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

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From: Douglas C. Smith (doug@dsmith.org)
Date: Sat Jan 08 2000 - 12:38:45 PST


Hi Jose,

Here it is:

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
well.

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 creen with the measurement
itself (current measurements).

Doug

-------------
"Cruz, Jose" wrote:
>
> Can you resend
>

-- 
-----------------------------------------------------------
    ___          _           Doug Smith
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      =========              Los Gatos, CA 95031-1457
   _ / \     / \ _           TEL/FAX: 408-356-4186/358-3799
 /  /\  \ ] /  /\  \         Mobile:  408-858-4528
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 \ _ /    ]    \ _ /         Website: http://www.dsmith.org
-----------------------------------------------------------

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