From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: SMD Cap Test Fixture
References: <3E225B37.A41@sneakemail.com> <3E230680.firstname.lastname@example.org> <3E23128D.email@example.com> <3E256E49.firstname.lastname@example.org>
Date: Thu, 16 Jan 2003 12:58:12 -0500
NNTP-Posting-Date: Thu, 16 Jan 2003 12:57:40 EST
Organization: Bell Sympatico
> Mike Monett wrote:
> > email@example.com 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,
> not much)
> 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 have a reciprocal counter and found the linearity was pretty good over
about 5 decades down to about 10pF.
Using frequency and a lookup table, the measurement can reach 9 decades,
but this may be academic since the large value electrolytics need a
different test jig anyway. So I might as well have a separate oscillator.
Then linearity is a very good thing, since it eliminates the lookup
> 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.
Excellent idea! Very clever.
Yes, I have some tools that became magnetized and now attract caps.
Sometimes it helps, sometimes it is a bit of a pain.
> http://inquisitor.i.am/ | mailto:firstname.lastname@example.org | Ian Stirling.