Subject: Re: SMD Cap Test Fixture
Date: Wed, 15 Jan 2003 22:09:17 +0000 (UTC)
References: <3E225B37.A41@sneakemail.com> <3E230680.email@example.com> <3E23128D.firstname.lastname@example.org> <3E256E49.email@example.com>
NNTP-Posting-Date: Wed, 15 Jan 2003 22:09:17 +0000 (UTC)
User-Agent: tin/1.5.14-20020926 ("Soil") (UNIX) (Linux/2.4.20 (i686))
Mike Monett wrote:
> firstname.lastname@example.org wrote:
>> Tie inputs together, and use it as a standard 2 inverter multivibrator,
>> I used this as it was what was in the bits box.
> Thanks, Ian. I didn't want to use that arrangement since both ends of the
> cap are floating and stray capacity would cause more error at low values.
True, but one end of the cap is effectively grounded through the small
impedance of the output buffer. (well, it has a bit of effect, but as long
as the timing resistor is quite a bit bigger than the output impedance,
All stray capacitance there does is to slightly slow the slew-rate of the
output, which has relatively little effect.
In practice, I found it worked well, especially if it was used with
a frequency counter capable of outputing reciprocals.
Then you just need to subtract a small constant.
I forgot to add...
The probe I used on the few occasions I needed to measure the value
of a SMD cap with this was simple.
A powerfull magnet from a disk drive (~5mm*20mm*10mm) was used as the
base for the fixture.
The magnet was mounted on end, with the 5mm edge up, so that the north-south
field kept the cap both pulled down, and aligned with the faces of the
A 0.5mm base of some plastic was put over the magnet, and a fixed electrode
was placed near the center of the magnet.
A second electrode that slid over the plastic base (I think I put a little
steel nut in so that the magnet would pull this onto the DUT) was used
to make the other contact.
I found that all capacitors I tried were sufficiently magnetic, I think
often due to the nickel plating in them.
http://inquisitor.i.am/ | mailto:email@example.com | Ian Stirling.
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