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: Tue, 06 May 1997 16:21:03 -0500
Approved: tesla-at-stic-dot-net
Subject:      Re: RE measuring inductance 
    Date:     Tue, 6 May 1997 08:39:37 +1200
       From:  "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization: Wellington Polytechnic, NZ
         To:  tesla-at-pupman-dot-com


HI Fr. McGahee, all,

<snip of previous replies etc>
> 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.

The only problem I have with that is that he was lighting the LED 
when the generator was shunt connected with the tank. You don't 
get more voltage than the generator can give with such a connection.

Malcolm 

> Robert Michaels correctly indicates that current flow is probably too
> low in your case. But the underlying cause for THAT being the case is
> probably that the *voltage* is below the required 2 volts or so for
> forward bias. Once the LED is forward biased, it acts as a current
> controlled device in which the voltage across it tends to remain
> constant (DC characteristic, of course).
> 
> Robert also correctly indicates that an incandescent bulb can make a
> good indicator. The bulb should be a low voltage and LOW CURRENT
> bulb. The type used in flashlights draw too much current. Look for
> something with a really thin filament. I used to use #47 panel lamps
> that were 6 volt rated. You can INCREASE the sensitivity using a
> technique that was pioneered by Nikola Tesla. (This technique should
> also work for LEDs, but it will only "help" one of them!)
> 
> Tesla's technique (which he devised for making very sensitive RF
> receivers) was to connect a sensitive low voltage incandescent light
> bulb up as usual to the circuit under test. Then he would attach a
> BATTERY (gotta be totally isolated.. don't try using a power supply
> here!) to one side of the lamp. The other side of the battery he
> would connect up to the OTHER side of the lamp via a series connected
> VARIABLE resistor. The key is to adjust the resistance so that in a
> darkened room you can just *barely* see the lamp turned on *solely*
> by the battery power. Then when you fire up the RF Generator, any
> CHANGE in currents will make the lamp dimmer or brighter. This is an
> example of BIASING. Do not make the lamp too bright with the biasing
> or it will become a *less* sensitive circuit.
> 
> Hope this sheds a little more light on the subject :)
> 
> Fr. Tom Mcgahee
> 
>