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From: email@example.com (Bill Sloman)
Subject: Re: digital voltmeter and ammeter
Date: 20 Jan 2003 00:34:49 -0800
References: <3E2AB7F8.firstname.lastname@example.org> <3E2ACBE3.email@example.com> <3E2AF137.firstname.lastname@example.org> <email@example.com> <3E2B345F.firstname.lastname@example.org>
NNTP-Posting-Date: 20 Jan 2003 08:34:49 GMT
Alex Graham wrote in message news:<3E2B345F.email@example.com>...
> lol well I just the MAX712 switch mode circuit. Will I have neighbours
> knocking on my door in protest now?
> so what do you mean by a filter?
The MAX712/MAX713 data sheet shows a 10uF electrolytic capacitor C3
across the battery in several of the circuit diagrams, most clearly on
page 16. This is half a filter, in that it provides an easy, local
by-pass for high frequency switching noise. The other half of the
filter is an inductor between the capacitor and the battery to add
extra impedance to inhibit high-frequency noise currents from going
through the battery or - God forbid! - the load.
Choosing the inductor is always tricky. I usually look through the
Farnell catalogue for the biggest inductor (highest inductance) that I
can afford/accomodate that will take the maximum current the source
can deliver, but if you know that a specific frequency is a problem
you just make sure that the inductive impedance of the inductor at the
frequency of interest is a good bit lower than the capacitative and
resistive impedance of the capacitor C3 at the same frequency.
If TV and radio interference is a real worry, you normally want to add
a "ferrite bead" or "ferrite chip" inductor in series with any wound
inductor - all wound inductors end up looking like capacitors at
sufficiently high frequencies (listed in their data sheets as their
"self resonant frequency") whereas as ferrite beads and chips are not
wound inductors and are engineered to look like resistors at very high
For high frequency attentuation you also to put a high frequency
capacitor - typically a 100nF multilayer ceramic - in parallel with
your 10uF electrolytic.
You can buy electrolytic capacitors where the equivalent series
resistance is specified at 100kHz, but even these "high frequency"
parts, aimed at the switching power supply market, start looking like
resistors at around 100kHz and higher, while multilayer ceramic disks
look like capacitors up to of the order of 100MHz.
This is the simplest form of LC network - an L-section filter. Check
the resonant frequency of the LC pair, and the reactive impedance of
the L and the C at resonance (it has to be the same) and make sure
that the resistive impedance involved is bigger, so that any resonance
is damped - most electrolytic capacitors do this for you, but every
now and again I have to add a low value resistor in series with the
A text-book on filter design will take you through progressively more
complicated structures, and exponentially more complicated mathematics
- there are good textbooks on the subject, but it is essentially
specialised post-graduate stuff.
Hope this helps. Doing filters and by-passing right is complicated,
and most of us get by with very rough rules of thumb.
Bill Sloman, Nijmegen
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