Re: OT: who has access to a professional capacimeter?
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I'm skeptical of black-box capacitance meters. Connect a square-wave source, resistor, and capacitor in series, with grounded end of source? and one end of cap in common, and watch charging/discharging of cap voltage?with an oscilloscope. Measure half-life of exponential decay, calculate time constant, and solve?for capacitance. Remember that total resistance is sum of resistor you put in circuit plus generator resistance of square wave source.
If you plan to do this a lot--especially with small caps--you should do some "calibration" runs with several accurately known small caps: find capacitance of o'scope probe.
On Monday, August 3, 2015 9:39 AM, "Jerry Lee Marcel jerryleemarcel@... [LTspice]" wrote:
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I wonder: Is there 'be trusted to buy those capacitance
meters?
This has already been answered. When the real capacitor is close
enough to the basic "one C- two R" model, they generally can be
trusted, although different measurement frequencies are likely to
give slightly different values. As for electrolytics, they cannot be
trusted to give an accurate value, because such a concept does not
exist; as I mentioned earlier, their capacitance varies with
frequency, but also electrolytic show significant DA (dielectric
absorption), a non linear phenomenon that makes the apparent
capacitance change over time. In addition, capacitance decreases
with time, but some regeneration can happen, and it also varies
significantly with temperature.
These issues are known from the beginning, that's why electrolytics
are not to be used in applications where their value is critical, as
in filters (although many cheap loudspeaker manufactures use NP
lytics in their passive x-overs).
From their inception, designers have commonly used a factor 10.
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