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## [Amps] meter internal R value, quick and easy.

 To: [Amps] meter internal R value, quick and easy. garyschafer@attbi.com (Gary Schafer) Thu, 01 Aug 2002 10:48:20 -0400
 ```That won't work exactly right. If the pot in parallel with the meter matches the meter resistance then the sum of the meter and parallel resistor is 1/2 of what the meter was alone. That will increase the series circuit current from what it was with just the meter so there will be more circuit current than exactly 1/2 the original current in the meter and the resistor. If you set the pot for 1/2 scale meter reading the pot and parallel resistor will each have a different current because the total series circuit resistance has changed and the circuit current is different. It would work if you had a second meter to monitor the series circuit current and readjusted the series pot to maintain the same circuit current. 73 Gary K4FMX skipp isaham wrote: > I was surprised to see that no one came up with > the classic simple answer regarding the meter > internal R post... > - > - > Two Potentiometers, a 9 volt battery and one or two > resistors will do the trick. > - > Assume the movement to be 50 uA unless otherwise > known "in stone" that it is another value. I have > surplus meters with full scale 1mA indicated, but > having micro amp movements. > - > One must construct a simple current loop using > a few simple parts. > - > Limit the max current to a "safe value" that will > not peg (slam) the movement hard (might actually > bend the meter needle). Pick a value like 47K min > as your soft limit value, placed in series with a > 250K linear pot and the meter movement. > Observe polarities... > - > Adjust the series linear pot for full scale deflection > of the meter. Leave the meter reading full > scale and apply a second pot as described > just below. > - > Place the second pot in parallel with the meter > and adjust it for half scale on the meter. A > 250 ohm pot is probably a good value to use. > - > With the 250 ohm pot set at some position > which drops the meter reading from full, to > half scale.... the meter and the 250 ohm pot > share equal current. Thus.... their values are > equal. Measure the value across the pot and > you have your meter movement resistance. > - > In step one, you might decrease the value of > the 47K resistor to obtain full scale meter > readings on higher current meter movements. > - > A 1 milli amp "mA" movement might use > a 4700 ohm resistor as your testing safe > limit value. > - > cheers > skipp > - > >Anyone know of a procedure where I can determine the internal resistance > >without damaging the meter movement? or, am i SOL? Some are uA meters, > >some volts and some watts -- so who knows what the intended range was. > >Thanks, > >Gerry, W1VE > > ________________________________________________________________ > GET INTERNET ACCESS FROM JUNO! > Juno offers FREE or PREMIUM Internet access for less! > Join Juno today! For your FREE software, visit: > http://dl.www.juno.com/get/web/. > _______________________________________________ > Amps mailing list > Amps@contesting.com > http://lists.contesting.com/mailman/listinfo/amps ```
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