[SI-LIST] : Method of measuring characteristics of a capacitor

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From: Doug Smith ([email protected])
Date: Fri Jan 07 2000 - 10:56:10 PST

I have included below the description of how to easily measure ESR,
inductance, and capacitance of a capacitor using a logic pulse generator
and a scope. This was included in an earlier post, but this time it is
in plain text.
Here is a quick way of measuring ESR, capacitance, and inductance of a
capacitor. Connect a 5 V logic pulse
generator through 100 Ohms to the resistor. Keep all leads shorter than
1/2 cm. I do this by putting a 100 Ohm
carbon comp 1/2 watt resistor into a BNC female jack with one lead
clipped so the body fully inserts into the
jack. I then solder the capacitor to the exposed end of the resistor
whose lead is clipped to a stub. The other
end of the cap solders to the BNC shell. The generator should have a 1
to 2 ns risetime. Give the capacitor one
pulse while looking at the first few tens of mv of the exponential rise.
You will see a peak, an offset, and an
upward slope. The peak is from e = L*di/dt, the offset from ESR, and the
slope from the capacitance. It is
pretty easy to calculate these parameters from the scope screen. Probe
quality is very important so do not use
one with a capacitive input impedance, a probe built from a 1000 Ohm
resistor and a piece of coax will work

Here is my attempt at an ASCII diagram of the scope waveform:

                        -- slope gives capacitance from i = c*dv/dt
           / -- --------
          / \ -- e=Ldi*dt
         / \ --
        / -- ----- --------
-------/ ----- ESR

When paralleling capacitors (large and small ones for greater
bandwidth), this method will alert you if the small
one is resonating with the inductance of the large one.
BTW, this month's article at www.dsmith.org is how to display
measurement error simultaneously on a scope screen with the measurement
itself (current measurements).


    ___          _          Doug Smith
     \          / )         Manager EMC Development & Test
      =========             Auspex Systems
   _ / \     / \ _          2300 Central Expressway
 /  /\  \ ] /  /\  \        Santa Clara, CA 95050-2516
|  q-----( )  |  o  |       Phone/FAX: 408-566-2157/2020
 \ _ /    ]    \ _ /        Email: [email protected]

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