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Re: RE measuring inductance

Subject:   Re: RE measuring inductance
  Date:    Tue, 6 May 1997 08:02:31 -0400
  From:   "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
    To:   "Tesla List" <tesla-at-pupman-dot-com>



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-poodle.pupman-dot-com
> Subject: RE: RE measuring inductance
> Date: Tuesday, May 06, 1997 12:41 AM
> 
> Subject:  RE: RE measuring inductance
>   Date:   Mon, 5 May 97 15:36:12 UT
>   From:  "William Noble" <William_B_Noble-at-msn-dot-com>
>     To:  "Tesla List" <tesla-at-pupman-dot-com>
> 
> 
> Your comment about 2 volts is suspicious - LED's are diodes which
emit
> when 
> forward biased - depending on the LED, there will be one or more
> "standard" 
> diode drops in the forward direction - most red LEDs are a single
diode,
> so 
> 0.7 volts will cause them to turn on.  I have some green LEDs here
that
> take 
> 1.4 volts (e.g. they have 2 diodes in series).  You can easily
determine
> what 
> you have by putting some current through the LED (with a suitable
> dropping 
> resistor) and then measuring the voltage across the LED.
> I would recommend using low power LEDs not the new super bright
ones for
> your 
> kind of work - you could use an OP amp circuit and a meter or just
a DVM
> too, 
> of course, but the LED is pretty simple.
> 
> [Bill]  snip 
> 
> The LED requires a bit more than 2 volts to become forward biased,
> and only then will current *begin* to flow through it. I have one
RF
> Generator that only puts out a measley 2 volts. And it has a
> disgustingly high output impedance. The LED will give decent
visible
> indication beginning at about 1/10th of a milliamp. The series
> resonant circuit has maximum current flow through it at resonance,
> but the LED will only light if the VOLTAGE across the LED exceeds
the
> forward bias point.
> 
> [Bill]  snip

Bill,
Silicon diodes have a .7 volt forward diode drop. Germanium diodes
have an even lower diode drop. LEDs have a forward diode drop that
varies between about 1.2 and 2.2 volts, depending upon what material
they are made of. You will find that LEDs of different colors have
different diode drops because they are not exactly identical
(obviously). 

LEDs do not have a larger drop because they have several diodes
inside (although there ARE LEDs that contain more than one LED chip).
The reason for the higher voltage drop in LEDs is that the material
they are made out of has a different set of characteristics than
silicon.

Lower POWER LEDs will not generally have a lower diode drop. What
they have is a greater EFFICIENCY. That is, for a given amount of
CURRENT they will emit more light.

Hope this helps.

Fr. Tom Mcgahee