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Case, Power, VRM, MMX

As the case may be A typical case will have 5 drive bays, a built-in 200 W power supply, a power switch, a reset button, and a key lock.

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You can never have too many drive bays. Sometimes, internal drive bays are found above the P/S, which I find extremely unappealing. Numerical LED's are out, turbo switches are out, but extra fans and cutouts are in. Try to get a case that accepts the new ATX form-factor motherboards and the AT boards. The motherboard mounting holes are in different positions for the two form-factors, but one case can easily have holes for both. Ask your vendor about a case for either ATX or Baby-AT. ATX motherboards and cases have a maximum of only seven slots, while the baby-AT designs can have up to eight.


Power (or Electrons are your friend)

The Power Supply is largely misunderstood by the general public. It is a fairly common electronic component, because of the prevalence of AC electrical power sources and the large number of components requiring regulated DC.

All modern P/S's are of the switching type. Basically, a switching P/S minimizes the ripple associated with standard rectifiers. This is a simplified explanation of how +5 VDC is produced

  1. The 60 or 50 Hz AC from your wall is rectified by a full-wave rectifier.
  2. The result is an unfiltered voltage (for instance, 18 volts), in positive and negative values, with common being chassis ground (connected to earth ground for most computers).
  3. This 18 V is then filtered, but it still ripples at 120 or 100 Hz. (Full wave rectifiers ripple at twice the input frequency.)
  4. In the switching portion of the P/S, a power transistor turns on and off at rapid intervals, perhaps at 20 kHz. The transistor has a square-wave output, and its conduction interval may be relatively short compared to its cut-off interval. How does the transistor know how long to stay on for each interval? Since the circuit's purpose is to produce regulated and filtered DC, a feedback signal from the output side controls the switching transistor's conduction interval. When the output voltage drops below nominal, the transistor gets a boost signal, causing it to conduct longer. When the voltage goes above nominal, the transistor gets a buck signal, causing it to conduct shorter.
  5. Capacitors in parallel and inductors in series complete the circuit to provide the filter. When the output is filtered, you have a perfectly regulated (+ or -) 12 or 5 or 3.3 volts, depending on the circuit.
  6. The end result: you have a regulated voltage that changes little as the load increases, and ripples imperceptibly at 20 kHz, suitable for TTL or CMOS circuits.

More often than not, your case will dictate the P/S you get. But most case vendors will offer a wide range of P/S brands, sizes, shapes, quality, power, and voltage options.

If you expect to get an ATX motherboard, be sure to coordinate your P/S, motherboard, and case so that the three will work together. Many ATX P/S's will not work with older AT-style motherboards, because the connector is different.


Spend the extra money for a UL-listed P/S Power is a good thing: get a 230+ W P/S Quiet is cool: get a P/S that has an automatic temperature sensor that will speed up its fan as temperature rises. ATX P/S's allow your computer to be shut off or turned on electronically through your OS or through a modem ring-on feature.



While we're talking about voltage regulators, a brief description of a voltage regulator module (VRM) is necessary.

Do not feel left out if you don't have a VRM per se. It may be kst another portion of your motherboard that is identified by one or more large aluminum heat sink(s) stapled to a few power transistors. Since nearly all PC P/S's made up to 1995 only had +/- 12 and +/- 5 volt supplies, the advent of the P54C 3.3 V Pentium, 486DX4, and Pentium Pro computers has complicated the power issue. Basically, any voltage below 5 V can be generated at will with power transistors and large heat sinks - on he motherboard, where they don't belong. To make matters worse, some new EDO RAM requires 3.3 V - avoid it like the plague because it's an all-or-nothing approach - you can't mix it with 5 V RAM of any kind.



The Pentium with MMX technology has an extremely finicky choice: 2.8 volts. Unfortunately, the majority of non-Intel motherboard manufacturers were caught off-guard for this chip. By my own estimate, perhaps 2% of Pentium motherboards currently (February 1997) on the desktops of the world can accept a 2.8 volt CPU. Everyone keeps saying, "Sure, the P55C will work in your motherboard because the only thing different is its instruction set and cache." This is misleading, because only an MMX Overdrive (with built-in VRM) will work in the 3.3 volt sockets.

