent series resistance (ESR) and

-Dielectric absorption

It can check aluminum electrolytic capacitor, film capacitor, ceramic, high voltage capacitor and etc.

Conclusion-Different capacitor manufacturer produced different type of quality of a capacitor. Perhaps the bad capacitor that I encountered are from the lowest grade one. A capacitor failure when under load is very rare. Using ESR capacitor meter alone can solve most of the electrolytic capacitor problem.

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An RC Snubber Network consists of two passive components, a Resistor and Capacitor. These components are connected in series across the output of switching components. The switching components are usually configured in a push-pull arrangement, and the active devices are generally either IGBT’s (Insulated Gate Bipolar Transistors) or MOSFET’s (Metal Oxide Semiconductor Field Effect Transistors).

In a Switch Mode Power Supply, the switching components and primary transformer windings have a parasitic Capacitance and Inductance associated with them. There combination forms an LC resonant circuit. When the switching components are gated, it is this parasitic based self resonance that creates ringing on the rising and falling edges of the switching waveform. The ringing appears as decaying amplitude oscillations. More commonly, when considered singularly they are referred to as Voltage spikes or Transients. The Voltage spikes can have an amplitude high enough to stress the switching components to eventual destruction. The ringing occurs at a frequency that is many times higher than the switching frequency. If an RF Spectrum Analyzer (Test equipment that shows voltage amplitudes in the frequency domain) is used to observe the ringing, it will be seen as Spurs (Spurious RF emissions) many times higher than the baseband frequency. These emissions can cause EMI (Electromagnetic Interference) / RFI (Radio Frequency Interference). Because of this, Noise and process problems can occur to Audio equipment, Communication Networks, Computer Systems, Radios, Televisions and Video Systems.

When designing a Snubber Network for switching circuits a practical approach is the best method for reducing Spurs. It involves the use of a DSO (Digital Storage Oscilloscope), Scientific Calculator, and a small selection of components. To work out the required values of Resistance and Capacitance for the Snubber Network it is necessary to determine the value of the parasitic Inductance, parasitic Capacitance, as well as the frequency of the Spurs. This can be achieved by taking measurements and calculating the unknown quantities. All necessary precautions should be taken while doing this to avoid contact with live circuits. High Voltages are normally present in Switch Mode Power Supplies and other types of Power Inverters. Contact with these Power Supplies can Kill, or cause serious injuries. If you are not a competent Electronics Technician or Electronics Engineer treat this information as Reference material only.

Spur attenuation, is achieved by performing the following steps:

1. Utilizing a DSO, measure the natural resonant frequency of the Spurs, and the peak Amplitude.

2. Connect a low value capacitor (100pF or less) across the switching device. Keep increasing the capacitance until the peak amplitude of the Spur is observed to have halved from what was originally observed (this occurs at the -6dB point). Take note of the Capacitor value used to achieve this, and divide this value by three. This value represents an approximation of the parasitic Capacitance.

3. Now that we know the frequency of the Spurs (f), and the parasitic Capacitance (C) of the switching circuit, we can calculate the Parasitic Inductance (L), where L = 1/ [(6.28 x f) squared] x C.

4. The Impedance (Z) of the switching circuit can now be calculated, where Z = square root (L/C). The Snubber resistor is selected to match this impedance. This allows maximum power transfer between source (Switching device) and load (Snubber network).

5. The Snubber Capacitor is generally made to be ten times the value of the parasitic capacitance. This provides a very high attenuation of the Spur (past the previously measured -6dB point).

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