The Ammeter and the Voltmeter — Question 1

Question 1

A shunt converts a sensitive movement into a high-current ammeter

A meter movement has full-scale current Ig=50.0 microampere and resistance Rg=2.00 kilohm. A parallel shunt is to produce a 5.00 A full-scale ammeter.

1. Find the shunt resistance and full-scale voltage burden across the instrument.

2. Find the effective ammeter resistance and shunt heating at full scale.

Question 1 - Solution

A shunt converts a sensitive movement into a high-current ammeter

(a) Method and calculation

$$V_g=I_gR_g=(50.0\times10^{-6})(2000)=.100\,\mathrm V.$$$$R_s=\frac{I_gR_g}{I_{\mathrm{FS}}-I_g}=\frac{.100}{5-.000050}=.0200002\,\Omega.$$

(b) Interpretation and further calculation

$$R_A=V_g/I_{\mathrm{FS}}=.0200\,\Omega.$$$$P_s=(I_{\mathrm{FS}}-I_g)V_g=.499995\,\mathrm W\simeq.500\,\mathrm W.$$

Physical check

The shunt carries almost all the current, but both parallel paths have the same voltage.