Also, the voltage requirement is pretty strict. Only a +/- 0.1 VDC variance is allowed. And I know I've seen motherboards with a 2.85 volt setting, which may not always deliver in-spec voltages. I think the wisdom is that once the chip is loading down the regulator, it will maintain 2.8 volts.

So you may want to either skip the OEM versions, or buy an in-line VRM to essentially make it into a kind of do-it-yourself Overdrive chip.

Since I said so much about the MMX Pentium (and voltage is the "hot" topic), I might as well editorialize some more. The MMX Pentium does not need a BIOS upgrade, unless you must have your BIOS detect the difference. You also do not need a new chipset (or motherboard design) to take advantage of this CPU, except perhaps its caching algorithms. One possible exception: if you have a SMP-designed motherboard and intend to use two MMX Pentium chips in it, a BIOS upgrade may be required. Intel knows very well that this will be an upgrade chip - it will be what Pentium owners will upgrade to for the next 3+ years when they're tired of their 100 MHz P54C. It's analogous to the DX4-100 or the Pentium Overdrive 83 for the 486 owners of today.


Back to VRM's

Interestingly, the Pentium Pro could be a plethora of different voltages, which it selects in the range of 2.1 to 3.5 volts in increments of 0.1 V, depending on the state of four pins on the

CPU. Your VRM performs a simple decode of these four bits, and provides the proper voltage to your CPU. Remember that some Pentium Pro's suck nearly 40 watts of precious electrons, and you're VRM will expend quite a lot of power just producing the proper voltage. Typical efficiency is less than 80%, so multiply your CPU's power by 1.3 to get a good idea of the amount of electrons your poor motherboard is drawing - just to un the CPU. You see why I recommend that extra fan? Have you ever put a 100 W light bulb in a tight, enclosed space?

For those Pentium guys and gals, your VRM may be relatively unsophisticated, requiring jumpers to change voltage, and may have only 3 or so choices for its voltage. You may not even have a VRM per se, only a fixed voltage regulator that produces a single voltage.

There's a lot of regulators out there, and two major types:switching (see my description of a rudimentary switching power supply - or is that a rudimentary description of a switching P/S?) and linear.

The linear regulators are cheap, more sensitive to current spikes, and extremely more wasteful of power. A linear 3.3 V regulator can be at most only 66% efficient if it has a 5 V input, while a switching (intelligent) regulator is typically 88% efficient.

The difference is the amount of heat produced and the degree to which the regulator reacts to current spikes - which can generate voltage droops in linear regulators.

By products of your friends, the electrons

Sure, we all love our electron buddies. They perform those neat tricks like brightening up our day (the monitor) and making that hard disk spin. They also make pesky RF in the noise bands (LF, MF, HF, VHF, UHF - you get the picture) and cause the verage molecular kinetic energy of components to increase. If you missed that one, I just said that they make things get hot. So what to do? Boycott those damned charged particles and elegate them to Bohr models of the atom? Nope, you gotta love your charged cousins. It's not their fault they have alter-egos.They come from co-dependent families of protons and the -neutral neutrons. Don't shun them, live in harmony with them.

Yes, take compassion on our entropy-inducing, thermodynamic, flowing, negative leptons and use your knowledge of the second law of thermodynamics and the refrigeration cycle to COOL THEM DOWN! It's just like my mom used to always say, "Billy, never forget that cool electrons are happy electrons and you will go far in life." So keep your room at 68 F (20 C) sing your air conditioner, heat pump, or swamp cooler of choice. Cooler is fine, but frozen hard disks may squeak or even not spin.

Suggestions to make our friends happier:

You may wonder why I spend so much bandwidth talking bout heat. Well, I had some cache chips on my old 486 otherboard that were very heat-sensitive. In Florida and Hawaii, I had to disable the secondary cache altogether, but during the winter in Idaho, I rarely had any problems.

I have seen ISA cards that have one or two fans stapled on them. This is great for cases that do not have a place or a second fan, as long as it can accept a full-length card (although some may be shorter, the ones I have seen are full-length). This will dramatically increase air flow and may only set you back US$ 20 to $ 40.

Allow your BIOS to spin down your hard disks when not in use. I have three hard disks, and two of them (which have a combined capacity of less than a gigabyte) get limited use. I let them sleep after 18 minutes of inactivity, which makes things much quieter and cooler. Do not use a $3000 piece of equipment merely to listen to CD's. If you like listening to CD's, buy a $200 portable CD player.