From: mmunson@animal.es.com (Mark Munson)
Subject: Re: 5v to 5.25v
Reply-To: mmunson@animal.es.com (Mark Munson)
Organization: Evans & Sutherland Computer Corp., Salt Lake City, UT
Date: Thu, 1 Oct 1992 23:13:51 GMT


Frank:

You're power supply won't give you 5.25 Volts directly.  It only 
supplies +/- 5 Volts and +/- 12 Volts.  So you've have to get the
5.25 Volts that you want from the 12 Volt line.

First off, you've have to remove the current 5 V line to the
CPU by cutting a trace on the accelerator card.  The new
power supply will be connected to the line where you just
cut it, supplying the chip with power from your new source.

If I recall correctly, the ZipGS (9 MHz) came with some circuitry built
in to boost the voltage up.  7 MHz ZipGS cards don't have the components,
but the circuit board shows where a transistor and a few resistors could
go.  If you have a 7 MHz zip card, find someone with a 9+ MHz version and
add the components that they have on thier board and you don't have on yours.

On a TWGS, you must add the extra circuitry yourself.  This can be done by
building a small breadboard with the compnents attached.

A sample circuit to get a variable voltage supply might look like the
following:


     12 V         12 V                 12 V = +12 Volt line (available from BUS line)
      |            |                      T = NPN type Transistor (2N2222)
      |            |                     R1 = Resistor ( ~ 100 Kilo-Ohm )
      R1           R3                    R2 = Micro miniature pot (variabl
      |            |                          (resistor  100 K Ohm)
      |     ___C---                 R3 & R4 = Resistor ( ~ 10 Kilo-Ohm )  
 --> R2 --B( T )                          B = Transistor Base lead
      |     ---E         [output]         C = Transistor Collector lead
      |         ---+---- 0 - ~5.3 Volts   E = Transistor Emitter lead
      |            |
      |            R4
      |            |
      |            |
     GND          GND


Current will flow through R1 and R2, with the voltage at point B beind calculated by

Vb = 12 V * (R2 / (R1 + R2))

When R2 is at its maximum value of 100 K, the voltage at point B will be half that of
the original 12 V, resulting in a Base lead voltage of 6 V.  The voltage at the 
Emitter lead of the transistor will follow that of the Base, but it will be ~ .6 to
.7 less.  As a result, the MAXIMUM voltage at the output should be 5.3 - 5.4 volts.
This should prevent you from "accidently" giving your expensive CPU too much power.

R2 can be manually tuned to vary the voltage from 0 to MAXIMUM.  Start with 5V, and
move up slowly.  You'll want to have a good digital voltmeter attached when you are
doing this.  An anolog voltmeter will do OK, if it has a high-precision scale (and
you have good eyes).

R3 represents the output load from the transistor circuit.  You can omit this if
desired, but it will be necessary to have some load when the circuit is being tested
prior to hooking this up to your CPU.

R3 and R4 are matched, so a complete failure of T will result in a maximum voltage
of ~ 6.0 V.  By increasing the value of R3 relative to R4, you can reduce this down
even further. With a new CPU being so expensive, why take chances that a 20 cent 
part might fail?

When purchasing your parts, you can get small 1/8 Watt resistors, because the power
level you are working with is small.  1/4 and 1/2 Watt resistors are OK, but they are
larger and bulkier, and will be a slight overkill for this application.

If you purchase a transistor with a metal package (not platic), you will see a small
square tab next to one of the leads.  The lead neares the tab is the Emitter, and if
you look at the transistor from the top (wires exiting at the bottom), then you can
orient the device as shown in the illustration above to locate the Collector and
Base leads.


If you go to a local electronics shop, you can purchase everything needed for less than
a dollar.  Radio shack will expect you to hand over several $$ for the same parts.


Built the circuit seperately from you computer.  Use and external power supply (car
battery or charger will do) to test the circuit.  When you know it works right, cut the
5 V power to your CPU and replace it with your new 'boosted' source.  Start increasing
the frequency and only increase the power if the CPU can't run at the current voltage.

I hope this helps you get going.  You may want to increase the resistor values to reduce
the overall power drain.  I don't know how much current the CPU will require, but it
couldn't be much.

[MGM]

[MGM]


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