🚀 go-pugleaf

In article <1989Dec28.234113.12838@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>In article <1077@swbatl.UUCP> adams@swbatl.UUCP (4237) writes:
>>Seems that with a 555, some judicious values of f and careful choice of
>>value and range of C that an inductance meter is almost trivial if one
>>can reasonably accurately measure C. 
>
>Also of relevance:  with a 555 and some resistors, plus a frequency counter
>(which is trivial to build), you *have* a capacitance meter.  Just build
>an RC oscillator and measure the output frequency.
>-- 
>1972: Saturn V #15 flight-ready|     Henry Spencer at U of Toronto Zoology
	There is a certain similarity in the equations of RC behavior and
those of LC behavior that leads to an extension of this idea; one '555
oscillator circuit uses a resistor to the output for charging the capacitor
and (of course) discharging it, making the '555 a true RC oscillator;
with a bit of buffering (I'd suggest a 74HC04 with several sections
paralleled), it would be simple to make an RL oscillator which, instead
of driving the '555 with a voltage on the capacitor, drives it with the
voltage on a resistor (i.e. the current in an inductor for which that
resistor is a current sense resistor).  The only weakness of this scheme
is that the resistor must be grounded to the midpoint of the '555 
power rails, instead of to the negative rail (as the capacitor in the
RC oscillator is).  As an RL oscillator, the output frequency will be
proportional to R and inversely proportional to L (with constants
that might depend on the individual '555).  The test currents in the
coil, of course, must be kept within the 74HC04's limits (for five
paralleled sections, this should be about 25 mA with no trouble).  The 
output resistance of the 'HC series gates is low enough (20 Ohms) that
the sense resistor can be chosen in the range of 50 Ohms to 100 kOhm,
or to 2MOhm with a CMOS '555.
	Lest I be unclear, a '555 is a timing circuit similar to the
Signetics NE555 and its generic imitators, and a 74HC04 is a fast
CMOS inverter with six individual sections, here used as a buffer
amplifier for its high frequency response and low output impedance,
as well as for its elegant saturation characteristics (jam them
little MOSFETs ON and watch the juice flow!).
	The best commercial LCR bridges use phase-shift measurements
with internal tone generators (low-distortion sine wave generators)
to measure the second parameter, i.e. both the L of a coil AND its
(stray) resistance, or the C of a capacitor AND its ESR, equivalent-
series-resistance, at the test frequency.  For critical work, they
might be well worth the few hundred dollars they cost.

I am known for my brilliance,           John Whitmore
 by those who do not know me well.