Re: [Phys-l] Ugh - RLC circuit issues

[Bernard Cleyet <bernardcleyet@redshift.com>]

"I'm driving the unknown coil, a known electrolytic cap (100-1000 uF range)
with a ScienceWorkshop freq. generator, and continuously measuring the
voltage across a resistor in series with the cap and coil, looking for the
highest voltage difference to find the highest current.  Any ideas?"


For starters never, never, never, use an electrolytic at greater than low audio frequencies.  If you must use this method, not a bad one, use caps designed for radio frequencies.  e.g. silver mica.  Electrolytics usually have a value rating of ~ +/- 100% and have significant INDUCTANCE and loss tangents.  This means use an RF generator, which in my days at school were ubiquitous. Of course, if one has the RF generator one can then use it directly w/ the LC circuit.  Using a std. cap. will enable one to determine the values instead of just the sqrt (LC), etc. 

I use a grid dip meter.


wiki:


http://en.wikipedia.org/wiki/Grid_dip_oscillator


How to:




http://www.qsl.net/iz7ath/web/02_brew/15_lab/02_dipper/pag01_eng.htm


bc, also has a set of absorption wave meters (RF and microwave).


p.s. I don't know if JD mentioned that the resonant circuit is loaded by the antenna and probably shifts the "natural" resonance of the LC circuit.




Josh Gates-fac wrote:

>  I'm on my last nerve with a couple of student independent lab project
> that won't behave - perhaps the combined wisdom of phys-l can guide me to
> the light at the end of the tunnel!
>
>  #1- they have a crystal radio kit, and they're trying to identify the
> frequency of the radio station that they're picking up.  I suggested that
> they'd get a good answer by determining the inductance of the coil and the
> capacitance of the variable cap at the correct tuning.  For this, they're
>  
cut

>