RE: RE measuring inductance

To: tesla-at-poodle.pupman-dot-com
Subject: RE: RE measuring inductance
From: Tesla List <tesla-at-stic-dot-net>
Date: Wed, 07 May 1997 00:28:04 -0500
Approved: tesla-at-stic-dot-net
Subject:       RE: RE measuring inductance
       Date:   Wed, 7 May 1997 08:04:53 +1200
       From:  "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization:  Wellington Polytechnic, NZ
         To:   tesla-at-pupman-dot-com


Hi all,
         Here's some workbench experience to add to this....

>   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.

Not true in my experience. I can just perceive a glow at around 1.5V 
for red and a measurement shows that current doesn't seriously start 
flowing until this voltage is reached.

> I have some green LEDs here that take 1.4 volts (e.g. they have 
> 2 diodes in series).

Nope. Just one diode. LEDs are bandgap devices and require  higher 
voltages than a standard P-N diode. It is approaching 2V for green/
yellow. The colour is related to bandgap energy which in turn is 
related to voltage drop across the device. Blue LEDs require a higher 
voltage again.

Malcolm

>  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
> 
>