adams@swbatl.UUCP (4237) (12/28/89)
I posted an article requesting info about capacitance meters a while
back. I got no response with the exception of a helpful message from Bob
Parnass and a request to send info on home-made capacitance meters.
So I went out and collected a lot of info on Capacitance and Inductance
Meters and Bridges.
I summarize that info at the end of this article in the hopes it will
help someone, or someone will find a better deal, or suggest
alternative manufacturers. I'm still undecided which meter I'd buy,
but building one is starting to look better and better, given
commercial cost and the apparent ease of doing so.
I have an article (Cap Checker by W.S Gardiner (VE6BGL) in Mar/87 73
Magazine ) with instructions for an extremely easy looking ( 1 pf to
50uF) analog capacitance meter. This meter uses a crystal oscillator,
divided down to various values by 7490's and then measures the
capactive resistance at an appropriate frequency.
C (unknown)
[~]----)|---+ Where [~] is the 7490's and stuff providing
| assorted frequencies {f: 1hz, 10hz, 100hz ... 1Mhz}
+--+------>|---+-->|-+ and )| is a cap, >| a diode, [M] a meter, etc.
_|_ | |
/// | .1 |
+------)|---------+ I assume the appropriate eqn is
| | Xc = -1/(2 pi f C)
| 1mA |
+-----[M]---------+
What I don't understand is the role of the diodes and the .1mF capacitor.
I also don't see how the 1mA full scale deflection was chosen. (i.e.
what happened to pi? )
I do see how easy it should be to measure inductance with a Q-meter;
and how easy such a meter is to build, given a known frequency and a
capacitance meter.
L R
+--()()()--/\/\/\-------+ Where Variable Cap C is tuned to Maximimum
| | | Voltage on Meter M, and Unknown Inductance
[~] | [M] L = 1/[ (2 pi f ) C ]
| C / | |
+-------)/|--------+ |
| / |
+-----------------------+
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.
It also would seem I could build an inductance meter into a "Cap Checker"
fairly easy, perhaps first measuring the the unknown resonant capacitance
at one of the available frequencies, then using that in the resonance equation,
or somehow producing a calibrated deflection.
Am I missing some large gotcha? How would one figure out values for the
meter part of a Q meter? Why are commercial units so expensive?
Damned if I understood much of this a week ago and my "Principles of
Electronic Instumentation" suggests a voltage divider to measure
capacitance, switching a volt meter between voltage across the source
and known cap values and using:
V / V = C / ( C + C)
known cap source unknown known unknown
Which doesn't seem to be what the Cap Checker is based upon.
It then goes on to say this won't work for inductance, because of
unknown "mutual inductance" between the known and unknown inductors and
recommends the Q meter approach. Say What?
I realize this is probally *old* hat to you guys, but it's getting
to be fun for this bored old systems grunt to learn these basics.
Apologize for thinking out loud here, hope someone else is interested.
Here is the list I worked up for prices and ranges.
With the Xmas holidays I couldn't get any info on Fluke, or full lines
from Beckman and some others. I also left off resolution and voltage, current
and resistance ranges, though most units had similar values ( ~ 200-20M Ohms
200mV-1000V, .200uA - 100ma, or 10A). If someone sees something much better,
please let me know. Check out the B+K 2906, and 388-HD.
--------------------------------------------------------------------------------
Capacitance Meters < $300 US
Elenco CM-1500 (D) .1pF - 20,000uF 59-C&S
Beckman Circuitmate CM20A (3.5D) 200pF - 20,000uF 140-VnS
Beckman Circuitmate DM25L (D) 2000pF - 20uF 100-Jen
American Reliance 360-D (D) 200pF - 20,000uF 140-AMR
American Reliance 460-D (D-I,R) 200pF - 20,000uF,200uH - 2H 170-AMR
Mercer 9670 (3.5D) .1pF - 20,000uF 105-VnS
B+K 820 (4D) .1pF - 1F 226-B+K
B+K 830 (4D-Ar) .1pF - .2F 275-B+K
B+K 2832 (3.5D-Bn,V,I,R) 2nF - 20uF 175-B+K
B+K 388-HD (3.5D-V,R,F) .1nF-20u, 20Hz -200kHz 139-B+K
B+K 2907 (3.5D-L,V,I,R) ? - 20u 90-B+K
B+K 2906 (3.5D-V,I,R,T) ?-20uF, -40-1832 Deg F 95-B+K
B+K 2904 (3.5D-L,R) ??? - 20uF 75-B+K
Heathkit IT-2240 (3D-B,L,K) 200pF - 2000uF,200uH - 2000H 200-HTK
Heathkit IB-5281 (A-B,L,R,K) 10pF - 10uF,10uH - 10H 90-HTK
Heathkit SM-2255 (3.5D) 200pF - 2000uF,200uH - 200H 90-HTK
Heathkit IM2320 (3.5D-V,I,R,K) 2nF - 20uF 68-HTK
Heathkit SM2320 Same as IM2320 Above,assembled 78-HTK
Elenco LC-1800 (3.5D-L,C,R) .1pF - 200uF,1uH -200H 125-C&S
IET LCR-620 (3.5D-L,C,R) .1pF - 200uF,1uH - 200H 269-IET
IET CM-500 (3.5D-Ar,D) 1pF - 200,000uF 299-IET
--------------------------------------------------------------------------------
C&S C&S sales Deerfield, IL 800-292-7711 (pp34 Jan90 Radio Electronics).
VnS Van Sickle Electrical 314-621-5000 St. Louis, MO
Jen Jenson Tools 602-968-6231 Phoenix, AZ
B+K B+K Precision 312-889-1448, Chicago, IL
MCR Mercer (Division of Simpson) 312-697-2265 Elgin, IL
HTK Heathkit 800-253-0570 (616-982-3200) Benton Harbor, MI
IET IET Labs, Inc. 516-334-5959 Westbury, NY
AM American Reliance, 800-654-9838 Rosemead CA
FLK John Fluke, 206-356-5400 Everett, WA
ELN Elenco Precision Electronics, 800-533-2441
X-Z[,Z]: Where X is N[.n]D and N[.n] is number of Digits in display, or A for
Analog and Z is Ar for Autoranging, B for Bridge, D for digital output
available, R for Resistance Measurement, V for Voltage Measurement ,
L for Inductance Measurement, T for Temperature Measurement, F for
Frequency Measurement, K for Kit, Bn for Bench Model
Prices provided by above sources rounded to nearest dollar. I gots no
connections with anybody here and have made little effort to
verify/shop around on prices.
--
uunet!swbatl!adams or adams@swbatl.swbt.com | Tom Adams: 314-235-7459
BOOKS WANTED: pre-1930 radio, electrical & scientific topics.henry@utzoo.uucp (Henry Spencer) (12/29/89)
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. Mind you, you need a bit of arithmetic to go from frequency to capacitance, but a $10 calculator solves that unless you're measuring lots of capacitors and speed is important. (Our old electronics wizard, now retired, pointed this out to me during a discussion of cheap test equipment.) It may not give you five-digit accuracy, but capacitor values are seldom that crucial (and can seldom be depended on to be that stable) anyway. -- 1972: Saturn V #15 flight-ready| Henry Spencer at U of Toronto Zoology 1989: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
whit@milton.acs.washington.edu (John Whitmore) (12/29/89)
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.