You will need
  • - copper or nichrome wire;
  • - power supply with adjustable output voltage;
  • - ammeter;
  • - ohmmeter;
  • the load, for example, tungsten.
To calculate the resistance of the shunt, use the following formula:
Rш = (Ra*Ia)/(I-Ia),
where Rш – the required resistance of the shunt; Ra – the resistance of the winding of the ammeter; I – upper value of the measured current; Ia is the current full deflection of the ammeter.
Determine the scale of the existing device maximum measuring current Ia. For example, its value is 100 µa and you need to measure current up to 25 A.
Find the magnitude of the winding resistance RA of the ammeter. It can be taken from the passport of the device or to measure with a ohmmeter (with acceptable error). Let this is equal to 1750 Ohms.
Substitute the values into the formula and get the result:
Rш = (1750*0,0001)/(25-0,0001)=0,007 Om
Now empirically using a model of the ohmmeter to find the length of the wire. The obtained value is small, and for the manufacture of the shunt will need to cut copper wire. It is more correct to use a solicitor with the appropriate shunt resistance value.
If in the future it is necessary to conduct measurements with a given error, the device with the installed shunt must be approved in metrological laboratory since the installation of the shunt lowers the measurement accuracy.
If for any reason it is impossible to find out the data of the measuring device, pick up shunt experimentally. It is desirable to confine small currents up to 5 A.
Sequentially connect the load, monitoring the ammeter and the test ammeter. Terminal of the test ammeter will short-circuit a piece of nichrome or copper of minimum length.
Set the power supply voltage to the control device showed a current value for which the selected shunt. Arrow test ammeter should not exceed the limit.
Slightly releasing the clamp one of the terminals of the ammeter, increase the length of wire between the terminals as long as the needle reaches the maximum scale. Lock the clamps on the test device, trim the excess wire. Turn off the power supply. Current meter range you need ready to use.