[R-390] Meters

Thu, 08 May 2003 05:49:03 -1000

Ahhhhh, therein lies the problem!  This meter only requires 17 millivolts 
across it to achieve full scale deflection.  This makes it a very sensitive 
meter.  Typical meters of this size are higher resistance and require 60-100 
millivolts or more for full scale deflection.  You can get the right 
resistance or current but getting them both together is what makes these 
meters 'special'.
One trick used is to get the right resistance with a shunt and use a small 
DC amplifier on a piece of perf board to drive the meter to get the right 
current.  But then, this isn't 'original', is it?

pete KH6GRT

>From: "Scott, Barry (Clyde B)" <[email protected]>
>To: "R390 (E-mail)" <[email protected]>
>Subject: RE: [R-390] Meters
>Date: Thu, 8 May 2003 09:50:11 -0500
>
>Okay, we can figure out the internal resistance.
>
>Now, assume I find a 1ma fs meter that has an internal resistance different
>from what I need.  Is it possible with a combination of series and parallel
>resistors to achieve the 17-ohm (or whatever it needs to be) resistance to
>make it swing full-scale and appear to have 17-ohms in the circuit?  I
>assume so but I don't like to assume...
>
>Barry(III) - N4BUQ
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