From: john@Neopha.44in88.net (John Todd)
Subject: Re: Cheap low frequency impedance bridge?
Date: 11 Oct 2002 00:29:18 GMT
User-Agent: slrn/0.9.5.5 (UNIX)
On Fri, 11 Oct 2002 01:07:48 +0200, Bill Sloman
>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
>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
>Bill Sloman, Nijmegen
You can get the Heathkit schematic at:
ftp://bama.sbc.edu/downloads/schematics/heath/ (I think!)
you will need the .djvu plugin, at:
John E. Todd <> email@example.com
Note: Ensure correct polarity prior to connection.