Caple C871i Uživatelský manuál stáhnout pdf (Strana 561)

Apple II Computer Info

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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]

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Apple II Computer Technical Information : Apple II Family Hardware Info

ftp://ground.ecn.uiowa.edu/2/apple2/miscinfo/hardware : May 2001 : 561 of 572

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