## PSN-L Email List Message

Subject: Re: Verticle Seismometer with Feedback, Transducer Question
From: Brett Nordgren brett3nt@.............
Date: Fri, 12 Sep 2008 21:11:08 -0400

```Gary,

To answer the questions you ask, you first need to measure the force
constant (converted to Newtons/Ampere) of your coil/magnet.  Then you will
want to decide what mass (in kg) you are expecting to use.  In many designs
the forcing coils are run at a rather low current---a few tens of
microamperes.  10 mA would represent a fairly large current, and would
probably be nowhere near the power limits of the coil. Whatever limits you
will have will almost certainly not involve coil power.

If the derivative branch capacitor is Cd Farads, and you want to obtain Av
Volts/ meter/sec as the 'generator constant' of the completed instrument,
and Gn is the measured coil constant in Newtons/Ampere, you can use the
relationship Av = Mass / (Cd Gn) to get a feel for how it is going to
look.  Many instruments provide a value for Av which is within a factor of
two of 1500 V / m/sec, though if you want Ad to be higher, you can also add
more gain following the feedback loop.  I have heard that some people have
found that more gain is desirable when using a 16-bit A/D rather than a
professionally more common 24-bit system.  It makes for a lower clipping
level, but will give better sensitivity for small motions.

Regards,
Brett

At 04:36 PM 9/12/2008 -0700, you wrote:

>I m collecting parts for a vertical seismometer and intend to use
>feedback. The feedback transducer I have is from a 12 hi-fi speaker. I
>have separated the voice-coil and magnet and now wondering what sort of
>current capability do I need to specify for the voice-coil driver.
>Certainly this voice-coil must have high power capacity when it was used
>as a speaker. DC resistance is 7 ohms. If you assume a 20 watt speaker and
>7 ohm resistance, then RMS current would be 1.7 amp. Has anyone built a
>transducer driver with this sort of capability. Next question what would
>be the voice coil force I would get. Sounds like I need to do some
>experiments. Any thoughts or experience is much appreciated.
>
>Gary
>
>

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