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Re: RE measuring inductance
Subject: Re: RE measuring inductance
Date: Sun, 4 May 1997 20:17:34 -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 measuring inductance
> Date: Saturday, May 03, 1997 12:47 AM
>
> Subject: RE measuring inductance
> Date: Thu, 01 May 1997 20:01:28 GMT
> From: robert.michaels-at-online.sme-dot-org (Robert Michaels)
> Organization: Society of Manufacturing Engineers
> To: tesla-at-pupman-dot-com
>
>
> My quick and dirty supposition is that the amount of
current
> which was able to pass thru the capacitor was not
sufficient
> to overcome the threshold of the LED.
>
> The device of choice for this sort of test is an rf
ammeter,
> or, failing that, a very small =incandescent= lamp.
>
> Are you sure the LED is good? Did you test it
> =after= the disappointing results you obtained?
>
> Likewise, the capacitor?
>
> Physics - now and forever,
>
> Robert Michaels - Detroit,
USA
>
>
>
> [ ... ]
>
> T>Instead of putting my signal generator away I tried doing the
same
> T>measurement again but this time with the coil, capacitor and led
in
> T>series.
> T>Since this is a series LCR circuit I expected the led to remain
dark
> T>except
> T>for at the resonant freq when it would be brightly illuminated.
> However
> T>the
> T>led remained dark at all freqs (even at 116 kilohertz). Anybody
out
> T>there
> T>know where I went wrong with my theory - I'm pretty sure all my
> T>connections
> T>were good.
>
> T>Thanks!
>
> T>Best Regards,
> T>Mark
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.
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