April 2015 Archives

Why Analog Meters?

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Why should I use analog meters?

  • Require no batteries
  • Can be left in circuit at all times
  • Instant or continuous reading/observation
  • Range can be set by configuration/specification/design

Analog meters may be used for a number of applications for measuring volts, amps, watts, ohms, etc. The biggest advantage is that one need not replace batteries to continue to observe electrical conditions.

Protection

In the case of voltmeters, one can protect the meter by using zener diodes. The range in a voltmeter is set by the coil resistance, as well as the multiplier or ballast resistor.

For ammeters, a pair of Schottky diodes wired back to back will protect the meter movement from large swings, with the hazard being that a 50 or 75 mV Full-Scale Deflection (FSD) meter movement may experience 300-400 mV as the diode starts to conduct, effectively acting like a zener diode.

  • Caveat: Ammeter shunts must be designed or rated for continuous-duty use to remain in circuit all the time. 
A standard shunt is only designed for 66% of the continuous-duty power dissipation requirements.  To maintain resistance rating, a larger shunt must be used that provides the same resistance over the desired range. One may calculate the range based on comparing 50mV shunts to 75 mV shunts and lining up the resistance to power ratings.

50mV is 66% of 75 mV; a 75A 75 mV shunt produces 50mV at 66% of the rated current (75A x 66% = 49.5A). Thus, a 75A 75mV intermittent-duty shunt is also  50mV 50A continuous-duty shunt (plus or minus a few percent).

To protect either in the case or situation of induced RF, liberal use of Schottky diodes and/or 10 - 1000 nF (0.01 uF, 0.1 uF, and 1 uF) capacitors will effectively short out the AC component.

A secondary caveat to using analog meters is that they are not known for accuracy; common meters were +/- 3% accurate, while high-accuracy meters were often +/- 1% accurate. Digital meters are far more accurate, but with accuracy come costs, and external power requirements.

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This page is an archive of entries from April 2015 listed from newest to oldest.

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