Re: LED: Light Emitting Diode Question

[wsquires@lonestar.utsa.edu (William H. Squires)]

In article <1994Mar7.005405.16729@presby.edu> sebugg@presby.edu (Stephen Buggie) writes:
>             LED LIGHT EMITTING DIODE:  WHAT OHM RESISTOR FOR +5V AND +12V?
>
>             QUANTUM LPS52 HARD DISK: WHAT AMPS REQUIRED ON EACH VOLTAGE?
>
>
>
>I am proceeding with my plan to move my external hard disk (Quantum LPS52
>3.5" HH) from its commercial external box to the classic Apple Disk ][
>casing.  The power will come from a surplus Apple ][ power supply connected
>by a 6 ft four conductor cord, which will supply the +5V and +12V power
>required.
>
>I plan to install two LED lamps on the front panel of the Disk ][ housing
>so that I can monitor the power.
>
>QUESTION: What resistor should be wired in series with the LED?  I expect that
>the resistor (ohm) value will differ for the two voltages: +5V and +12V.
>The higher voltage will require a higher ohms resistance, correct?  Please
>specify ohms values for each of the two voltages, +5v and +12v.  I presume
>that
>I can use even a tiny resistor in series with the LED; perhaps even the 1/8
>watt size; is this correct?
>

  You can figure the resistance value needed with this simple equation:

  Rled = (Vs - Vd) / Id

where Vs is the source voltage, Vd is the forward voltage drop of the LED
(usually around 1.6-1.8 volts or so), and Id is the maximum current rating
of the LED, usually around 10-20 mA or so. Thus if you have an LED with a
Vd of 1.7, and an Id of 10 mA, you would use a

  Rled = (5 - 1.7) / .01 = 330 ohms, a common value. This is for 5 volts;

For 12 volts, this resistor will be 1030 ohms. Use a 1.1 kilohm resistor, the
nearest common value. The power rating of the resistor is:

Prled = (Vd^2)/Rled where Prled is the power dissipation of Rled above. Thus
you would need at least an 8.76 mW rating on the 330 ohm resistor, and a
2.63 mW rating on the 1.1 kilohm resistor. Thus both resistors can safely be
of the little 1/8 W (125 mW) rating without problem.

The Vd (aften called Vf or Vgamma), and the Id ratings of the LEDs should be
available from the manufacturer's specs. Radio Shack prints these ratings on
packages of single LEDs (though, not on their 20 LED assortments).

>Second, I plan to install two fuse-holders on the back panel of the Disk ][,
>a fuse on the +5v line and a second fuse on the +12v line.
>
>QUESTION:  What will be the optimum fuse values for the +5v line and the +12v
>line?  The answer will depend on the current requirement of the Quantum LPS52
>hard disk drive.  I _guess_ that the +5v line powers the electronics while the
>+12v line powers the motor.  But does anyone know how much current this
>Quantum
>pulls from the two voltage lines?   If the current is known, then I will
>install fuses just slightly larger than the amperage requirements of the
>disk drive.
>
>Please e-mail me at:     sebugg@cs1.presby.edu   Thanks.   Stephen Buggie
>

  If you're going to use an old Apple // power supply, you don't need the
fuses. This is a switching power supply, and is fully capable of
protecting itself from no-load, and short-circuit conditions! You just need
a power supply that's hefty enough for the job. The power supply has a
feedback path from the output to the input oscillator that generates the
kilohertz-range frequency that drives the transformer. If no load, the
open-circuit voltage saturates the osc. circuit, preventing it from working,
thus the supply "turns off". When a short occurs, the voltage drop is 0,
thus the oscillator doesn't have any power to operate, and, once again, the
supply protects itself. Note however, that the circuit it's powering isn't
protected -- not if it's got a short!
  Spare yourself the trouble of drilling holes in the thick metal case of the
Disk ][ housing, and consider yourself lucky!
  Another option is to use the supply that came with it (the HD), and put it
in another Disk ][ housing, or in a little project box from Radio Shack (those
little plastic boxes are much easier to drill through!!), and use the ribbon
cable from the old Disk ][ to carry the power from the supply to the drive.
Just parallel some of the wires in the cable for greater current-carrying
capability!
  Hope this helps!