FANS, 5V LED ( was Re: Gecko current set resistor

Hi Dave, Listies,

Read the previous thread (6 mo. ago?) on using a 12 or 24V DC fan (and
appropriate series resistor) to discharge cap and indicate charge
remaining. You want a fan anyway!

My collection of old PC power supply fans are typically 170 - 250 ma. A
small resistor drops the need voltage from a 37 or 74v power supply.
IDENTICAL fans can be used in series.

Another good way as Mariss published (see files) is to use a relay to
switch in a BIG bleeder resistor when the AC power is shut off. I'm
happy with my fans!

HTH

Alan KM6VV

turbulatordude wrote:

>
> Modern LED's are designed to use the 20mA and 5V as the basis for
> determining resistors.
>
> There are some different styles like ones with internal resistors and
> ones with much lower power. The Garden variety is 5v-20mA
>
> I assume you are referring to the 1.7 volt drop across the LED so
> I=5/R
> but 5-1.7=3.3 volts for power
> so I=3.3/R or 20mA=3.3/R
> In many circuits you worry abourt wasting power, but my premise is
> that at 1 amp, you are not worried about an 0.02 drain.
>
> You can power one with 40mA or lots more if you pulse the voltage,
> makes it REALLY bright. if you don't get the pulse correct, it makes
> it's life REALLY short. The reasoning is that amps will heat the
> LED if left on too long. and if the pulses are faster than your eye
> can seperate the pulses it will appear REALLY bright. and with
> cooling between pulses, you do not risk the LED by over heating.
>
> And you are correct that 5v-20mA will yield a bright light. One
> question is how big is that Cap and how long will it take to bleed it
> down ? and how bright will the LED be when there is still enough
> charge to make you jump ?
>
> An LED will light quite nicely at lower voltages, so as the cap
> discharges you will see a dimming as the voltage reduces. At some
> point you will need to be in a very dark room to see the faint glow.
>
> An LED should work up to about 40mA, but any spikes will soon destroy
> it. Also, you should have a faint glow at about 1mA. but you do
> need about 1.7 volts to light it (1.9 for orange)
>
> At what voltage will your cap still shock you ?
>
> if you light at 20mA at 80 volts, your LED will glow dim down to very
> low voltages and will blead faster than a higher resistor.
>
> My use is not to just indicate, but to blead down the cap and read
> voltages all the way to the almost safe level.
>
> besides it LOOKs like your machine has a lot of power when it is
> glowing brightly. power ummmm......
>
> Dave
>
> --- In CAD_CAM_EDM_DRO@y..., JanRwl@A... wrote:
> > In a message dated 21-May-02 12:11:17 Central Daylight Time,
> > davemucha@j... writes:
> >
> >
> > > Figure 20mA for the current and 5V for the LED
> > >
> > >
> > >
> >
> > 5 VDC is WAY too much for LED's (except those special ones with VR
> > "on-chip")!!!
> >
> > 20 mA is BRIGHT for a T1-3/4. "Modern" LED's work fine on 10 or 15
> mA!!!!!
> >
> >
> > [Non-text portions of this message have been removed]
>
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