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From: email@example.com (Bill Sloman)
Subject: Re: Cheap low frequency impedance bridge?
Date: 11 Oct 2002 14:47:46 -0700
NNTP-Posting-Date: 11 Oct 2002 21:47:46 GMT
firstname.lastname@example.org (john jardine) wrote in message news:<email@example.com>...
> "Bill Sloman" wrote in message news:...
> > I've got a component at work that looks like a resistance in series with a
> > capacitor, and I've got to monitor the capacitance fairly precisely to look
> > for degradation in use - the device is set up so that the resistive
> > impedance (which is what we are interested in) almost dominates the
> > impedance at the measurement frequency, but the capacitance can get eroded
> > in service, and we are going to have put numbers on this.
> > The measurement frequency range is 2kHz to 200kHz, and we'll probably
> > monitor down to rather lower frequencies if we can, to boost the reactive
> > impedance to more or less match the resistive component.
> > So I need an impedance bridge. I don't need - and couldn't pay for - an RF
> > mpedance bridge going into the MHz range.
> > The resistive impedance range runs from to 10R to 10k, but the region up to
> > 100R is the most interesting. The capacitance runs from about 1uF to about
> > 100uF, and the higher values are frequency dependent (just to make life
> > interesting and to exclude LCR measuring gear that doesn't give you control
> > over the excitation frequency).
> > The excitation voltage should be about 5mV rms - we've used more (up to
> > about 100mV) without seeing any gross problems, but more precise
> > measurements may need the lower drive level.
> > We've done some preliminary work on an HP 4192A bridge in a nearby
> > university lab, at vast expense. The HP4192A was just what I wanted, but
> > Agilent don't make it any more, and my boss couldn't afford to buy it if
> > they did.
> > Anybody know of anything cheap that offers frequencies up to 200kHz? I used
> > a signal generator and an oscilloscope for the intial development, a couple
> > of years ago, but for this work we really need something rather more
> > precise.
> > ----
> > Bill Sloman, Nijmegen
> You've got a real stonker of a measuring problem here. I make the D
> values ranging from '8'(2kHz-1u-10r) to '.000008'(200kHz-100u-10k) and
> consequent phase angles of 83deg to an incredible 46udeg.
Well, we don't have a problem at the 10k/100uF/200kHz end of the
range, and while I've still got 100uF, I don't have a problem at 10R
either. What I need to do is get the frequency down to 150Hz so I've
got 45 degrees of phase shift and can measure the 100uF *precisely*,
and see any small progressive erosion in the capacitance long before
it starts giving me trouble where I'm actually measuring the 10R, and
maybe predict the service life of the gadget within a period
substantially shorter than the service life.
In fact I had to go down to 10Hz during the exploratory measurements
to get a sensible phase angle, but we had over-done it to some tune on
that particular component.
> The HP4284 'LCR meter' (D down to .000001) or the HP4278 'C meter'(D
> down to .00001) could make a stab at it but at enormous cost. The fine
> phase(time) resolution needed, implies a lot of precision digital
> signal processing and consequent expense.
Tom Bruhns has already recommended the Agilent 4284 (I e-mailed him
directly) and it would be just what I want - but at nearly $14,000 my
boss was *not* sympathetic. The Agilent 4263 at a third of the price
would probably do, but the boss is looking for an even cheaper
> Perhaps the best option is to restrict the test range to say a 'Q' of
> 1000 max and go for an older second hand HP LCR meter, say £1500, or a
> HP4342 'Q' meter for a few hundred pounds.(Even then Q meter has a
> lower limit of 22kHz +inductor Q problems.)
Tom says that what they normally use in their lab is an old 4274, but
I've not found any used examples on the web so far. MicroLease came up
with an Agilent 4285, which is just one digit off, but it won't go
Thanks for the response!
Bill Sloman, Nijmegen
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