You will need
- — ammeter;
- the conductor of the cross sections of the known material;
- — table of specific resistances.
Connect a shunt in parallel with the ammeter to expand the possibilities of its measurement. In this case, a shunt current passes through the primary, and that part which is to be measured, passes through the ammeter. Rated current in the network calculate the formula.
To calculate the shunt, find out the maximum amperage that will have to be measured by the device. To do this, measure the voltage at the current source U in volts and divide it by the total resistance of the circuit R in ohms. All measurements performed by tester if the current is constant in this case, consider the polarity of the device. Find the rated current in the circuit by dividing voltage resistance I=U/R. Look at the scale of the ammeter and find out the maximum current it can measure.
Find the resistance of the shunt. To do this, measure your own resistance-ammeter R1 in ohms, and find the resistance of the shunt by dividing the product of the maximum current that can be measured by the device I1 and its resistance R1 to the rated current in the network I (R=(I1∙R1)/I).
Example. Need to measure the current in the circuit, where the maximum value can reach up to 20 A. it is proposed to use the ammeter with the maximum possible measurement current of 100 mA and a resistance of 200 Ohms. The resistance of the shunt in this case is R=(0,1∙200)/20= 1 Om.
As the graft use the standard resistors. If those are not available, make a shunt yourself. For the manufacture of shunts it is best to take conductors of copper or other material with high conductivity. To calculate the desired length of the conductor shunt l, take the wire the cross sections S and find out the resistivity ρ of the material from which made this device. Then, the resistance R, times the cross section of the conductor, measured in mm2, and divide by its resistivity, expressed in Ohm∙ mm2/m, taken from a special table l=R∙S/ρ.
To manufacture the shunt to the ammeter from the above example of a copper wire 0.2 mm2, take its length, which calculate according to the formula l=1∙0,2/0,0175=11,43 m. the same principle use and bypass any other part of the circuit.