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- Aug 30, 2004
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PRACTICAL ELECTRONICS MINI-REFERENCE
Last Updated: Nov 20, 2005
Introduction:
The idea behind this writeup is to introduce the practical aspects of Electronics design. In the hardware world, one sees a big discrepancy between the quality functionally similar components. Take for example motherboards, a good motherboard is a consequence of several factors, the most important being the use of high quality components. Mismatched or poorly chosen components can play havoc with stability. Generally, QC catches boards which are defective at stock settings plus a little safety margin. That, however may not reach the enthusiast regimn. In this short writeup, I hope to introduce aspects which will allow you to critically examine a few components used on your equipment for quality.
I assume that you all know how common passive and active elements behave, if you don't I'll include a few links in the last section.
Capacitors:
Capacitors come in several flavours. Why the heck do we need caps made of different materials? If it is to span a wide range of capacitance, why should there be an overlap? I mean, why do we need Teflon 0.001 uF and Tantalum 0.001uF caps? Won't one material suffice? The answer is NO. Ever material has its own characteristics, drawbacks and advantages. So, each design has its own unique match of caps.
- Important points to look for when shopping for a Capacitor :
- Equivalent Series Resistance (ESR)
A practical capacitor is not the ideal textbook model we see. Accompanied with capacitance are stray resistance and inductance.These parasitic quantities are detrimental to the operation of the capacitor. They cause the Capacitor to behave as an R-L-C ckt, instead of a true Capacitor. If you see ringing (spurious oscillations) in your ckt, its beacause of a poor capacitor. While the inductance is usually controllable (although its more pronounced in metal-foil types), parasitic resistance is bloody hard to eliminate. So we have an LC filter/oscillator, with a resonant freq of f=1/2*Pi*sqrt(L*C).
Although the ESR concept is truly valid only for power dissipation calculations at a specific frequency and temperature, ESR can be reasonably constant over some frequency and temperature range. ESR usually begins to rise above 100KHz., reducing allowable capacitor current. For some capacitor types ESR rises with temperature. Be very careful not to envision a real capacitor as "an ideal capacitor in series with a fixed resistor of ‘ESR’ ohms"! [Although good polypropylene capacitors may behave nearly this way over a very wide frequency range]
The official definition of ESR is ..."ESR is the sum of in-phase AC resistance. It includes resistance of the dielectric, plate material, electrolytic solution, and terminal leads at a particular frequency."- Quality Factor (Q)
The quality factor Q, is a dimensionless number that is equal to the capacitor’s reactance divided by the capacitor’s parasitic resistance (ESR). The value of Q changes greatly with frequency as both reactance and resistance change with frequency. The reactance of a capacitor changes tremendously with frequency or with the capacitance value, and therefore the Q value could vary by a great amount. Q is important in tuned circuits because they are more damped and have a broader tuning point as the Q goes down. Q = 1/R*Xc where Xc is the capacitive reactance (Xc = 2*Pi*f*C) and R is ESR. Q is proportional to the inverse of the amount of energy dissipated in the capacitor. Thus, ESR rating of a capacitor is inversely related to its quality. Higher the Q better the capacitor.
- Dissipation Factor (D)
The inverse of Q is the dissipation factor (d). Thus, D = ESR/Xc and the higher the ESR the more losses in the capacitor, the more energy dissipated. If too much energy is dissipated in the capacitor, it heats up, which changes the capacitance and may result in a complete failure. Electrolytic caps are more prone to such failure. The leaky cap phenomenon is as a result of this.
- Ripple Current Rating
Ripple current is the RMS value of the capacitor current in an application where the voltage across the capacitor is small (less than ~5% of DC rating). For switching supplies the voltage change across the capacitor may be much less than this. RMS capacitor current typically is not specified at a particular frequency and thus should be carefully considered. [The term "ripple" originated with vacuum tube capacitor input power supplies, and may not have any meaning in the context of some modern capacitor applications]. It also is a measure of energy dissipation as E= I^2*ESR.
- Tips and Tricks
* Be very careful with electrolyic Caps! Even a small value cap like 2uF can blow up dangerously if you apply a DC voltage of the wrong polarity or apply an AC voltage to it.
* Go with Tantalum Caps for low ESR and less lekage.
* Mylar Capacitors have low ESR but poor Thermal stability.
* For good temprature stability, low lekage and good dielectric absorption, go with Teflon or Polystyrene. Remember that these Caps are bulky.
* Metallized Polyester film or Polypropylene Film caps "self heal", i.e if a hole develops in the metal film, the high current density at high voltages will fuse the metal, bridging ay pinhole defects. Good for high voltage, extremely poor for low voltage operation. Not recommended for Audio coupling.
* Extended Foil, Silvered Mica and Ceramic caps are suitable for high frequency applications as the ESR can be controlled.
* High K-type Caps are not recommended! They have mediocer leakage, poor dielectric dissipation and thermal stability. Avoid at all costs! Sadly, due to their compact nature, a lot of products use them. If you see them on your MoBo, you are in for a bit of a stability battle as temps increase.
* Always add bypass capacitors across supply lines (supply line to GND) to minimize oscillations. The standard rule would be to use a 2uF Tantalum Cap in conjunction with a 20uF Electrolytic Cap for every 3 to 5 IC's. See if your MoBo has any spots where you can add caps, trace the connections back and add Electrolytic caps if possible.
* If you see spurious oscillations, adding a small 5Ohm resistor in SERIES with the bypass capacitor may help dampen it.
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