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Hardware Hacking The hammer and tongs school of Overclocking. (NOT for the beginner and you assume all risks)


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Old 31st January, 2006, 04:51 AM
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Post Practical Electronics mini-guide

WORK IN PROGRESS
Still to come:
*Inductors, Resistors and IC's (Opamps, Voltage regulators and miscillaneous IC's).
*Measuring rail voltages of your PSU when under full load.
*Simple voltage step up/down circuits.
*Very basic PCB fabrication tricks.

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. Most of this is taken from my experience in designing circuits (not much, but a bit more than most other people).

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 leakage.
* Mylar Capacitors have low ESR but poor Thermal stability.
* For good temprature stability, low leakage 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 5 Ohm resistor in SERIES with the bypass capacitor may help dampen it.


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Old 31st January, 2006, 06:03 AM
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When I was younger I loved blowing up caps by overvolting (don't try this at home I'm a trained idoit).
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Old 31st January, 2006, 06:24 AM
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When you've finished Super Nade PM me and I'll copy this over to our FAQ and drop commenting post as it is enshrined for AOA eternity...":O}
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Old 31st January, 2006, 03:30 PM
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Sure thing Daniel, wow, lots of writing to do on Sunday!

QSDT, blowing up caps is fun. I stopped doing it when I blew up an innocuous looking 12V 47uF electrolytic cap and had the dielectric fluid mess up my shirt
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Old 31st January, 2006, 03:52 PM
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SN, there appear to be rather large portions of the above info that are copied verbatim from other sites. This is permissible, so long as the original source is cited. Could you provide links to the original sources, please?
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Old 31st January, 2006, 06:36 PM
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The last section will be devoted towards references. Most of the definitions have been taken/referred to from the IEEE Electronics handbook. I'll also include links to other sites which refer to them. Other than that, this is exactly a copy of what was posted at ocforums.com, by someone called Super Nade

Hope you don't mind me copying over some stuff I wrote up at ocforums to these forums? (I'm on a long sabbatical)
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Old 31st January, 2006, 06:57 PM
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Quote:
Originally Posted by Super Nade
Hope you don't mind me copying over some stuff I wrote up at ocforums to these forums? (I'm on a long sabbatical)
Haven't got a problem with that at all, in fact I prefer to have it here as opposed to elsewhere; gives people an excuse to stay here!

Just want to be sure that proper credit is given where it is due, that's all.
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Old 31st January, 2006, 10:12 PM
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Quote:
Originally Posted by Super Nade

Hope you don't mind me copying over some stuff I wrote up at ocforums to these forums? (I'm on a long sabbatical)
Not as long as my own! ":O}
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Old 1st February, 2006, 05:17 PM
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I had no idea that capacitors were this complicated. Thank you, Super Nade!
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Old 2nd February, 2006, 12:01 AM
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Good stuff,

For our applications, bypass capacitors close to the noise source are probably the most important. Replacing electrolytics removed by the manufacturer can also help, as long as you choose the correct values and match the requirements.

The impact of ESL of capacitors used in switching power supplies would be good to cover too...

Nice write up so far tho

Tom
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Old 2nd February, 2006, 03:50 AM
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Thanks for the kind words folks.
At the moment, I'm having a hellish time designing a balanced photodetector for my experiment. Its the bloody caps that are killing my setup. Getting the right caps is very very important in analog electronics. Nothing ever works as its supposed to. I've been struggling with my design for several months. Horrible ringing issues. I'm moving over to a 4 layer PCB. Its gonna be more expensive, but I hope things will work out correctly.
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